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  • 1. Standard Atomic Weights Based on the assigned relative mass of C = 12. For the sake of completeness, all known elements are included in the list. Sev- 12eral of those more recently discovered are represented only by the unstable isotopes. In each case, the values in parentheses inthe atomic weight column are the mass numbers of the most stable isotopes. Atomic Atomic Atomic Atomic Name Symbol No. Weight Valence Name Symbol No. Weight ValenceActinium Ac 89 227.028 ... Mercury Hg 80 200.59 1,2Aluminum Al 13 26.9815 3 (hydrargyrum)Americium Am 95 (243) 3,4,5,6 Molybdenum Mo 42 95.94 3,4,6Antimony Sb 51 121.75 3,5 Neodymium Nd 60 144.24 3 (stibium) Neon Ne 10 20.1179 0Argon Ar 18 39.948 0 Neptunium Np 93 237.0482 4,5,6Arsenic As 33 74.9216 3,5 Nickel Ni 28 58.69 2,3Astatine At 85 (210) 1,3,5,7 Niobium Nb 41 92.9064 3,5Barium Ba 56 137.33 2 (columbium)Berkelium Bk 97 (247) 3,4 Nitrogen N 7 14.0067 3,5Beryllium Be 4 9.0122 2 Nobelium No 102 (259) ...Bismuth Bi 83 208.980 3,5 Osmium Os 76 190.2 2,3,4,8Boron B 5 10.81 3 Oxygen O 8 15.9994 2Bromine Br 35 79.904 1,3,5,7 Palladium Pd 46 106.42 2,4,6Cadmium Cd 48 112.41 2 Phosphorus P 15 30.9738 3,5Calcium Ca 20 40.08 2 Platinum Pt 78 195.08 2,4Californium Cf 98 (251) ... Plutonium Pu 94 (244) 3,4,5,6Carbon C 6 12.011 2,4 Polonium Po 84 (209) ...Cerium Ce 58 140.12 3,4 Potassium K 19 39.0983 1Cesium Cs 55 132.9054 1 (kalium)Chlorine Cl 17 35.453 1,3,5,7 Praseodymium Pr 59 140.908 3Chromium Cr 24 51.996 2,3,6 Promethium Pm 61 (145) 3Cobalt Co 27 58.9332 2,3 Protactinium Pa 91 231.0359 ...Columbium Radium Ra 88 226.025 2 (see Niobium) Radon Rn 86 (222) 0Copper Cu 29 63.546 1,2 Rhenium Re 75 186.207 ...Curium Cm 96 (247) 3 Rhodium Rh 45 102.906 3Dysprosium Dy 66 162.50 3 Rubidium Rb 37 85.4678 1Einsteinium Es 99 (252) ... Ruthenium Ru 44 101.07 3,4,6,8Erbium Er 68 167.26 3 Samarium Sm 62 150.36 2,3Europium Eu 63 151.96 2,3 Scandium Sc 21 44.9559 3Fermium Fm 100 (257) ... Selenium Se 34 78.96 2,4,6Fluorine F 9 18.9984 1 Silicon Si 14 28.0855 4Francium Fr 87 (223) 1 Silver Ag 47 107.868 1Gadolinium Gd 64 157.25 3 (argentum)Gallium Ga 31 69.72 2,3 Sodium Na 11 22.9898 1Germanium Ge 32 72.59 4 (natrium)Gold Au 79 196.967 1,3 Strontium Sr 38 87.62 2 (aurum) Sulfur S 16 32.06 2,4,6Hafnium Hf 72 178.49 4 Tantalum Ta 73 180.9479 5Helium He 2 4.0026 0 Technetium Tc 43 (98) 6,7Holmium Ho 67 164.930 3 Tellurium Te 52 127.60 2,4,6Hydrogen H 1 1.0079 1 Terbium Tb 65 158.925 3Indium In 49 114.82 3 Thallium Tl 81 204.383 1,3Iodine I 53 126.905 1,3,5,7 Thorium Th 90 232.038 4Iridium Ir 77 192.22 3,4 Thulium Tm 69 168.934 3Iron Fe 26 55.847 2,3 Tin Sn 50 118.71 2,4 (ferrum) (stannum)Krypton Kr 36 83.80 0 Titanium Ti 22 47.88 3,4Lanthanum La 57 138.906 3 Tungsten W 74 183.85 6Lawrencium Lr 103 (260) ... (wolfram)Lead Pb 82 207.2 2,4 Uranium U 92 238.029 4,6 (plumbum) Vanadium V 23 50.9415 3,5Lithium Li 3 6.941 1 Xenon Xe 54 131.29 0Lutetium Lu 71 174.967 3 Ytterbium Yb 70 173.04 2,3Magnesium Mg 12 24.305 2 Yttrium Y 39 88.9059 3Manganese Mn 25 54.9380 2,3,4,6,7 Zinc Zn 30 65.39 2Mendelevium Md 101 (258) ... Zirconium Zr 40 91.224 4Modified and reproduced, with permission from Lide DR (editor-in-chief): CRC Handbook of Chemistry and Physics,83rd ed. CRC Press, 2002–2003.
  • 2. a LANGE medical bookReview ofMedical Physiologytwenty-second editionWilliam F. Ganong, MDJack and DeLoris Lange Professor of Physiology EmeritusUniversity of CaliforniaSan FranciscoLange Medical Books/McGraw-HillMedical Publishing DivisionNew York Chicago San Francisco Lisbon London Madrid Mexico CityMilan New Deli San Juan Seoul Singapore Sydney Toronto
  • 3. Review of Medical Physiology, Twenty-Second EditionCopyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved. Printed in the United States ofAmerica. Except as permitted under the United States Copyright Act of 1976, no part of this publicationmay be reproduced or distributed in any form or by any means, or stored in a data base or retrieval system, withoutthe prior written permission of the publisher.Previous editions copyright © 2003, 2001 by The McGraw-Hill Companies, Inc.; copyright © 1999, 1997, 1995,1993, 1991, by Appleton & Lange; copyright © 1963 through 1989 by Lange Medical Publications.1234567890 DOC/DOC 098765ISBN 0-07-144040-2ISSN 0892-1253 Notice Medicine is an ever-changing science. As new research and clinical experience broaden our knowledge, changes in treatment and drug therapy are required. The author and the publisher of this work have checked with sources believed to be reliable in their efforts to provide information that is complete and generally in accord with the standards accepted at the time of publication. However, in view of the possibility of human error or changes in medical sciences, neither the author nor the publisher nor any other party who has been involved in the preparation or publication of this work warrants that the information contained herein is in every respect accurate or complete, and they disclaim all responsibility for any errors or omissions or for the results obtained from use of the information contained in this work. Readers are encouraged to confirm the information contained herein with other sources. For example and in particular, readers are advised to check the product information sheet included in the package of each drug they plan to administer to be certain that the information contained in this work is accurate and that changes have not been made in the recommended dose or in the contraindications for administration. This recommendation is of particular importance in connection with new or infrequently used drugs.The book was set in Adobe Garamond by Rainbow Graphics.The editors were Janet Foltin, Harriet Lebowitz, and Regina Y. Brown.The production supervisor was Catherine H. Saggese.The cover designer was Mary McKeon.The art manager was Charissa Baker.The index was prepared by Katherine Pitcoff.RR Donnelley was printer and binder.This book is printed on acid-free paper.
  • 4. ContentsPreface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiSECTION I. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1. The General & Cellular Basis of Medical Physiology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Introduction 1 Transport Across Cell Membranes 28 General Principles 1 The Capillary Wall 35 Functional Morphology of the Cell 8 Intercellular Communication 36 Structure & Function of Homeostasis 48 DNA & RNA 18 Aging 48 Section I References 49SECTION II. PHYSIOLOGY OF NERVE & MUSCLE CELLS . . . . . . . . . . . . . . . . . . . . . . . . . 51 2. Excitable Tissue: Nerve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 Introduction 51 Properties of Mixed Nerves 60 Nerve Cells 51 Nerve Fiber Types & Function 60 Excitation & Conduction 54 Neurotrophins 61 Ionic Basis of Excitation Neuroglia 63 & Conduction 58 3. Excitable Tissue: Muscle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 Introduction 65 Cardiac Muscle 78 Skeletal Muscle 65 Morphology 78 Morphology 65 Electrical Properties 78 Electrical Phenomena Mechanical Properties 78 & Ionic Fluxes 68 Metabolism 81 Contractile Responses 68 Pacemaker Tissue 81 Energy Sources & Metabolism 74 Smooth Muscle 82 Properties of Skeletal Muscles Morphology 82 in the Intact Organism 75 Visceral Smooth Muscle 82 Multi-Unit Smooth Muscle 84 4. Synaptic & Junctional Transmission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 Introduction 85 Principal Neurotransmitter Systems 94 Synaptic Transmission 85 Synaptic Plasticity & Learning 116 Functional Anatomy 85 Neuromuscular Transmission 116 Electrical Events in Postsynaptic Neuromuscular Junction 116 Neurons 88 Nerve Endings in Smooth & Cardiac Inhibition & Facilitation Muscle 118 at Synapses 91 Denervation Hypersensitivity 119 Chemical Transmission of Synaptic Activity 94 iii
  • 5. iv / CONTENTS 5. Initiation of Impulses in Sense Organs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 Introduction 121 Generation of Impulses in Different Nerves 123 Sense Organs & Receptors 121 “Coding” of Sensory Information 124 The Senses 121 Section II References 127SECTION III. FUNCTIONS OF THE NERVOUS SYSTEM . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 6. Reflexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 Introduction 129 Polysynaptic Reflexes: The Withdrawal Reflex 134 Monosynaptic Reflexes: General Properties of Reflexes 137 The Stretch Reflex 129 7. Cutaneous, Deep, & Visceral Sensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138 Introduction 138 Temperature 142 Pathways 138 Pain 142 Touch 141 Other Sensations 147 Proprioception 142 8. Vision. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148 Introduction 148 Responses in the Visual Pathways & Cortex 160 Anatomic Considerations 148 Color Vision 163 The Image-Forming Mechanism 152 Other Aspects of Visual Function 166 The Photoreceptor Mechanism 156 Eye Movements 168 9. Hearing & Equilibrium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 Introduction 171 Hearing 176 Anatomic Considerations 171 Vestibular Function 183 Hair Cells 175 10. Smell & Taste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185 Introduction 185 Taste 188 Smell 185 Receptor Organs & Pathways 188 11. Alert Behavior, Sleep, & the Electrical Activity of the Brain. . . . . . . . . . . . . . . . . . . . . . . . . . 192 Introduction 192 Evoked Cortical Potentials 193 The Thalamus & the Cerebral The Electroencephalogram 194 Cortex 192 Physiologic Basis of the EEG, Consciousness, The Reticular Formation & the Reticular & Sleep 196 Activating System 192 12. Control of Posture & Movement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202 Introduction 202 Spinal Integration 207 General Principles 202 Medullary Components 210 Corticospinal & Corticobulbar Midbrain Components 211 System 203 Cortical Components 212 Anatomy & Function 203 Basal Ganglia 213 Posture-Regulating Systems 206 Cerebellum 217
  • 6. CONTENTS / v 13. The Autonomic Nervous System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 Introduction 223 Chemical Transmission at Autonomic Anatomic Organization of Autonomic Junctions 223 Outflow 223 Responses of Effector Organs to Autonomic Nerve Impulses 226 14. Central Regulation of Visceral Function. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232 Introduction 232 Relation to Cyclic Phenomena 235 Medulla Oblongata 232 Hunger 235 Hypothalamus 233 Thirst 240 Anatomic Considerations 233 Control of Posterior Pituitary Secretion 242 Hypothalamic Function 234 Control of Anterior Pituitary Secretion 248 Relation to Autonomic Function 234 Temperature Regulation 251 Relation to Sleep 235 15. Neural Basis of Instinctual Behavior & Emotions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256 Introduction 256 Other Emotions 259 Anatomic Considerations 256 Motivation & Addiction 260 Limbic Functions 256 Brain Chemistry & Behavior 261 Sexual Behavior 257 16. “Higher Functions of the Nervous System”: Conditioned Reflexes, Learning, & Related Phenomena . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266 Introduction 266 Learning & Memory 266 Methods 266 Functions of the Neocortex 272 Section III References 276SECTION IV. ENDOCRINOLOGY, METABOLISM, & REPRODUCTIVE FUNCTION . . . 279 17. Energy Balance, Metabolism, & Nutrition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279 Introduction 279 Protein Metabolism 292 Energy Metabolism 279 Fat Metabolism 298 Intermediary Metabolism 282 Nutrition 311 Carbohydrate Metabolism 285 18. The Thyroid Gland . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317 Introduction 317 Effects of Thyroid Hormones 323 Anatomic Considerations 317 Regulation of Thyroid Secretion 326 Formation & Secretion Clinical Correlates 328 of Thyroid Hormones 317 Transport & Metabolism of Thyroid Hormones 321 19. Endocrine Functions of the Pancreas & Regulation of Carbohydrate Metabolism . . . . . . . . . 333 Introduction 333 Fate of Secreted Insulin 335 Islet Cell Structure 333 Effects of Insulin 336 Structure, Biosynthesis, & Secretion Mechanism of Action 338 of Insulin 334 Consequences of Insulin Deficiency 340
  • 7. vi / CONTENTS Insulin Excess 344 Effects of Other Hormones & Exercise Regulation of Insulin Secretion 345 on Carbohydrate Metabolism 351 Glucagon 348 Hypoglycemia & Diabetes Mellitus in Humans 353 Other Islet Cell Hormones 350 20. The Adrenal Medulla & Adrenal Cortex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 356 Introduction 356 Physiologic Effects of Adrenal Morphology 356 Glucocorticoids 369 Adrenal Medulla 358 Pharmacologic & Pathologic Effects Structure & Function of Medullary of Glucocorticoids 370 Hormones 358 Regulation of Glucocorticoid Regulation of Adrenal Medullary Secretion 372 Secretion 361 Effects of Mineralocorticoids 375 Adrenal Cortex 361 Regulation of Aldosterone Secretion 377 Structure & Biosynthesis of Role of Mineralocorticoids in the Adrenocortical Hormones 361 Regulation of Salt Balance 380 Transport, Metabolism, & Excretion Summary of the Effects of of Adrenocortical Hormones 366 Adrenocortical Hyper- Effects of Adrenal Androgens & Hypofunction in Humans 380 & Estrogens 368 21. Hormonal Control of Calcium Metabolism & the Physiology of Bone . . . . . . . . . . . . . . . . . 382 Introduction 382 The Parathyroid Glands 390 Calcium & Phosphorus Metabolism 382 Calcitonin 393 Bone Physiology 383 Effects of Other Hormones & Humoral Agents on Vitamin D & the Calcium Metabolism 395 Hydroxycholecalciferols 387 22. The Pituitary Gland . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396 Introduction 396 Physiology of Growth 404 Morphology 396 Pituitary Insufficiency 408 Intermediate-Lobe Hormones 397 Pituitary Hyperfunction in Humans 409 Growth Hormone 398 23. The Gonads: Development & Function of the Reproductive System . . . . . . . . . . . . . . . . . . . 411 Introduction 411 Gametogenesis & Ejaculation 424 Sex Differentiation & Development 411 Endocrine Function of the Testes 428 Chromosomal Sex 411 Control of Testicular Function 431 Embryology of the Human Abnormalities of Testicular Function 433 Reproductive System 413 The Female Reproductive System 433 Aberrant Sexual Differentiation 414 The Menstrual Cycle 433 Puberty 418 Ovarian Hormones 438 Precocious & Delayed Puberty 420 Control of Ovarian Function 444 Menopause 421 Abnormalities of Ovarian Function 447 Pituitary Gonadotropins & Prolactin 421 Pregnancy 448 The Male Reproductive System 424 Lactation 451 Structure 424
  • 8. CONTENTS / vii 24. Endocrine Functions of the Kidneys, Heart, & Pineal Gland . . . . . . . . . . . . . . . . . . . . . . . . . 454 Introduction 454 Hormones of the Heart & Other Natriuretic The Renin-Angiotensin System 454 Factors 460 Erythropoietin 459 Pineal Gland 462 Section IV References 465SECTION V. GASTROINTESTINAL FUNCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 467 25. Digestion & Absorption. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 467 Introduction 467 Lipids 473 Carbohydrates 467 Absorption of Water & Electrolytes 475 Proteins & Nucleic Acids 471 Absorption of Vitamins & Minerals 477 26. Regulation of Gastrointestinal Function. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 479 Introduction 479 Exocrine Portion of the Pancreas 497 General Considerations 479 Liver & Biliary System 498 Gastrointestinal Hormones 482 Small Intestine 504 Mouth & Esophagus 488 Colon 508 Stomach 491 Section V References 512SECTION VI. CIRCULATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 515 27. Circulating Body Fluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 515 Introduction 515 Red Blood Cells 532 Blood 515 Blood Types 537 Bone Marrow 515 Plasma 539 White Blood Cells 516 Hemostasis 540 Immunity 520 Lymph 546 Platelets 531 28. Origin of the Heartbeat & the Electrical Activity of the Heart . . . . . . . . . . . . . . . . . . . . . . . . 547 Introduction 547 Cardiac Arrhythmias 554 Origin & Spread of Cardiac Electrocardiographic Findings in Other Cardiac Excitation 547 & Systemic Diseases 561 The Electrocardiogram 549 29. The Heart as a Pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 565 Introduction 565 Cardiac Output 570 Mechanical Events of the Cardiac Cycle 565 30. Dynamics of Blood & Lymph Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 577 Introduction 577 Capillary Circulation 590 Functional Morphology 577 Lymphatic Circulation & Interstitial Fluid Biophysical Considerations 581 Volume 593 Arterial & Arteriolar Circulation 587 Venous Circulation 595 31. Cardiovascular Regulatory Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 597 Introduction 597 Systemic Regulation by Hormones 600 Local Regulation 597 Systemic Regulation by the Nervous System 602 Substances Secreted by the Endothelium 598
  • 9. viii / CONTENTS 32. Circulation Through Special Regions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 611 Introduction 611 Brain Metabolism & Oxygen Cerebral Circulation 611 Requirements 619 Anatomic Considerations 611 Coronary Circulation 620 Cerebrospinal Fluid 612 Splanchnic Circulation 623 The Blood-Brain Barrier 614 Cutaneous Circulation 625 Cerebral Blood Flow & Placental & Fetal Circulation 627 Its Regulation 616 33. Cardiovascular Homeostasis in Health & Disease . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 630 Introduction 630 Inflammation & Wound Healing 635 Compensations for Gravitational Shock 636 Effects 630 Hypertension 641 Exercise 632 Heart Failure 643 Section VI References 644SECTION VII. RESPIRATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 647 34. Pulmonary Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 647 Introduction 647 Gas Exchange in the Lungs 660 Properties of Gases 647 Pulmonary Circulation 661 Anatomy of the Lungs 649 Other Functions of the Respiratory System 664 Mechanics of Respiration 650 35. Gas Transport Between the Lungs & the Tissues. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 666 Introduction 666 Carbon Dioxide Transport 669 Oxygen Transport 666 36. Regulation of Respiration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 671 Introduction 671 Chemical Control of Breathing 672 Neural Control of Breathing 671 Nonchemical Influences on Respiration 678 Regulation of Respiratory Activity 672 37. Respiratory Adjustments in Health & Disease . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 681 Introduction 681 Hypercapnia & Hypocapnia 692 Effects of Exercise 681 Other Respiratory Abnormalities 692 Hypoxia 683 Diseases Affecting the Pulmonary Circulation 694 Hypoxic Hypoxia 684 Effects of Increased Barometric Pressure 694 Other Forms of Hypoxia 690 Artificial Respiration 695 Oxygen Treatment 691 Section VII References 697SECTION VIII. FORMATION & EXCRETION OF URINE . . . . . . . . . . . . . . . . . . . . . . . . . . 699 38. Renal Function & Micturition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 699 Introduction 699 Tubular Function 708 Functional Anatomy 699 Water Excretion 713 Renal Circulation 702 Acidification of the Urine Glomerular Filtration 705 & Bicarbonate Excretion 720
  • 10. CONTENTS / ix Regulation of Na+ & Cl− Excretion 723 Effects of Disordered Renal Function 725 Regulation of K+ Excretion 724 The Bladder 726 Diuretics 72439. Regulation of Extracellular Fluid Composition & Volume. . . . . . . . . . . . . . . . . . . . . . . . . . . 729 Introduction 729 Defense of Specific Ionic Composition 730 Defense of Tonicity 729 Defense of H+ Concentration 730 Defense of Volume 729 Section VIII References 738 Self-Study: Objectives, Essay Questions, & Multiple-Choice Questions (black edges) . . . . . . 739 Answers to Quantitative & Multiple-Choice Questions (black edges). . . . . . . . . . . . . . . . . . . 807 Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 811 General References 811 Some Standard Respiratory Symbols 821 Normal Values & the Statistical Equivalents of Metric, United States, Evaluation of Data 811 & English Measures 821 Abbreviations & Symbols Commonly Greek Alphabet 822 Used in Physiology 814 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 823 Standard Atomic Weights . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Inside Front Cover Ranges of Normal Values in Human Whole Blood, Plasma, or Serum . . . . . . . . Inside Back Cover
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  • 12. PrefaceThis book is designed to provide a concise summary of mammalian and, particularly, of human physiology thatmedical students and others can use by itself or can supplement with readings in other texts, monographs, and re-views. Pertinent aspects of general and comparative physiology are also included. Summaries of relevant anatomicconsiderations will be found in each section, but this book is written primarily for those who have some knowledgeof anatomy, chemistry, and biochemistry. Examples from clinical medicine are given where pertinent to illustratephysiologic points. In many of the chapters, physicians desiring to use this book as a review will find short discus-sions of important symptoms produced by disordered function. Review of Medical Physiology also includes a self-study section to help students review for Board and other exami-nations and an appendix that contains general references, a discussion of statistical methods, a glossary of abbrevia-tions, acronyms, and symbols commonly used in physiology, and several useful tables. The index is comprehensiveand specifically designed for ease in locating important terms, topics, and concepts. In writing this book, the author has not been able to be complete and concise without also being dogmatic. I be-lieve, however, that the conclusions presented without detailed discussion of the experimental data on which theyare based are supported by the bulk of the current evidence. Much of this evidence can be found in the papers citedin the credit lines accompanying the illustrations. Further discussions of particular subjects and information on sub-jects not considered in detail can be found in the references listed at the end of each section. Information about ser-ial review publications that provide up-to-date discussion of various physiologic subjects is included in the note ongeneral references in the appendix. In the interest of brevity and clarity, I have in most instances omitted the namesof the many investigators whose work made possible the view of physiology presented here. This omission is in noway intended to slight their contributions, but including their names and specific references to original paperswould greatly increase the length of the book. In this twenty-second edition, as in previous editions, the entire book has been revised, with a view to eliminat-ing errors, incorporating suggestions of readers, updating concepts, and discarding material that is no longer rele-vant. In this way, the book has been kept concise while remaining as up-to-date and accurate as possible. Since thelast edition, research on the regulation of food intake has continued at a rapid pace, and this topic has been ex-panded in the current edition. So has consideration of mitochondria and molecular motors, with emphasis on theubiquity of the latter. Chapter 38 on renal function has been reorganized as well as updated. The section on estro-gen receptors has been revised in terms of the complexity of the receptor and the way this relates to “tailor-made”estrogens used in the treatment of disease. Other topics on which there is new information include melanopsin,pheromones related to lactation, von Willebrand factor, and the complexity of connexons. The self-study section has been updated, with emphasis placed on physiology in relation to disease, in keepingwith the current trend in the United States Medical Licensing Examinations (USMLE). I am greatly indebted to the many individuals who helped with the preparation of this book. Those who were es-pecially helpful in the preparation of the twenty-second edition include Drs. Stephen McPhee, Dan Stites, DavidGardner, Igor Mitrovic, Michael Jobin, Krishna Rao, and Johannes Werzowa. Andrea Chase provided invaluablesecretarial assistance, and, as always, my wife made important contributions.Special thanks are due to Jim Ransom,who edited the first edition of this book over 42 years ago and now has come back to make helpful and worthwhilecomments on the two most recent editions. Many associates and friends provided unpublished illustrative materials,and numerous authors and publishers generously granted permission to reproduce illustrations from other booksand journals. I also thank all the students and others who took the time to write to me offering helpful criticismsand suggestions. Such comments are always welcome, and I solicit additional corrections and criticisms, which maybe addressed to me at Department of Physiology University of California San Francisco, CA 94143-0444 USA Since this book was first published in 1963, the following translations have been published: Bulgarian, Chinese(2 independent translations), Czech (2 editions), French (2 independent translations), German (4 editions), Greek(2 editions), Hungarian, Indonesian (4 editions), Italian (9 editions), Japanese (17 editions), Korean, Malaysian, xi
  • 13. xii / PREFACEPolish (2 editions), Portuguese (7 editions), Serbo-Croatian, Spanish (19 editions), Turkish (2 editions), andUkranian. Various foreign English language editions have been published, and the book has been recorded in Eng-lish on tape for the blind. The tape recording is available from Recording for the Blind, Inc., 20 Rozsel Road,Princeton, NJ 08540 USA. For computer users, the book is now available, along with several other titles in theLange Medical Books series, in STAT!-Ref, a searchable Electronic Medical Library (http://www.statref.com), fromTeton Data Systems, P.O. Box 4798 Jackson, WY 83001 USA. More information about this and other Lange andMcGraw-Hill books, including addresses of the publisher’s international offices, is available on McGraw-Hill’s website, www.AccessMedBooks.com. William F. Ganong, MDSan FranciscoMarch 2005
  • 14. SECTION I Introduction The General & Cellular Basis of Medical Physiology 1INTRODUCTION closely resembles that of the primordial oceans in which, presumably, all life originated.In unicellular organisms, all vital processes occur in a In animals with a closed vascular system, the ECF issingle cell. As the evolution of multicellular organisms divided into two components: the interstitial fluid andhas progressed, various cell groups have taken over par- the circulating blood plasma. The plasma and the cel-ticular functions. In humans and other vertebrate ani- lular elements of the blood, principally red blood cells,mals, the specialized cell groups include a gastrointesti- fill the vascular system, and together they constitute thenal system to digest and absorb food; a respiratory total blood volume. The interstitial fluid is that part ofsystem to take up O2 and eliminate CO2; a urinary sys- the ECF that is outside the vascular system, bathing thetem to remove wastes; a cardiovascular system to dis- cells. The special fluids lumped together as transcellulartribute food, O2, and the products of metabolism; a re- fluids are discussed below. About a third of the totalproductive system to perpetuate the species; and body water (TBW) is extracellular; the remaining twonervous and endocrine systems to coordinate and inte- thirds is intracellular (intracellular fluid).grate the functions of the other systems. This book isconcerned with the way these systems function and theway each contributes to the functions of the body as a Body Compositionwhole. In the average young adult male, 18% of the body This chapter presents general concepts and princi- weight is protein and related substances, 7% is mineral,ples that are basic to the function of all the systems. It and 15% is fat. The remaining 60% is water. The dis-also includes a short review of fundamental aspects of tribution of this water is shown in Figure 1–1.cell physiology. Additional aspects of cellular and mole- The intracellular component of the body water ac-cular biology are considered in the relevant chapters on counts for about 40% of body weight and the extracel-the various organs. lular component for about 20%. Approximately 25% of the extracellular component is in the vascular systemGENERAL PRINCIPLES (plasma = 5% of body weight) and 75% outside theOrganization of the Body blood vessels (interstitial fluid = 15% of body weight). The total blood volume is about 8% of body weight.The cells that make up the bodies of all but the simplestmulticellular animals, both aquatic and terrestrial, exist Measurement of Body Fluid Volumesin an “internal sea” of extracellular fluid (ECF) en-closed within the integument of the animal. From this It is theoretically possible to measure the size of each offluid, the cells take up O2 and nutrients; into it, they the body fluid compartments by injecting substancesdischarge metabolic waste products. The ECF is more that will stay in only one compartment and then calcu-dilute than present-day seawater, but its composition lating the volume of fluid in which the test substance is 1
  • 15. 2 / CHAPTER 1 Since 14,000 mL is the space in which the sucrose was Stomach Intestines distributed, it is also called the sucrose space. Volumes of distribution can be calculated for any Skin substance that can be injected into the body, provided Blood plasma: Lungs Kidneys the concentration in the body fluids and the amount 5% body weight removed by excretion and metabolism can be accurately Extra- measured. cellular Although the principle involved in such measure- fluid: ments is simple, a number of complicating factors must Interstitial fluid:20% body weight 15% body weight be considered. The material injected must be nontoxic, must mix evenly throughout the compartment being measured, and must have no effect of its own on the distribution of water or other substances in the body. In addition, either it must be unchanged by the body dur- ing the mixing period, or the amount changed must be known. The material also should be relatively easy to measure. Plasma Volume, Total Blood Volume, & Red Cell Volume Intracellular fluid: Plasma volume has been measured by using dyes that 40% body weight become bound to plasma protein—particularly Evans blue (T-1824). Plasma volume can also be measured by injecting serum albumin labeled with radioactive io- dine. Suitable aliquots of the injected solution and plasma samples obtained after injection are counted in a scintillation counter. An average value is 3500 mL (5% of the body weight of a 70-kg man, assuming unit density). If one knows the plasma volume and the hematocrit (ie, the percentage of the blood volume that is made up of cells), the total blood volume can be calculated by multiplying the plasma volume by 100Figure 1–1. Body fluid compartments. Arrows repre-sent fluid movement. Transcellular fluids, which consti- 100 − hematocrittute a very small percentage of total body fluids, are Example: The hematocrit is 38 and the plasma vol-not shown. ume 3500 mL. The total blood volume is 100distributed (the volume of distribution of the injected 3500 × 100 − 38 = 5645 mLmaterial). The volume of distribution is equal to theamount injected (minus any that has been removed The red cell volume (volume occupied by all thefrom the body by metabolism or excretion during the circulating red cells in the body) can be determined bytime allowed for mixing) divided by the concentration subtracting the plasma volume from the total bloodof the substance in the sample. Example: 150 mg of su- volume. It may also be measured independently by in-crose is injected into a 70-kg man. The plasma sucrose jecting tagged red blood cells and, after mixing has oc-level after mixing is 0.01 mg/mL, and 10 mg has been curred, measuring the fraction of the red cells that isexcreted or metabolized during the mixing period. The tagged. A commonly used tag is 51Cr, a radioactive iso-volume of distribution of the sucrose is tope of chromium that is attached to the cells by incu- bating them in a suitable chromium solution. Isotopes 150 mg − 10 mg of iron and phosphorus (59Fe and 32P) and antigenic 0.01 mg/mL = 14,000 mL tagging have also been employed.
  • 16. THE GENERAL & CELLULAR BASIS OF MEDICAL PHYSIOLOGY / 3Extracellular Fluid Volume of water, the ratio of TBW to body weight varies with the amount of fat present. TBW is somewhat lower inThe ECF volume is difficult to measure because the women than men, and in both sexes, the values tend tolimits of this space are ill defined and because few sub- decrease with age (Table 1–1).stances mix rapidly in all parts of the space while re-maining exclusively extracellular. The lymph cannot beseparated from the ECF and is measured with it. Manysubstances enter the cerebrospinal fluid (CSF) slowly Units for Measuringbecause of the blood–brain barrier (see Chapter 32). Concentration of SolutesEquilibration is slow with joint fluid and aqueous In considering the effects of various physiologically im-humor and with the ECF in relatively avascular tissues portant substances and the interactions between them,such as dense connective tissue, cartilage, and some the number of molecules, electric charges, or particlesparts of bone. Substances that distribute in ECF appear of a substance per unit volume of a particular bodyin glandular secretions and in the contents of the gas- fluid are often more meaningful than simply the weighttrointestinal tract. Because they are separated from the of the substance per unit volume. For this reason, con-rest of the ECF, these fluids—as well as CSF, the fluids centrations are frequently expressed in moles, equiva-in the eye, and a few other special fluids—are called lents, or osmoles.transcellular fluids. Their volume is relatively small. Perhaps the most accurate measurement of ECF vol-ume is that obtained by using inulin, a polysaccharidewith a molecular weight of 5200. Mannitol and sucrose Moleshave also been used to measure ECF volume. A gener- A mole is the gram-molecular weight of a substance, ie,ally accepted value for ECF volume is 20% of the body the molecular weight of the substance in grams. Eachweight, or about 14 L in a 70-kg man (3.5 L = plasma; mole (mol) consists of approximately 6 × 1023 mole-10.5 L = interstitial fluid). cules. The millimole (mmol) is 1/1000 of a mole, and the micromole (mmol) is 1/1,000,000 of a mole. Thus, 1 mol of NaCl = 23 + 35.5 g = 58.5 g, and 1 mmol =Interstitial Fluid Volume 58.5 mg. The mole is the standard unit for expressing the amount of substances in the SI unit system (see Ap-The interstitial fluid space cannot be measured directly, pendix).since it is difficult to sample interstitial fluid and since The molecular weight of a substance is the ratio ofsubstances that equilibrate in interstitial fluid also equi- the mass of one molecule of the substance to the masslibrate in plasma. The volume of the interstitial fluid of one twelfth the mass of an atom of carbon-12. Sincecan be calculated by subtracting the plasma volume molecular weight is a ratio, it is dimensionless. The dal-from the ECF volume. The ECF volume/intracellular ton (Da) is a unit of mass equal to one twelfth the massfluid volume ratio is larger in infants and children than of an atom of carbon-12, and 1000 Da = 1 kilodaltonit is in adults, but the absolute volume of ECF in chil- (kDa). The kilodalton, which is sometimes expresseddren is, of course, smaller than in adults. Therefore, de- simply as K, is a useful unit for expressing the molecu-hydration develops more rapidly and is frequently more lar mass of proteins. Thus, for example, one can speaksevere in children. of a 64-K protein or state that the molecular mass of the protein is 64,000 Da. However, since molecularIntracellular Fluid VolumeThe intracellular fluid volume cannot be measured di-rectly, but it can be calculated by subtracting the ECF Table 1–1. Total body water (as percentagevolume from the TBW. TBW can be measured by the of body weight) in relation to age and sex.same dilution principle used to measure the other bodyspaces. Deuterium oxide (D2O, heavy water) is most Age (years) Male (%) Female (%)frequently used. D2O has slightly different propertiesfrom those of H2O, but in equilibration experiments 10–18 59 57for measuring body water it gives accurate results. Tri- 18–40 61 51tium oxide (3H2O) and aminopyrine have also beenused for this purpose. 40–60 55 47 The water content of lean body tissue is constant at Over 60 52 4671–72 mL/100 g of tissue, but since fat is relatively free
  • 17. 4 / CHAPTER 1weight is a dimensionless ratio, it is incorrect to say that Buffersthe molecular weight of the protein is 64 kDa. Intracellular and extracellular pH are generally main- tained at very constant levels. For example, the pH ofEquivalents the ECF is 7.40, and in health, this value usually varies less than ±0.05 pH unit. Body pH is stabilized by theThe concept of electrical equivalence is important in buffering capacity of the body fluids. A buffer is a sub-physiology because many of the important solutes in stance that has the ability to bind or release H+ in solu-the body are in the form of charged particles. One tion, thus keeping the pH of the solution relatively con-equivalent (eq) is 1 mol of an ionized substance divided stant despite the addition of considerable quantities ofby its valence. One mole of NaCl dissociates into 1 eq acid or base. One buffer in the body is carbonic acid.of Na+ and 1 eq of Cl–. One equivalent of Na+ = 23 g; This acid is only partly dissociated into H+ and bicar-but 1 eq of Ca2+ = 40 g/2 = 20 g. The milliequivalent bonate: H2CO3 ← H+ + HCO3–. If H+ is added to a so- →(meq) is 1/1000 of 1 eq. lution of carbonic acid, the equilibrium shifts to the left Electrical equivalence is not necessarily the same as and most of the added H+ is removed from solution. Ifchemical equivalence. A gram equivalent is the weight OH– is added, H+ and OH– combine, taking H+ out ofof a substance that is chemically equivalent to 8.000 g solution. However, the decrease is countered by moreof oxygen. The normality (N) of a solution is the num- dissociation of H2CO3, and the decline in H+ concen-ber of gram equivalents in 1 liter. A 1 N solution of hy- tration is minimized. Other buffers include the blooddrochloric acid contains 1 + 35.5 g/L = 36.5 g/L. proteins and the proteins in cells. The quantitative as- pects of buffering and the respiratory and renal adjust-pH ments that operate with buffers to maintain a stable ECF pH of 7.40 are discussed in Chapter 39.The maintenance of a stable hydrogen ion concentra-tion in the body fluids is essential to life. The pH of asolution is the logarithm to the base 10 of the reciprocal Diffusionof the H+ concentration ([H+]), ie, the negative loga-rithm of the [H+]. The pH of water at 25 °C, in which Diffusion is the process by which a gas or a substance inH+ and OH– ions are present in equal numbers, is solution expands, because of the motion of its particles,7.0 (Figure 1–2). For each pH unit less than 7.0, the to fill all of the available volume. The particles (mole-[H+] is increased tenfold; for each pH unit above 7.0, it cules or atoms) of a substance dissolved in a solvent areis decreased tenfold. in continuous random movement. A given particle is equally likely to move into or out of an area in which it is present in high concentration. However, since there are more particles in the area of high concentration, the total number of particles moving to areas of lower con- H+ concentration centration is greater; ie, there is a net flux of solute par- (mol/L) pH ticles from areas of high to areas of low concentration. 10 −1 1 The time required for equilibrium by diffusion is pro- 10 −2 2 portionate to the square of the diffusion distance. The ACIDIC 10 −3 3 magnitude of the diffusing tendency from one region to 10 −4 4 another is directly proportionate to the cross-sectional 10 −5 5 area across which diffusion is taking place and the con- 10 −6 6 For pure water, centration gradient, or chemical gradient, which is 10 −7 7 [H+] = 10−7 mol/L the difference in concentration of the diffusing sub- 10 −8 8 10 −9 9 stance divided by the thickness of the boundary (Fick’s law of diffusion). Thus, ALKALINE 10 −10 10 10 −11 11 ∆c 10 −12 12 J = –DA ∆x 10 −13 13 10 −14 14 where J is the net rate of diffusion, D is the diffusion coefficient, A is the area, and ∆c/∆x is the concentra-Figure 1–2. pH. (Reproduced, with permission, from tion gradient. The minus sign indicates the direction ofAlberts B et al: Molecular Biology of the Cell, 4th ed. Gar- diffusion. When considering movement of moleculesland Science, 2002.) from a higher to a lower concentration, ∆c/∆x is nega-
  • 18. THE GENERAL & CELLULAR BASIS OF MEDICAL PHYSIOLOGY / 5tive, so multiplying by –DA gives a positive value. The of solutions. In an ideal solution, osmotic pressure (P)permeabilities of the boundaries across which diffusion is related to temperature and volume in the same way asoccurs in the body vary, but diffusion is still a major the pressure of a gas:force affecting the distribution of water and solutes. nRT P= VOsmosisWhen a substance is dissolved in water, the concentra- where n is the number of particles, R is the gas con-tion of water molecules in the solution is less than that stant, T is the absolute temperature, and V is the vol-in pure water, since the addition of solute to water re- ume. If T is held constant, it is clear that the osmoticsults in a solution that occupies a greater volume than pressure is proportionate to the number of particles indoes the water alone. If the solution is placed on one solution per unit volume of solution. For this reason,side of a membrane that is permeable to water but not the concentration of osmotically active particles is usu-to the solute, and an equal volume of water is placed on ally expressed in osmoles. One osmole (osm) equals thethe other, water molecules diffuse down their concen- gram-molecular weight of a substance divided by thetration gradient into the solution (Figure 1–3). This number of freely moving particles that each moleculeprocess—the diffusion of solvent molecules into a re- liberates in solution. The milliosmole (mosm) isgion in which there is a higher concentration of a 1/1000 of 1 osm.solute to which the membrane is impermeable—is If a solute is a nonionizing compound such as glu-called osmosis. It is an important factor in physiologic cose, the osmotic pressure is a function of the numberprocesses. The tendency for movement of solvent mole- of glucose molecules present. If the solute ionizes andcules to a region of greater solute concentration can be forms an ideal solution, each ion is an osmotically ac-prevented by applying pressure to the more concen- tive particle. For example, NaCl would dissociate intotrated solution. The pressure necessary to prevent sol- Na+ and Cl– ions, so that each mole in solution wouldvent migration is the osmotic pressure of the solution. supply 2 osm. One mole of Na2SO4 would dissociate Osmotic pressure, like vapor pressure lowering, into Na+, Na+, and SO42–, supplying 3 osm. However,freezing-point depression, and boiling-point elevation, the body fluids are not ideal solutions, and although thedepends on the number rather than the type of particles dissociation of strong electrolytes is complete, the num-in a solution; ie, it is a fundamental colligative property ber of particles free to exert an osmotic effect is reduced owing to interactions between the ions. Thus, it is actu- ally the effective concentration (activity) in the body fluids rather than the number of equivalents of an elec- Semipermeable trolyte in solution that determines its osmotic effect. membrane Pressure This is why, for example, 1 mmol of NaCl per liter in the body fluids contributes somewhat less than 2 mosm of osmotically active particles per liter. The more con- centrated the solution, the greater the deviation from an ideal solution. The osmolal concentration of a substance in a fluid is measured by the degree to which it depresses the freezing point, with 1 mol of an ideal solution depress- ing the freezing point 1.86 °C. The number of millios-Figure 1–3. Diagrammatic representation of osmosis. moles per liter in a solution equals the freezing point depression divided by 0.00186. The osmolarity is theWater molecules are represented by small open circles, number of osmoles per liter of solution (eg, plasma),solute molecules by large solid circles. In the diagram whereas the osmolality is the number of osmoles peron the left, water is placed on one side of a membrane kilogram of solvent. Therefore, osmolarity is affected bypermeable to water but not to solute, and an equal vol- the volume of the various solutes in the solution andume of a solution of the solute is placed on the other. the temperature, while the osmolality is not. Osmoti-Water molecules move down their concentration gradi- cally active substances in the body are dissolved inent into the solution, and, as shown in the diagram on water, and the density of water is 1, so osmolal concen-the right, the volume of the solution increases. As indi- trations can be expressed as osmoles per liter (osm/L) ofcated by the arrow on the right, the osmotic pressure is water. In this book, osmolal (rather than osmolar) con-the pressure that would have to be applied to prevent centrations are considered, and osmolality is expressedthe movement of the water molecules. in milliosmoles per liter (of water).
  • 19. 6 / CHAPTER 1 Note that although a homogeneous solution con- and other fluid and electrolyte abnormalities. Hyperos-tains osmotically active particles and can be said to have molality can cause coma (hyperosmolar coma; seean osmotic pressure, it can exert an osmotic pressure Chapter 19). Because of the predominant role of theonly when it is in contact with another solution across a major solutes and the deviation of plasma from an idealmembrane permeable to the solvent but not to the solution, one can ordinarily approximate the plasma os-solute. molality within a few milliosmoles per liter by using the following formula, in which the constants convert the clinical units to millimoles of solute per liter:Osmolal Concentration of Plasma: Tonicity Osmolality = 2[Na+] + 0.055[Glucose] + 0.36[BUN]The freezing point of normal human plasma averages (mosm/L) (mEq/L) (mg/dL) (mg/dL)–0.54 °C, which corresponds to an osmolal concentra-tion in plasma of 290 mosm/L. This is equivalent to an BUN is the blood urea nitrogen. The formula is alsoosmotic pressure against pure water of 7.3 atm. The os- useful in calling attention to abnormally high concen-molality might be expected to be higher than this, be- trations of other solutes. An observed plasma osmolalitycause the sum of all the cation and anion equivalents in (measured by freezing-point depression) that greatly ex-plasma is over 300. It is not this high because plasma is ceeds the value predicted by this formula probably indi-not an ideal solution and ionic interactions reduce the cates the presence of a foreign substance such asnumber of particles free to exert an osmotic effect. Ex- ethanol, mannitol (sometimes injected to shrinkcept when there has been insufficient time after a sud- swollen cells osmotically), or poisons such as ethyleneden change in composition for equilibrium to occur, all glycol or methanol (components of antifreeze).fluid compartments of the body are in or nearly in os-motic equilibrium. The term tonicity is used to de- Regulation of Cell Volumescribe the osmolality of a solution relative to plasma.Solutions that have the same osmolality as plasma are Unlike plant cells, which have rigid walls, animal cellsaid to be isotonic; those with greater osmolality are membranes are flexible. Therefore, animal cells swellhypertonic; and those with lesser osmolality are hypo- when exposed to extracellular hypotonicity and shrinktonic. All solutions that are initially isosmotic with when exposed to extracellular hypertonicity. However,plasma (ie, that have the same actual osmotic pressure cell swelling activates channels in the cell membraneor freezing-point depression as plasma) would remain that permit increased efflux of K+, Cl–, and small or-isotonic if it were not for the fact that some solutes dif- ganic solutes referred to collectively as organic os-fuse into cells and others are metabolized. Thus, a 0.9% molytes. Water follows these osmotically active parti-saline solution remains isotonic because there is no net cles out of the cell, and the cell volume returns tomovement of the osmotically active particles in the so- normal. Ion channels and other membrane transportlution into cells and the particles are not metabolized. proteins are discussed in detail in a later section of thisOn the other hand, a 5% glucose solution is isotonic chapter.when initially infused intravenously, but glucose is me-tabolized, so the net effect is that of infusing a hypo- Nonionic Diffusiontonic solution. Some weak acids and bases are quite soluble in cell It is important to note the relative contributions of membranes in the undissociated form, whereas theythe various plasma components to the total osmolal cross membranes with difficulty in the ionic form.concentration of plasma. All but about 20 of the Consequently, if molecules of the undissociated sub-290 mosm in each liter of normal plasma are con- stance diffuse from one side of the membrane to thetributed by Na+ and its accompanying anions, princi- other and then dissociate, there is appreciable netpally Cl– and HCO3–. Other cations and anions make a movement of the undissociated substance from one siderelatively small contribution. Although the concentra- of the membrane to the other. This phenomenon,tion of the plasma proteins is large when expressed in which occurs in the gastrointestinal tract (see Chaptergrams per liter, they normally contribute less than 25) and kidneys (see Chapter 38), is called nonionic2 mosm/L because of their very high molecular weights. diffusion.The major nonelectrolytes of plasma are glucose andurea, which in the steady state are in equilibrium with Donnan Effectcells. Their contributions to osmolality are normallyabout 5 mosm/L each but can become quite large in When an ion on one side of a membrane cannot diffusehyperglycemia or uremia. The total plasma osmolality through the membrane, the distribution of other ionsis important in assessing dehydration, overhydration, to which the membrane is permeable is affected in a
  • 20. THE GENERAL & CELLULAR BASIS OF MEDICAL PHYSIOLOGY / 7predictable way. For example, the negative charge of a gradient for Cl– exactly balanced by the oppositely di-nondiffusible anion hinders diffusion of the diffusible rected electrical gradient, and the same holds true forcations and favors diffusion of the diffusible anions. K+. Third, since there are more proteins in plasma thanConsider the following situation, in interstitial fluid, there is a Donnan effect on ion movement across the capillary wall (see below). X Y m K+ K+ Forces Acting on Ions Cl− Cl− The forces acting across the cell membrane on each ion Prot− can be analyzed mathematically. Chloride ions are pre- sent in higher concentration in the ECF than in the cellin which the membrane (m) between compartments X interior, and they tend to diffuse along this concentra-and Y is impermeable to Prot– but freely permeable to tion gradient into the cell. The interior of the cell isK+ and Cl–. Assume that the concentrations of the an- negative relative to the exterior, and chloride ions areions and of the cations on the two sides are initially pushed out of the cell along this electrical gradient. Anequal. Cl– diffuses down its concentration gradient equilibrium is reached at which Cl– influx and Cl– ef-from Y to X, and some K+ moves with the negatively flux are equal. The membrane potential at which thischarged Cl– because of its opposite charge. Therefore equilibrium exists is the equilibrium potential. Its [K+X] > [K+Y] magnitude can be calculated from the Nernst equa- tion, as follows: Furthermore, RT [Cl −] ECl = In o− [K+X] + [Cl−X] + [Prot−X] > [K+Y] + [Cl−Y] FZCl [Cli ]ie, more osmotically active particles are on side X than whereon side Y. Donnan and Gibbs showed that in the presence of a ECl = equilibrium potential for Cl−nondiffusible ion, the diffusible ions distribute them- R = gas constantselves so that at equilibrium, their concentration ratios T = absolute temperatureare equal: F = the faraday (number of coulombs per mole of charge) [K+X] [Cl−Y] ZCl = valence of Cl− (−1) = [Clo−] = Cl− concentration outside the cell [K+Y] [Cl−X] [Cli−] = Cl− concentration inside the cellCross-multiplying, Converting from the natural log to the base 10 log [K+X] [Cl−X] = [K+Y] [Cl−Y] and replacing some of the constants with numerical val- ues, the equation becomesThis is the Gibbs–Donnan equation. It holds for anypair of cations and anions of the same valence. [Cli−] ECl = 61.5 log at 37 °C The Donnan effect on the distribution of ions has [Clo−]three effects in the body. First, because of proteins(Prot–) in cells, there are more osmotically active parti- Note that in converting to the simplified expressioncles in cells than in interstitial fluid, and since animal the concentration ratio is reversed because the –1 va-cells have flexible walls, osmosis would make them lence of Cl– has been removed from the expression.swell and eventually rupture if it were not for Na+–K+ ECl, calculated from the values in Table 1–2, isadenosine triphosphatase (ATPase) pumping ions back –70 mV, a value identical to the measured restingout of cells (see below). Thus, normal cell volume and membrane potential of –70 mV. Therefore, no forcespressure depend on Na+–K+ ATPase. Second, because other than those represented by the chemical and elec-at equilibrium the distribution of permeant ions across trical gradients need be invoked to explain the distribu-the membrane (m in the example used here) is asym- tion of Cl– across the membrane.metric, an electrical difference exists across the mem- A similar equilibrium potential can be calculated forbrane whose magnitude can be determined by the K+ :Nernst equation (see below). In the example used here,side X will be negative relative to side Y. The charges RT [Ko+] [Ko+] EK = In = 61.5 log at 37 °Cline up along the membrane, with the concentration FZK [Ki+] [Ki+]
  • 21. 8 / CHAPTER 1Table 1–2. Concentration of some ions inside on the outside and anions on the inside. This conditionand outside mammalian spinal motor neurons. is maintained by Na+–K+ ATPase, which pumps K+ back into the cell and keeps the intracellular concentra- Concentration tion of Na+ low. The Na+–K+ pump is also electrogenic, (mmol/L of H2O) because it pumps three Na+ out of the cell for every two K+ it pumps in; thus, it also contributes a small amount Equilibrium to the membrane potential by itself. It should be em- Inside Outside Potential phasized that the number of ions responsible for the Ion Cell Cell (mV) membrane potential is a minute fraction of the total number present and that the total concentrations of Na+ 15.0 150.0 +60 positive and negative ions are equal everywhere except K+ 150.0 5.5 −90 along the membrane. Na+ influx does not compensate − for the K+ efflux because the K+ channels (see below) Cl 9.0 125.0 −70 make the membrane more permeable to K+ than toResting membrane potential = −70 mV Na+.where FUNCTIONAL MORPHOLOGY EK = equilibrium potential for K+ OF THE CELL ZK = valence of K+ (+1) Revolutionary advances in the understanding of cell [Ko+] = K+ concentration outside the cell structure and function have been made through use of [Ki+] = K+ concentration inside the cell the techniques of modern cellular and molecular biol- R, T, and F as above ogy. Major advances have occurred in the study of em- bryology and development at the cellular level. Devel-In this case, the concentration gradient is outward and opmental biology and the details of cell biology arethe electrical gradient inward. In mammalian spinal beyond the scope of this book. However, a basic knowl-motor neurons, EK is –90 mV (Table 1–2). Since the edge of cell biology is essential to an understanding ofresting membrane potential is –70 mV, there is some- the organ systems in the body and the way they func-what more K+ in the neurons than can be accounted for tion.by the electrical and chemical gradients. The specialization of the cells in the various organs The situation for Na+ is quite different from that for is very great, and no cell can be called “typical” of allK+ and Cl–. The direction of the chemical gradient for cells in the body. However, a number of structures (or-Na+ is inward, to the area where it is in lesser concen- ganelles) are common to most cells. These structurestration, and the electrical gradient is in the same direc- are shown in Figure 1–4. Many of them can be isolatedtion. ENa is +60 mV (Table 1–2). Since neither EK nor by ultracentrifugation combined with other techniques.ENa is at the membrane potential, one would expect the When cells are homogenized and the resulting suspen-cell to gradually gain Na+ and lose K+ if only passive sion is centrifuged, the nuclei sediment first, followedelectrical and chemical forces were acting across the by the mitochondria. High-speed centrifugation thatmembrane. However, the intracellular concentration of generates forces of 100,000 times gravity or moreNa+ and K+ remain constant because there is active causes a fraction made up of granules called the micro-transport of Na+ out of the cell against its electrical and somes to sediment. This fraction includes organellesconcentration gradients, and this transport is coupled such as the ribosomes and peroxisomes.to active transport of K+ into the cell (see below).Genesis of the Membrane Potential Cell MembraneThe distribution of ions across the cell membrane and The membrane that surrounds the cell is a remarkablethe nature of this membrane provide the explanation structure. It is made up of lipids and proteins and isfor the membrane potential. The concentration gradi- semipermeable, allowing some substances to passent for K+ facilitates its movement out of the cell via K+ through it and excluding others. However, its perme-channels, but its electrical gradient is in the opposite ability can also be varied because it contains numerous(inward) direction. Consequently, an equilibrium is regulated ion channels and other transport proteins thatreached in which the tendency of K+ to move out of the can change the amounts of substances moving across it.cell is balanced by its tendency to move into the cell, It is generally referred to as the plasma membrane.and at that equilibrium there is a slight excess of cations The nucleus is also surrounded by a membrane of this
  • 22. THE GENERAL & CELLULAR BASIS OF MEDICAL PHYSIOLOGY / 9 Secretory granules Golgi apparatus Centrioles Rough endoplasmic Smooth reticulum endoplasmic reticulum Lysosomes Nuclear envelope Lipid droplets Mitochondrion Globular heads NucleolusFigure 1–4. Diagram showing a hypothetical cell in the center as seen with the light microscope. It is surroundedby various organelles. (After Bloom and Fawcett. Reproduced, with permission, from Junqueira LC, Carneiro J, Kelley RO:Basic Histology, 9th ed. McGraw-Hill, 1998.)type, and the organelles are surrounded by or made up prokaryotes (cells such as bacteria in which there is noof a membrane. nucleus), the membranes are relatively simple, but in Although the chemical structures of membranes and eukaryotes (cells containing nuclei), cell membranestheir properties vary considerably from one location to contain various glycosphingolipids, sphingomyelin, andanother, they have certain common features. They are cholesterol.generally about 7.5 nm (75 Å) thick. The chemistry of Many different proteins are embedded in the mem-proteins and lipids is discussed in Chapter 17. The brane. They exist as separate globular units and manymajor lipids are phospholipids such as phosphatidyl- pass through the membrane (integral proteins),choline and phosphatidylethanolamine. The shape of whereas others (peripheral proteins) stud the insidethe phospholipid molecule is roughly that of a clothes- and outside of the membrane (Figure 1–5). Thepin (Figure 1–5). The head end of the molecule con- amount of protein varies with the function of the mem-tains the phosphate portion and is relatively soluble in brane but makes up on average 50% of the mass of thewater (polar, hydrophilic). The tails are relatively in- membrane; ie, there is about one protein molecule persoluble (nonpolar, hydrophobic). In the membrane, 50 of the much smaller phospholipid molecules. Thethe hydrophilic ends of the molecules are exposed to proteins in the membranes carry out many functions.the aqueous environment that bathes the exterior of the Some are cell adhesion molecules that anchor cells tocells and the aqueous cytoplasm; the hydrophobic ends their neighbors or to basal laminas. Some proteinsmeet in the water-poor interior of the membrane. In function as pumps, actively transporting ions across the
  • 23. 10 / CHAPTER 1 1–6). Proteins may be myristolated, palmitoylated, or prenylated (ie, attached to geranylgeranyl or farnesyl groups). The protein structure—and particularly the enzyme content—of biologic membranes varies not only from cell to cell but also within the same cell. For example, some of the enzymes embedded in cell membranes are different from those in mitochondrial membranes. In epithelial cells, the enzymes in the cell membrane on the mucosal surface differ from those in the cell mem- brane on the basal and lateral margins of the cells; ie, the cells are polarized. This is what makes transport across epithelia possible (see below). The membranes are dynamic structures, and their constituents are being constantly renewed at different rates. Some proteins are anchored to the cytoskeleton, but others move laterallyFigure 1–5. Biologic membrane. The phospholipid in the membrane. For example, receptors move in themolecules each have two fatty acid chains (wavy lines) membrane and aggregate at sites of endocytosis (seeattached to a phosphate head (open circle). Proteins are below).shown as irregular colored globules. Many are integral Underlying most cells is a thin, fuzzy layer plusproteins, which extend through the membrane, but pe- some fibrils that collectively make up the basementripheral proteins are attached to the inside (not shown) membrane or, more properly, the basal lamina. Theand outside of the membrane, sometimes by glyco- basal lamina and, more generally, the extracellular ma-sylphosphatidylinositol (GPI) anchors. trix are made up of many proteins that hold cells to- gether, regulate their development, and determine their growth. These include collagens, laminins (see below), fibronectin, tenascin, and proteoglycans.membrane. Other proteins function as carriers, trans-porting substances down electrochemical gradients by Mitochondriafacilitated diffusion. Still others are ion channels,which, when activated, permit the passage of ions into Over a billion years ago, aerobic bacteria were engulfedor out of the cell. The role of the pumps, carriers, and by eukaryotic cells and evolved into mitochondria,ion channels in transport across the cell membrane is providing the eukaryotic cells with the ability to formdiscussed below. Proteins in another group function as the energy-rich compound ATP by oxidativereceptors that bind neurotransmitters and hormones, phosphenylation. Mitochondria perform other func-initiating physiologic changes inside the cell. Proteins tions, including a role in the regulation of apoptosisalso function as enzymes, catalyzing reactions at the (see below), but oxidative phosphorylation is the mostsurfaces of the membrane. In addition, some glycopro- crucial. Hundreds to thousands of mitochondria are inteins function in antibody processing and distinguish- each eukaryotic cell. In mammals, they are generallying self from nonself (see Chapter 27). sausage-shaped (Figure 1–4). Each has an outer mem- The uncharged, hydrophobic portions of the pro- brane, an intermembrane space, an inner membrane,teins are usually located in the interior of the mem- which is folded to form shelves (cristae), and a centralbrane, whereas the charged, hydrophilic portions are lo- matrix space. The enzyme complexes responsible forcated on the surfaces. Peripheral proteins are attached oxidative phosphorylation are lined up on the cristaeto the surfaces of the membrane in various ways. One (Figure 1–7).common way is attachment to glycosylated forms of Consistent with their origin from aerobic bacteria,phosphatidylinositol. Proteins held by these glyco- the mitochondria have their own genome. There issylphosphatidylinositol (GPI) anchors (Figure 1–5) much less DNA in the mitochondrial genome than ininclude enzymes such as alkaline phosphatase, various the nuclear genome (see below), and 99% of the pro-antigens, a number of cell adhesion molecules, and teins in the mitochondria are the products of nuclearthree proteins that combat cell lysis by complement (see genes, but mitochondrial DNA is responsible for cer-Chapter 27). Over 40 GPI-linked cell surface proteins tain key components of the pathway for oxidative phos-have now been described. Other proteins are lipidated, phorylation. Specifically, human mitochondrial DNAie, they have specific lipids attached to them (Figure is a double-stranded circular molecule containing
  • 24. THE GENERAL & CELLULAR BASIS OF MEDICAL PHYSIOLOGY / 11 Lipid membrane Cytoplasmic or external face of membrane O N Gly Protein COOH N -Myristoyl H S-Cys Protein NH2 S -Palmitoyl O S-Cys Protein NH2 Geranylgeranyl S-Cys Protein NH2 Farnesyl O C C CH2 C C CH O O GPI anchor O C O P O Inositol O C Protein (Glycosylphosphatidylinositol) H2 O Hydrophobic domain Hydrophilic domainFigure 1–6. Protein linkages to membrane lipids. Some are linked by their amino terminals, others by their car-boxyl terminals. Many are attached via glycosylated forms of phosphatidylinositol (GPI anchors). (Reproduced, withpermission, from Fuller GM, Shields D: Molecular Basis of Medical Cell Biology. McGraw-Hill, 1998.)16,569 base pairs (compared with over a billion in nu- Sperms contribute few, if any, mitochondria to theclear DNA). It codes for 13 protein subunits that are zygote, so the mitochondria come almost entirely fromassociated with proteins encoded by nuclear genes to the ovum and their inheritance is almost exclusivelyform four enzyme complexes plus two ribosomal and maternal. Mitochondria have no effective DNA repair22 transfer RNAs (see below) that are needed for pro- system, and the mutation rate for mitochondrial DNAtein production by the intramitochondrial ribosomes. is over 10 times the rate for nuclear DNA. A large The enzyme complexes responsible for oxidative number of relatively rare diseases have now been tracedphosphorylation illustrate the interactions between the to mutations in mitochondrial DNA. These include forproducts of the mitochondrial genome and the nuclear the most part disorders of tissues with high metabolicgenome. For example, complex I, reduced nicotinamide rates in which energy production is defective as a resultadenine dinucleotide dehydrogenase (NADH), is made of abnormalities in the production of ATP.up of 7 protein subunits coded by mitochondrial DNAand 39 subunits coded by nuclear DNA. The origin of Lysosomesthe subunits in the other complexes is shown in Figure1–7. Complex II, succinate dehydrogenase-ubiquinone In the cytoplasm of the cell there are large, somewhatoxidoreductase, complex III, ubiquinone-cytochrome c irregular structures surrounded by membrane. The in-oxidoreductase, and complex IV, cytochrome c oxidase, terior of these structures, which are called lysosomes, isact with complex I coenzyme Q, and cytochrome c to more acidic than the rest of the cytoplasm, and externalconvert metabolites to CO2 and water. In the process, material such as endocytosed bacteria as well as worn-complexes I, III, and IV pump protons (H+) into the out cell components are digested in them. Some of theintermembrane space. The protons then flow through enzymes involved are listed in Table 1–3.complex V, ATP synthase, which generates ATP. ATP When a lysosomal enzyme is congenitally absent,synthase is unique in that part of it rotates in the gene- the lysosomes become engorged with the material thesis of ATP. enzyme normally degrades. This eventually leads to one
  • 25. 12 / CHAPTER 1 H+ H+ H+ H+ Intramemb space CoQ Cyt c Inner mito membrane AS Matrix space ADP ATP Complex I II III IV V Subunits from 7 0 1 3 2 mDNA Subunits from 39 4 10 10 14 nDNAFigure 1–7. Formation of ATP by oxidative phosphorylation in mitochondria. As enzyme complexes I through IVconvert 2-carbon metabolic fragments to CO2 and H2O), protons (H+) are pumped into the intermembrane space.The proteins diffuse back to the matrix space via complex V, ATP synthase, in which ADP is converted to ATP. Theenzyme complexes are made up of subunits coded by mitochondrial DNA (mDNA) and nuclear DNA (nDNA), andthe figures document the contribution of each DNA to the complexes. See text for further details.of the lysosomal storage diseases. For example, α- the peroxisome. The matrix contains more than 40 en-galactosidase A deficiency causes Fabry’s disease, and β- zymes, which operate in concert with enzymes outsidegalactocerebrosidase deficiency causes Gaucher’s dis- the peroxisome to catalyze a variety of anabolic andease. These diseases are rare, but they are serious and catabolic reactions. Several years ago, a number of syn-can be fatal. Another example is the lysosomal storage thetic compounds were found to cause proliferation ofdisease called Tay–Sachs disease, which causes mental peroxisomes by acting on receptors in the nuclei ofretardation and blindness. cells. These receptors (PPARs) are members of the nu- clear receptor superfamily, which includes receptors forPeroxisomes steroid hormones, thyroid hormones, certain vitamins, and a number of other substances (see below). WhenPeroxisomes are found in the microsomal fraction of activated, they bind to DNA, producing changes in thecells. They are 0.5 mm in diameter and are surrounded production of mRNAs. Three PPAR receptors—α, β,by a membrane. This membrane contains a number of and γ—have been characterized. PPAR-α and PPAR-γperoxisome-specific proteins that are concerned with have received the most attention because PPAR-γ’s aretransport of substances into and out of the matrix of activated by feeding and initiate increases in enzymes involved in energy storage, whereas PPAR-α’s are acti- vated by fasting and increase energy-producing enzymeTable 1–3. Some of the enzymes found activity. Thiazolidinediones are synthetic ligands forin lysosomes and the cell components PPAR-γ’s and they increase sensitivity to insulin,that are their substrates. though their use in diabetes has been limited by their toxic side effects. Fibrates, which lower circulating Enzyme Substrate triglycerides, are ligands for PPAR-α’s. Ribonuclease RNA Cytoskeleton Deoxyribonuclease DNA All cells have a cytoskeleton, a system of fibers that not Phosphatase Phosphate esters only maintains the structure of the cell but also permits Glycosidases Complex carbohydrates; glyco- it to change shape and move. The cytoskeleton is made sides and polysaccharides up primarily of microtubules, intermediate filaments, and microfilaments, along with proteins that anchor Arylsulfatases Sulfate esters them and tie them together. In addition, proteins and Collagenase Proteins organelles move along microtubules and microfilaments from one part of the cell to another propelled by molec- Cathepsins Proteins ular motors.
  • 26. THE GENERAL & CELLULAR BASIS OF MEDICAL PHYSIOLOGY / 13 Microtubules (Figures 1–8 and 1–9) are long, hol- ble that organelles cannot move. Mitotic spindles can-low structures with 5-nm walls surrounding a cavity not form, and the cells die.15 nm in diameter. They are made up of two globular Intermediate filaments are 8–14 nm in diameterprotein subunits: α- and β-tubulin. A third subunit, γ- and are made up of various subunits. Some of these fila-tubulin, is associated with the production of micro- ments connect the nuclear membrane to the cell mem-tubules by the centrosomes (see below). The α and β brane. They form a flexible scaffolding for the cell andsubunits form heterodimers (Figure 1–9), which aggre- help it resist external pressure. In their absence, cellsgate to form long tubes made up of stacked rings, with rupture more easily; and when they are abnormal in hu-each ring usually containing 13 subunits. The tubules mans, blistering of the skin is common.also contain other proteins that facilitate their forma- Microfilaments (Figure 1–8) are long solid fiberstion. The assembly of microtubules is facilitated by 4–6 nm in diameter that are made up of actin. Notwarmth and various other factors, and disassembly is fa- only is actin present in muscle (see Chapter 3), but itcilitated by cold and other factors. The end where as- and its mRNA are present in all types of cells. It is thesembly predominates is called the + end, and the end most abundant protein in mammalian cells, sometimeswhere disassembly predominates is the – end. Both accounting for as much as 15% of the total protein inprocesses occur simultaneously in vitro. the cell. Its structure is highly conserved; for example, Because of their constant assembly and disassembly, 88% of the amino acid sequences in yeast and rabbitmicrotubules are a dynamic portion of the cell skeleton. actin are identical. Actin filaments polymerize and de-They provide the tracks along with several different poidymerize in vivo, and it is not uncommon to findmolecular motors for transport vesicles, organelles such polymerization occurring at one end of the filamentas secretory granules, and mitochondria from one part while depolymerization is occurring at the other end.of the cell to another. They also form the spindle, The fibers attach to various parts of the cytoskeletonwhich moves the chromosomes in mitosis. Micro- (Figure 1–10). They reach to the tips of the microvillitubules can transport in both directions. on the epithelial cells of the intestinal mucosa. They are Microtubule assembly is prevented by colchicine also abundant in the lamellipodia that cells put outand vinblastine. The anticancer drug paclitaxel when they crawl along surfaces. The actin filaments in-(Taxol) binds to microtubules and makes them so sta- teract with integrin receptors and form focal adhesion MF MTFigure 1–8. Left: Electron micrograph of the cytoplasm of a fibroblast, showing microfilaments (MF) and micro-tubules (MT). (Reproduced, with permission, from Junqueira LC, Carneiro J: Basic Histology, 10th ed. McGraw-Hill, 2003.)Right: Distribution of microtubules in fibroblasts. The cells are treated with a fluorescently labeled antibody to tubu-lin, making microtubules visible as the light-colored structures. (Reproduced, with permission, from Connolly J et al:Immunofluorescent staining of cytoplasmic and spindle microtubules in mouse fibroblasts with antibody to τ protein.Proc Natl Acad Sci U S A 1977;74:2437.)
  • 27. 14 / CHAPTER 1 α-Tubulin 24 nm β-Tubulin 5 nm (–) End (+) End Cross section Longitudinal section Tubulin heterodimersFigure 1–9. Microtuble, showing assembly by addition of α- and β-tubulin dimers and disassembly by removal ofthese units. (Modified from Borison WF, Boupaep EL: Medical Physiology, Saunders, 2003).complexes, which serve as points of traction with the and then bends its neck while the other head swingssurface over which the cell pulls itself. In addition, forward and binds, producing almost continuoussome molecular motors use microfilaments as tracks. movement. Some kinesins are associated with mitosis and meiosis. Other kinesins perform different func- tions, including, in some instances, moving cargo to theMolecular Motors – end of microtubules.The molecular motors that move proteins, organelles, Dyneins have two heads, with their neck piecesand other cell parts (their cargo) to all parts of the cell embedded in a complex of proteins (Figure 1–11). Cy-are 100–500-kDa ATPases. They attach to their cargo toplasmic dynein has a function like that of conven-and their heads bind to microtubules or actin polymers. tional kinesin, except that it moves particles and mem-Hydrolysis of ATP in their heads causes the molecules branes to the – end of the microtubules. Axonemalto move. There are two types of molecular motors: dynein oscillates and is responsible for the beating ofthose producing motion along microtubules and those flagella and cilia (see below). The multiple forms ofproducing motion along actin (Table 1–4). Examples myosin in the body are divided into 18 classes. Theare shown in Figure 1–11, but each type is a member of heads of myosin molecules bind to actin and producea superfamily, with many forms throughout the animal motion by bending their neck regions (myosin II) orkingdom. walking along microfilaments, one head after the other The conventional form of kinesin is a double- (myosin V). In these ways, they perform functions asheaded molecule that moves its cargo toward the + ends diverse as contraction of muscle (see Chapter 3) andof microtubules. One head binds to the microtubule cell migration. Glycophorin C Anion exchanger Membrane Actin (Band 3) Ankyrin 4.1 Adducin 4.1 4.2 Actin α chain β chain Spectrin 4.9 Tropomyosin TropomodulinFigure 1–10. Membrane-cytoskeleton attachments in the red blood cell, showing the various proteins that anchoractin microfilaments to the membrane. Some are identified by numbers (4.1, 4.2, 4.9), whereas others have receivednames. (Reproduced, with permission, from Luna EJ, Hitt AL: Cytoskeleton-plasma membrane interactions. Science1992;258:955.)
  • 28. THE GENERAL & CELLULAR BASIS OF MEDICAL PHYSIOLOGY / 15Table 1–4. Examples of molecular motors. cell division. In multinucleate cells, a centrosome is near each nucleus. Microtubule-based Conventional kinesin Cilia Dyneins Cells have various types of projections. True cilia are dynein-driven motile processes that are used by unicel- Actin-based lular organisms to propel themselves through the water Myosins I–V and by multicellular organisms to propel mucus and other substances over the surface of various epithelia. They resemble centrioles in having an array of nine tubular structures in their walls, but they have in addi-Centrosomes tion a pair of microtubules in the center, and two rather than three microtubules are present in each of the nineNear the nucleus in the cytoplasm of eukaryotic animal circumferential structures. The basal granule, on thecells is a centrosome. The centrosome is made up of other hand, is the structure to which each cilium is an-two centrioles and surrounding amorphous pericentri- chored. It has nine circumferential triplets, like a centri-olar material. The centrioles are short cylinders ole, and there is evidence that basal granules and centri-arranged so that they are at right angles to each other. oles are interconvertible.Microtubules in groups of three run longitudinally inthe walls of each centriole (Figure 1–4). Nine of these Cell Adhesion Moleculestriplets are spaced at regular intervals around the cir-cumference. Cells are attached to the basal lamina and to each other The centrosomes are microtubule-organizing cen- by cell adhesion molecules (CAMs) that are promi-ters (MTOCs) that contain γ-tubulin. The micro- nent parts of the intercellular connections describedtubules grow out of this γ-tubulin in the pericentriolar below. These adhesion proteins have attracted great at-material. When a cell divides, the centrosomes dupli- tention in recent years because they are important incate themselves, and the pairs move apart to the poles embryonic development and formation of the nervousof the mitotic spindle, where they monitor the steps in system and other tissues; in holding tissues together in Cargo Light chains 4 nm Conventional kinesin 80 nm Cytoplasmic dynein Cargo-binding domain Head 1 Head 2 Head 2 Head 1 ADP ADP ATP Actin Myosin VFigure 1–11. Three examples of molecular motors. Conventional kinesin is shown attached to cargo, in this case amembrane-bound organelle. The way that myosin V “walks” along a microtuble is also shown. Note that the headsof the motors hydrolyze ATP and use the energy to produce motion.
  • 29. 16 / CHAPTER 1adults; in inflammation and wound healing; and in themetastasis of tumors. Many pass through the cell mem-brane and are anchored to the cytoskeleton inside thecell. Some bind to like molecules on other cells (ho-mophilic binding), whereas others bind to other mole- Tightcules (heterophilic binding). Many bind to laminins, a junctionfamily of large cross-shaped molecules with multiple re- (zonula occludens)ceptor domains in the extracellular matrix. Nomenclature in the CAM field is somewhat Zonulachaotic, partly because the field is growing so rapidly adherensand partly because of the extensive use of acronyms, asin other areas of modern biology. However, the CAMs Desmosomescan be divided into four broad families: (1) integrins,heterodimers that bind to various receptors; (2) adhe-sion molecules of the IgG superfamily of immuno-globulins; (3) cadherins, Ca2+-dependent moleculesthat mediate cell-to-cell adhesion by homophilic reac- Gaptions; and (4) selectins, which have lectin-like domains junctionsthat bind carbohydrates. The functions of CAMs ingranulocytes and platelets are described in Chapter 27,and their roles in inflammation and wound healing arediscussed in Chapter 33. Hemidesmosome The CAMs not only fasten cells to their neighbors,but they also transmit signals into and out of the cell. Figure 1–12. Intercellular junctions in the mucosa ofCells that lose their contact with the extracellular ma- the small intestine. Focal adhesions are not shown intrix via integrins have a higher rate of apoptosis (see detail.below) than anchored cells, and interactions betweenintegrins and the cytoskeleton are involved in cellmovement. these junctions are a significant part of overall ion andIntercellular Connections solute flux. In addition, tight junctions prevent the movement of proteins in the plane of the membrane,Two types of junctions form between the cells that helping to maintain the different distribution of trans-make up tissues: junctions that fasten the cells to one porters and channels in the apical and basolateral cellanother and to surrounding tissues, and junctions that membranes that make transport across epithelia possi-permit transfer of ions and other molecules from one ble (see above and Chapters 25 and 38).cell to another. The types of junctions that tie cells to- In epithelial cells, each zonula adherens is usually agether and endow tissues with strength and stability in- continuous structure on the basal side of the zonula oc-clude the tight junction, which is also known as the cludens, and it is a major site of attachment for intracel-zonula occludens. The desmosome and zonula ad- lular microfilaments. It contains cadherins.herens (Figure 1–12) hold cells together, and the Desmosomes are patches characterized by apposedhemidesmosome and focal adhesion attach cells to thickenings of the membranes of two adjacent cells. At-their basal laminas. Tight junctions between epithelial tached to the thickened area in each cell are intermedi-cells are also essential for transport of ions across epithe- ate filaments, some running parallel to the membranelia. The junction by which molecules are transferred is and others radiating away from it. Between the twothe gap junction. membrane thickenings. The intercellular space contains Tight junctions characteristically surround the api- filamentous material that includes cadherins and the ex-cal margins of the cells in epithelia such as the intestinal tracellular portions of several other transmembrane pro-mucosa, the walls of the renal tubules, and the choroid teins.plexus. They are made up of ridges—half from one cell Hemidesmosomes look like half-desmosomes thatand half from the other—which adhere so strongly at attach cells to the underlying basal lamina and are con-cell junctions that they almost obliterate the space be- nected intracellularly to intermediate filaments. How-tween the cells. They permit the passage of some ions ever, they contain integrins rather than cadherins. Focaland solute, and the degree of this “leakiness” varies. Ex- adhesions also attach cells to their basal laminas. Astracellular fluxes of ions and solute across epithelia at noted above, they are labile structures associated with
  • 30. THE GENERAL & CELLULAR BASIS OF MEDICAL PHYSIOLOGY / 17actin filaments inside the cell, and they play an impor- the particular connexin subunits that make up connex-tant role in cell movement. ons determine their permeability and selectivity.Gap Junctions Nucleus & Related StructuresAt gap junctions, the intercellular space narrows from A nucleus is present in all eukaryotic cells that divide. If25 nm to 3 nm, and units called connexons in the mem- a cell is cut in half, the anucleate portion eventually diesbrane of each cell are lined up with one another (Figure without dividing. The nucleus is made up in large part1–13). Each connexon is made up of six protein sub- of the chromosomes, the structures in the nucleus thatunits called connexins. They surround a channel that, carry a complete blueprint for all the heritable specieswhen lined up with the channel in the corresponding and individual characteristics of the animal. Except inconnexon in the adjacent cell, permits substances to germ cells, the chromosomes occur in pairs, one origi-pass between the cells without entering the ECF. The nally from each parent (see Figure 23–2). Each chro-diameter of the channel is normally about 2 nm, which mosome is made up of a giant molecule of deoxyri-permits the passage of ions, sugars, amino acids, and bonucleic acid (DNA). The DNA strand is about 2 mother solutes with molecular weights up to about 1000. long, but it can fit in the nucleus because at intervals itGap junctions thus permit the rapid propagation of is wrapped around a core of histone proteins to form aelectrical activity from cell to cell (see Chapter 4) and nucleosome. There are about 25 million nucleosomesthe exchange of various chemical messengers. However, in each nucleus. Thus, the structure of the chromo-the gap junction channels are not simply passive, non- somes has been likened to a string of beads. The beadsspecific conduits. At least 20 different genes code for are the nucleosomes, and the linker DNA betweenconnexins in humans, and mutations in these genes can them is the string. The whole complex of DNA andlead to diseases that are highly selective in terms of the proteins is called chromatin. During cell division, thetissues involved and the type of condition produced. coiling around histones is loosened, probably by acety-For instance, X-linked Charcot–Marie–Tooth disease lation of the histones, and pairs of chromosomes be-is a peripheral neuropathy associated with mutation of come visible, but between cell divisions only clumps ofone particular connexin gene. Experiments in mice in chromatin can be discerned in the nucleus. The ulti-which particular connexins are deleted by gene manipu- mate units of heredity are the genes on the chromo-lation or replaced with different connexins confirm that somes (see below). Each gene is a portion of the DNA molecule. During normal cell division by mitosis, the chro- mosomes duplicate themselves and then divide in such a way that each daughter cell receives a full complement (diploid number) of chromosomes. During their finalPresynaptic maturation, germ cells undergo a division in which halfmembrane 4 nm the chromosomes go to each daughter cell (see ChapterGap 23). This reduction division (meiosis) is actually a two-(extracellular 2 nm stage process, but the important consideration is that asspace) a result of it, mature sperms and ova contain half thePostsynaptic 5 nm normal number (the haploid number) of chromo-membrane somes. When a sperm and ovum unite, the resultant cell (zygote) has a full diploid complement of chromo- 8 nm somes, one-half from the female parent and one-half Connexon from the male. The chromosomes undergo recombina- (the gap junction unit) tion, which mixes maternal and paternal genes. The nucleus of most cells contains a nucleolus (Fig-Figure 1–13. Gap junction. Note that each connexon ure 1–4), a patchwork of granules rich in ribonucleicis made up of six subunits and that each connexon in acid (RNA). In some cells, the nucleus contains severalthe membrane of one cell lines up with a connexon in of these structures. Nucleoli are most prominent andthe membrane of the neighboring cell, forming a chan- numerous in growing cells. They are the site of synthe-nel through which substances can pass from one cell to sis of ribosomes, the structures in the cytoplasm inanother without entering the ECF. (Reproduced, with which proteins are synthesized (see below).permission, from Kandel ER, Schwartz JH, Jessell TM [edi- The interior of the nucleus has a skeleton of fine fil-tors]: Principles of Neural Science, 4th ed. McGraw-Hill, aments that are attached to the nuclear membrane, or2000.) envelope (Figure 1–4), which surrounds the nucleus.
  • 31. 18 / CHAPTER 1This membrane is a double membrane, and spaces be- The Golgi apparatus, which is involved in process-tween the two folds are called perinuclear cisterns. ing proteins formed in the ribosomes, and secretoryThe membrane is permeable only to small molecules. granules, vesicles, and endosomes are discussed belowHowever, it contains nuclear pore complexes. Each in the context of protein synthesis and secretion.complex has eightfold symmetry and is made up ofabout 100 proteins organized to form a tunnel through STRUCTURE & FUNCTION OF DNA & RNAwhich transport of proteins and mRNA occurs. Thereare many transport pathways, and proteins called im- The Genomeportins and exportins have been isolated and charac- DNA is found in bacteria, in the nuclei of eukaryoticterized. A protein named Ran appears to play an orga- cells, and in mitochondria. It is made up of two ex-nizing role. Much current research is focused on tremely long nucleotide chains containing the basestransport into and out of the nucleus, and a more de- adenine (A), guanine (G), thymine (T), and cytosinetailed understanding of these processes should emerge (C) (Figure 1–14). The chemistry of these purine andin the near future. pyrimidine bases and of nucleotides is discussed in Chapter 17. The chains are bound together by hydro-Endoplasmic Reticulum gen bonding between the bases, with adenine bondingThe endoplasmic reticulum is a complex series of to thymine and guanine to cytosine. The resultant dou-tubules in the cytoplasm of the cell (Figure 1–4). The ble-helical structure of the molecule is shown in Figureinner limb of its membrane is continuous with a seg- 1–15. An indication of the complexity of the moleculement of the nuclear membrane, so in effect this part of is the fact that the DNA in the human haploid genomethe nuclear membrane is a cistern of the endoplasmic (the total genetic message) is made up of 3 × 109 basereticulum. The tubule walls are made up of membrane. pairs.In rough, or granular, endoplasmic reticulum, gran- DNA is the component of the chromosomes thatules called ribosomes are attached to the cytoplasmic carry the “genetic message,” the blueprint for all theside of the membrane, whereas in smooth, or agranu- heritable characteristics of the cell and its descendants.lar, endoplasmic reticulum, the granules are absent. Each chromosome contains a segment of the DNAFree ribosomes are also found in the cytoplasm. The double helix. The genetic message is encoded by the se-granular endoplasmic reticulum is concerned with pro- quence of purine and pyrimidine bases in the nu-tein synthesis and the initial folding of polypeptide cleotide chains. The text of the message is the order inchains with the formation of disulfide bonds. The which the amino acids are lined up in the proteinsagranular endoplasmic reticulum is the site of steroid manufactured by the cell. The message is transferred tosynthesis in steroid-secreting cells and the site of detoxi- ribosomes, the sites of protein synthesis in the cyto-fication processes in other cells. As the sarcoplasmic plasm, by RNA. RNA differs from DNA in that it isreticulum (see Chapter 3), it plays an important role in single-stranded, has uracil in place of thymine, and itsskeletal and cardiac muscle. sugar moiety is ribose rather than 2′-deoxyribose (see Chapter 17). The proteins formed from the DNA blue-Ribosomes print include all the enzymes, and these in turn control the metabolism of the cell. A gene used to be defined asThe ribosomes in eukaryotes measure approximately the amount of information necessary to specify a single22 by 32 nm. Each is made up of a large and a small protein molecule. However, the protein encoded by asubunit called, on the basis of their rates of sedimenta- single gene may be subsequently divided into severaltion in the ultracentrifuge, the 60S and 40S subunits. different physiologically active proteins. In addition,The ribosomes are complex structures, containing different mRNAs can be formed from a gene, with eachmany different proteins and at least three ribosomal mRNA dictating formation of a different protein.RNAs (see below). They are the sites of protein synthe- Genes also contain promoters, DNA sequences that fa-sis. The ribosomes that become attached to the endo- cilitate the formation of RNA. Mutations occur whenplasmic reticulum synthesize all transmembrane pro- the base sequence in the DNA is altered by ionizing ra-teins, most secreted proteins, and most proteins that are diation or other mutagenic agents.stored in the Golgi apparatus, lysosomes, and endo-somes. All these proteins have a hydrophobic signal The Human Genomepeptide at one end. The polypeptide chains that formthese proteins are extruded into the endoplasmic reticu- When the human genome was finally mapped severallum. The free ribosomes synthesize cytoplasmic pro- years ago, there was considerable surprise that it con-teins such as hemoglobin (see Chapter 27) and the pro- tained only about 30,000 genes and not the 50,000 orteins found in peroxisomes and mitochondria. more that had been expected. Yet humans differ quite
  • 32. THE GENERAL & CELLULAR BASIS OF MEDICAL PHYSIOLOGY / 19 O N NH G 5 CH2 N NH2 N NH2 O O N P C H H CH2 O H H O N O H3C H O NH T P H H CH2 N O O H H NH2 H O N O N P A H H CH2 O H H N N H O O P O H H H H 3 H P OFigure 1–14. Segment of the structure of the DNA molecule in which the purine and pyrimidine bases adenine (A),thymine (T), cytosine (C), and guanine (G) are held together by a phosphodiester backbone between 2′-deoxyribosylmoieties attached to the nucleobases by an N-glycosidic bond. Note that the backbone has a polarity (ie, a 5′ and a3′ direction). (Reproduced, with permission, from Murray RK et al: Harper’s Illustrated Biochemistry, 26th ed. McGraw-Hill,2003.)markedly from their nearest simian relatives. The expla- ous restriction enzymes, so that restriction fragmentnation appears to be that rather than a greater number length polymorphism (RFLP) occurs in the DNAof genes in humans, there is a greater number of fragments from different individuals. Consequently,mRNAs—perhaps as many as 85,000. The implica- analysis of RFLP in a population gives a pattern that istions of this increase are discussed below. in effect a DNA fingerprint. The value of DNA finger- printing has been improved by additional specialized techniques. The chance of obtaining identical DNADNA Polymorphism patterns by using these techniques in individuals whoThe protein-coding portions of the genes (exons) make are not identical twins varies with the number of en-up only 3% of the human genome; the remaining 97% zymes used, the relatedness of the individuals, andis made up of introns (see below) and other DNA of other factors, and there has been debate about the ap-unsettled or unknown function. This 97% is some- propriate statistics to use for analysis. However, thetimes called junk DNA. A characteristic of human possibility that an RFLP match is due to chance hasDNA is its structural variability from one individual to been estimated at 1 in 100,000 to 1 in 1,000,000. Fur-another. Most of the variations occur in noncoding re- thermore, RFLP analysis can be carried out on smallgions, but they can also occur in coding regions, where specimens of semen, blood, or other tissue, and multi-they can be silent or expressed as a detectable alteration ple copies of pieces of DNA can be made by using thein a protein. A common form of these variations is vari- polymerase chain reaction (PCR), an ingenious tech-able repetition of base pairs (tandem repeats) from one nique for making DNA copy itself. Therefore, DNAto hundreds of times. This variation alters the length of fingerprinting is of obvious value in investigatingthe DNA chain between points where it is cut by vari- crimes and determining paternity, although reliable
  • 33. 20 / CHAPTER 1 hand, cells with high telomerase activity, which in- cludes most cancer cells, can in theory keep multiplying indefinitely. Not surprisingly, there has been consider- able interest in the telomerase mechanism, both in terms of aging and in terms of cancer. However, it now seems clear that the mechanism for replicating chromo- some ends is complex, and much additional research will be needed before a complete understanding isMinor groove achieved and therapeutic applications emerge. S A T S P P 3.4 nm Meiosis P In germ cells, reduction division (meiosis) takes place P P during maturation. The net result is that one of each S G C S pair of chromosomes ends up in each mature germ cell;Major groove consequently, each mature germ cell contains half the amount of chromosomal material found in somatic cells. Therefore, when a sperm unites with an ovum, the resulting zygote has the full complement of DNA, half of which came from the father and half from the mother. The chromosomal events that occur at the time of fertilization are discussed in detail in Chapter 23. The term “ploidy” is sometimes used to refer to the 2.0 nm number of chromosomes in cells. Normal resting diploid cells are euploid and become tetraploid justFigure 1–15. Double-helical structure of DNA, with before division. Aneuploidy is the condition in which aadenine (A) bonding to thymine (T) and cytosine (C) to cell contains other than the haploid number of chro-guanine (G). (Reproduced, with permission, from Murray mosomes or an exact multiple of it, and this conditionRK et al: Harper’s Illustrated Biochemistry, 26th ed. McGraw- is common in cancerous cells.Hill, 2003.) Cell Cycletechniques must be used and the results interpreted Obviously, the initiation of mitosis and normal cell di-with care. RFLP analysis is also of value in studying an- vision depends on the orderly occurrence of events dur-imal and human evolution and in identifying the chro- ing what has come to be called the cell cycle. A dia-mosomal location of genes causing inherited diseases. gram of these events is shown in Figure 1–16. There is intense interest in the biochemical machinery that pro-Mitosis duces mitosis, in part because of the obvious possibilityAt the time of each somatic cell division (mitosis), thetwo DNA chains separate, each serving as a templatefor the synthesis of a new complementary chain. DNApolymerase catalyzes this reaction. One of the doublehelices thus formed goes to one daughter cell and one G2 Mitosisgoes to the other, so the amount of DNA in eachdaughter cell is the same as that in the parent cell. S phase Pre-Telomeres start G1Cell replication involves not only DNA polymerase but Post- starta special reverse transcriptase that synthesizes the short G1repeats of DNA that characterize the ends (telomeres) Startof chromosomes. Without this transcriptase and relatedenzymes known collectively as telomerase, somaticcells lose DNA as they divide 40–60 times and then be- Figure 1–16. Sequence of events during the cellcome senescent and undergo apoptosis. On the other cycle.
  • 34. THE GENERAL & CELLULAR BASIS OF MEDICAL PHYSIOLOGY / 21of its relation to cancer. When DNA is damaged, entry protein (translation). This process occurs in the ribo-into mitosis is inhibited, giving the cell time to repair somes. tRNA attaches the amino acids to mRNA. Thethe DNA; failure to repair damaged DNA leads to can- mRNA molecules are smaller than the DNA molecules,cer. The cell cycle is regulated by proteins called cyclins and each represents a transcript of a small segment ofand cyclin-dependent protein kinases, which phos- the DNA chain. The molecules of tRNA contain onlyphorylate other proteins. However, the regulation is 70–80 nitrogenous bases, compared with hundreds incomplex, and a detailed analysis of it is beyond the mRNA and 3 billion in DNA.scope of this book. It is worth noting that DNA is responsible for the maintenance of the species; it passes from one genera-Transcription & Translation tion to the next in germ cells. RNA, on the other hand, is responsible for the production of the individual; itThe strands of the DNA double helix not only replicate transcribes the information coded in the DNA andthemselves, but also serve as templates by lining up forms a mortal individual, a process that has been calledcomplementary bases for the formation in the nucleus “budding off from the germ line.”of messenger RNA (mRNA), transfer RNA (tRNA),the RNA in the ribosomes (rRNA), and various otherRNAs. The formation of mRNA is called transcription Genes(Figure 1–17) and is catalyzed by various forms of RNApolymerase. Usually after some posttranscriptional Information is accumulating at an accelerating rateprocessing (see below), mRNA moves to the cytoplasm about the structure of genes and their regulation. Theand dictates the formation of the polypeptide chain of a structure of a typical eukaryotic gene is shown in dia- DNA RNA strand formed on DNA strand (transcription) tRNA Amino acid adenylate Posttranscrip- Chain separation tional modification Activating enzyme Messenger RNA Coding triplets for A3 A A1 A4 2 Posttranslational modification Translation Ribosome tRNA-amino acid-adenylate A3 A A1 complex A4Figure 1–17. Diagrammatic outline of protein synthesis. The nucleic acids are represented as lines with multipleshort projections representing the individual bases.
  • 35. 22 / CHAPTER 1grammatic form in Figure 1–18. It is made up of a there is evidence that sequences in this region can alsostrand of DNA that includes coding and noncoding re- affect the function of other genes.gions. In eukaryotes, unlike prokaryotes, the portionsof the genes that dictate the formation of proteins are Regulation of Gene Expressionusually broken into several segments (exons) separatedby segments that are not translated (introns). A pre- Each nucleated somatic cell in the body contains themRNA is formed from the DNA, and then the introns full genetic message, yet there is great differentiationand sometimes some of the exons are eliminated in the and specialization in the functions of the various typesnucleus by posttranscriptional processing, so that the of adult cells. Only small parts of the message are nor-final mRNA, which enters the cytoplasm and code for mally transcribed. Thus, the genetic message is nor-protein, is made up of exons (Figure 1–19). Introns are mally maintained in a repressed state. However, geneseliminated and exons are joined by several different are controlled both spatially and temporally. Whatprocesses. The introns of some genes are eliminated by turns on genes in one cell and not in other cells? Whatspliceosomes, complex units that are made up of small turns on genes in a cell at one stage of development andRNAs and proteins. Other introns are eliminated by not at other, inappropriate stages? What maintains or-self-splicing by the RNA they contain. Two different derly growth in cells and prevents the uncontrolledmechanisms produce self-splicing. RNA can catalyze growth that we call cancer? Obviously, DNA sequencesother reactions as well and there is great interest today such as the TATA box promote orderly transcription ofin the catalytic activity of RNA. the gene of which they are a part (cis regulation). How- Because of introns and splicing, more than one ever, the major key to selective gene expression is themRNA is formed from the same gene. As noted above, proteins that bind to the regulatory regions of the genethe formation of multiple proteins from one gene is and increase or shut off its activity. These transcriptionperhaps one of the explanations of the surprisingly factors are products of other genes and hence mediatesmall number of genes in the human genome. Other trans regulation. They are extremely numerous and in-physiologic functions of the introns are still unsettled, clude activated steroid hormone receptors and manythough they may foster changes in the genetic message other factors.and thus aid evolution. It is common for stimuli such as neurotransmitters Near the transcription start site of the gene is a pro- that bind to the cell membrane to initiate chemicalmoter, which is the site at which RNA polymerase and events that activate immediate-early genes. These inits cofactors bind. It often includes a thymidine–ade- turn produce transcription factors that act on othernine–thymidine–adenine (TATA) sequence (TATA genes. The best-characterized immediate-early genes arebox), which ensures that transcription starts at the c-fos, and c-jun. The proteins produced by these genes,proper point. Farther out in the 5′ region are regula- c-Fos, c-Jun, and several related proteins, form ho-tory elements, which include enhancer and silencer se- modimer or heterodimer transcription factors that bindquences. It has been estimated that each gene has an av- to a specific DNA regulatory sequence called an activa-erage of five regulatory sites. Regulatory sequences are tor protein-1 (AP-1) site (Figure 1–20). Some of thesometimes found in the 3′-flanking region as well, and dimers enhance transcription, and others inhibit it. The Basal Transcription Poly(A) Regulatory promoter start site addition region region site Exon Exon DNA 5 CAAT TATA AATAAA 3 5 Intron 3 Noncoding Noncoding region regionFigure 1–18. Diagram of the components of a typical eukaryotic gene. The region that produces introns andexons is flanked by noncoding regions. The 5′-flanking region contains stretches of DNA that interact with proteinsto facilitate or inhibit transcription. The 3′-flanking region contains the poly(A) addition site. (Modified from MurrayRK et al: Harper’s Illustrated Biochemistry, 26th ed. McGraw-Hill, 2003.
  • 36. THE GENERAL & CELLULAR BASIS OF MEDICAL PHYSIOLOGY / 23 Flanking DNA Introns Exons Second messengers Protein kinase CGENE Nuclear Transcription Flanking membrane DNA Cap FOSPRE- Poly(A)mRNA + FOS JUN DNA JUN + RNA POL IIPROCESSING Poly(A) TGAGTCA AP-1 sitemRNA Poly(A) TranslationPREPROHORMONE Signal peptide Signal peptide sequence cleavagePROHORMONE Proteolysis and/or Fragment C C C H glycosylation Zn ZnHORMONE C C C H Sugar Cys-Cys zinc finger Cys-His zinc fingerFigure 1–19. Transcription, posttranscriptional modi-fication of mRNA, translation in the ribosomes, and Figure 1–20. Top: Activation of genes by second mes-posttranslational processing in the formation of hor- sengers. Increased protein kinase C causes production ofmones and other proteins. Cap, cap site. (Modified from c-Fos and c-Jun by immediate-early genes. The c-Fos–c-Baxter JD: Principles of endocrinology. In: Cecil Textbook of Jun heterodimer binds to an AP-1 site, in this case acti-Medicine, 16th ed. Wyngaarden JB, Smith LH Jr [editors]. vating RNA polymerase II (Pol II) and increasing transcrip-Saunders, 1982.) tion of other genes. (Courtesy of DG Gardner.) Bottom: Zinc fingers. The curved lines represent polypeptide chains of proteins that bind to DNA, and the straight linesappearance of c-Fos, c-Jun, and related proteins is such indicate coordinate binding of zinc to cysteines (C) ora common sign of cell activation that immunocyto- cysteines and histidines (H). (Reproduced, with permission,chemistry for them or measurement of their mRNAs is from Murray RK et al: Harper’s Illustrated Biochemistry, 26thused to determine which cells in the nervous system ed. McGraw-Hill, 2003.)and elsewhere are activated by particular stimuli. Over 80% of the known transcription factors haveone of four DNA-binding motifs. The most common is other to form a coiled coil. Extensions of the dimer be-the zinc finger motif, in which characteristically yond the zippered region are rich in arginine and lysine,shaped complexes are formed by coordinate binding of and these bind to DNA. The other common DNA-Zn2+ between two cysteine and two histidine residues or binding motifs are helix-turn-helix and helix-loop-helixbetween four cysteine residues. Various transcription structures.factors contain 2–37 of these zinc fingers, which medi- It is now possible by using molecular biologic tech-ate the binding to DNA. Another motif is the leucine niques to augment the function of particular genes, tozipper, in which α-helical regions of dimers have regu- transfer human genes into animals, and to disrupt thelarly spaced leucine residues that interact with one an- function of single genes (gene knockout). The gene
  • 37. 24 / CHAPTER 1knockout technique is currently being used in numer- tRNA attaches to both. As the amino acids are added inous experiments. the order dictated by the triplet code, the ribosome moves along the mRNA molecule like a bead on a string. Translation stops at one of three stop, or non-Protein Synthesis sense, codons (UGA, UAA, or UAG), and the polypep-The process of protein synthesis is a complex but fasci- tide chain is released. The tRNA molecules are usednating one that, as noted above, involves four steps: again. The mRNA molecules are also reused approxi-transcription, posttranscriptional modification, transla- mately 10 times before being replaced.tion, and posttranslational modification. The various Typically, more than one ribosome occurs on asteps are summarized in simplified form in Figure 1–19. given mRNA chain at a time. The mRNA chain plus its When suitably activated, transcription of the gene collection of ribosomes is visible under the electron mi-starts at the cap site (Figure 1–19) and ends about croscope as an aggregation of ribosomes called a polyri-20 bases beyond theAATAAA sequence. The RNA bosome (polysome).transcript is capped in the nucleus by addition of Although mRNA is formed in the nucleus, individ-7-methylguanosine triphosphate to the 5′ end; this cap ual strands of mRNA can be moved along the cy-is necessary for proper binding to the ribosome (see toskeleton to various parts of the cell and, in the pres-below). A poly(A) tail of about 100 bases is added to ence of suitable ribosomes, synthesize proteins in thethe untranslated segment at the 3′ end. The function of local area within the cell. The role of this process in thethe poly(A) tail is still being debated, but it may help function of dendrites is discussed in Chapter 4.maintain the stability of the mRNA. The pre-mRNA At least in theory, synthesis of particular proteinsformed by capping and addition of the poly(A) tail is can be stopped by administering antisense oligonu-then processed by elimination of the introns (Figure cleotides, short synthetic stretches of bases comple-1–19), and once this posttranscriptional modification is mentary to segments of the bases on the mRNA for thecomplete, the mature mRNA moves to the cytoplasm. protein. These bind to the mRNA, blocking transla-Posttranscriptional modification of the pre-mRNA is a tion. Early results with this technology were disap-regulated process, and, as noted above, differential pointing because of nonspecific binding and immunesplicing can occur, with the formation of more than responses, but research continues and there is hope forone mRNA from a single pre-mRNA. products that will be useful in the treatment of a variety When a definitive mRNA reaches a ribosome in the of diseases, including cancer.cytoplasm, it dictates the formation of a polypeptidechain. Amino acids in the cytoplasm are activated Posttranslational Modificationby combination with an enzyme and adenosinemonophosphate (adenylate), and each activated amino After the polypeptide chain is formed, it is modified toacid then combines with a specific molecule of tRNA. the final protein by one or more of a combination of re-There is at least one tRNA for each of the 20 unmodi- actions that include hydroxylation, carboxylation, gly-fied amino acids found in large quantities in the body cosylation, or phosphorylation of amino acid residues;proteins of animals (see Chapter 17), but some amino cleavage of peptide bonds that converts a largeracids have more than one tRNA. The tRNA–amino polypeptide to a smaller form; and the folding andacid–adenylate complex is next attached to the mRNA packaging of the protein into its ultimate, often com-template, a process that occurs in the ribosomes. This plex configuration.process is shown diagrammatically in Figure 1–17. The It has been claimed that a typical eukaryotic cell syn-tRNA “recognizes” the proper spot to attach on the thesizes about 10,000 different proteins during its life-mRNA template because it has on its active end a set of time. How do these proteins get to the right locationsthree bases that are complementary to a set of three in the cell? Synthesis starts in the free ribosomes. Mostbases in a particular spot on the mRNA chain. The ge- proteins that are going to be secreted or stored in or-netic code is made up of such triplets, sequences of ganelles and most transmembrane proteins have at theirthree purine or pyrimidine bases (or both); each triplet amino terminal a signal peptide (leader sequence)stands for a particular amino acid. that guides them into the endoplasmic reticulum. The Translation starts in the ribosomes with an AUG sequence is made up of 15–30 predominantly hy-(transcribed from ATG in the gene), which codes for drophobic amino acid residues. The signal peptide,methionine. The amino terminal amino acid is then once synthesized, binds to a signal recognition parti-added, and the chain is lengthened one amino acid at a cle (SRP), a complex molecule made up of six polypep-time. The mRNA attaches to the 40S subunit of the ri- tides and 7S RNA, one of the small RNAs. The SRPbosome during protein synthesis; the polypeptide chain stops translation until it binds to a translocon, a porebeing formed attaches to the 60S subunit; and the in the endoplasmic reticulum that is a heterotrimeric
  • 38. THE GENERAL & CELLULAR BASIS OF MEDICAL PHYSIOLOGY / 25structure made up of Sec 61 proteins. The ribosome Although most secreted polypeptides and proteinsalso binds, and the signal peptide leads the growing have a leader sequence that targets them to the endo-peptide chain into the cavity of the endoplasmic reticu- plasmic reticulum and are secreted by exocytosis (seelum (Figure 1–21). The signal peptide is next cleaved below), a growing list of proteins that are secreted lack afrom the rest of the peptide by a signal peptidase while signal sequence. In humans these include the cytokinesthe rest of the peptide chain is still being synthesized. interleukin-1α (IL-1α) and IL-1β, three growth factors, The signals that direct nascent proteins to some of and various factors involved in hemostasis. Secretionthe other parts of the cell are fashioned in the Golgi ap- probably occurs via ATP-dependent membrane trans-paratus (see below) and involve specific modifications porters. There is a large family of these ATP-binding-of the carbohydrate residues on glycoproteins. cassette (ABC) transport proteins, and they transport ions and other substances as well as proteins between or-Secreted Proteins ganelles and across cell membranes. In general, they are made up of two cytoplasmic ATP-binding domains andMany and perhaps all proteins that are secreted by cells two membrane domains, each of which probably spansare synthesized as larger proteins, and polypeptide se- the membrane and in general contains six long α-helicalquences are cleaved off from them during maturation. sequences (Figure 1–23). The cystic fibrosis trans-In the case of the hormones, these larger forms are membrane conductance regulator (CFTR) is one ofcalled preprohormones and prohormones (Figures those ABC transport proteins that also has a region for1–19 and 1–22). Parathyroid hormone (see Chapter regulation by cyclic adenosine monophosphate (cAMP).21) is a good example. It is synthesized as a molecule It transports Cl– and is abnormal in individuals withcontaining 115 amino acid residues (preproparathyroid cystic fibrosis (see Chapter 37).hormone). The leader sequence, 25 amino acid residuesat the amino terminal, is rapidly removed to formproparathyroid hormone. Before secretion, an addi- Protein Foldingtional six amino acids are removed from the amino ter- Protein folding is an additional posttranslational modi-minal to form the secreted molecule. The function of fication. It is a complex process that is dictated primar-the six-amino-acid fragment is unknown. ily by the sequence of the amino acids in the polypep- tide chain. In some instances, however, nascent proteins associate with other proteins called chaper- 5 ones, which prevent inappropriate contacts with other 3 proteins and ensure that the final “proper” conforma- N UAA tion of the nascent protein is reached. Misfolded pro- SRP teins and other proteins targeted for degradation are N N N N conjugated to ubiquitin and broken down in the or- ganelles called 26S proteasomes (see Chapter 17). C C C C N N Apoptosis N In addition to dividing and growing under genetic con- trol, cells can die and be absorbed under genetic control.Figure 1–21. Translation of protein into endoplasmic This process is called programmed cell death, or apop-reticulum according to the signal hypothesis. The ribo- tosis (Gr apo “away” + ptosis “fall”). It can be called “cellsomes synthesizing a protein move along the mRNA suicide” in the sense that the cell’s own genes play an ac- tive role in its demise. It should be distinguished fromfrom the 5′ to the 3′ end. When the signal peptide of a necrosis (“cell murder”), in which healthy cells are de-protein destined for secretion, the cell membrane, or stroyed by external processes such as inflammation.lysosomes emerges from the large unit of the ribo- Apoptosis is a very common process during develop-some, it binds to a signal recognition particle (SRP), and ment and in adulthood. In the central nervous system,this arrests further translation until it binds to the large numbers of neurons are produced and then dietranslocon on the endoplasmic reticulum. N, amino end during the remodeling that occurs during developmentof protein; C, carboxyl end of protein. (Reproduced, with and synapse formation (see Chapter 4). In the immunepermission, from Perara E, Lingappa VR: Transport of pro- system, apoptosis gets rid of inappropriate clones of im-teins into and across the endoplasmic reticulum mem- munocytes (see Chapter 27) and is responsible for thebrane. In: Protein Transfer and Organelle Biogenesis. Das RC, lytic effects of glucocorticoids on lymphocytes (seeRobbins PW [editors]. Academic Press, 1988.) Chapter 20). Apoptosis is also an important factor in
  • 39. 26 / CHAPTER 1 Number of Number of amino acids amino acids in precursor Precursor peptide in hormones Prepropressophysin 166 9 AVP Prepro TRH 242 3 TRH TRH TRH TRH TRH TRH Preproenkephalin A * ** 267 5, 7, 8 Met-enk Leu-enk *Met-enk octapeptide **Met-enk heptapeptide Prepro-opiomelanocortin 39 ACTH 265 31β-Endorphin MSH ACTH MSH End Preprodynorphin (preproenkephalin B) 236 17 Dynorphin Leu-enk N-end DynFigure 1–22. Examples of large precursors (preprohormones) for small peptide hormones. See also Figure 14–12.TRH, thyrotropin-releasing hormone; AVP, arginine vasopressin; Met-enk, met-enkephalin; Leu-enk, leu-enkephalin;MSH, melanocyte-stimulating hormone; ACTH, adrenocorticotropic hormone; End, β-endorphin; Dyn, dynorphin; N-end, neoendorphin.processes such as removal of the webs between the fin- Apoptosis can be triggered by external and internalgers in fetal life and regression of duct systems in the stimuli. One ligand that activates receptors triggeringcourse of sexual development in the fetus (see Chapter apoptosis is Fas, a transmembrane protein that projects23). In adults, it participates in the cyclic breakdown of from natural killer cells and T lymphocytes (see Chap-the endometrium that leads to menstruation (see Chap- ter 27) but also exists in a circulating form. Another ister 23). In epithelia, cells that lose their connections to tumor necrosis factor (TNF).the basal lamina and neighboring cells undergo apopto- Between initiating stimuli and caspase activation is asis. This is responsible for the death of the enterocytes complex network of excitatory and inhibitory intracel-sloughed off the tips of intestinal villi (see Chapter 26). lular proteins. One of the important pathways goesAbnormal apoptosis probably occurs in autoimmune through the mitochondria, which release cytochrome cdisease, neurodegenerative diseases, and cancer. It is in- and a protein called smac/DIABLO. Cytochrome cteresting that apoptosis occurs in invertebrates, includ- acts with several cytoplasmic proteins to facilitate cas-ing nematodes and insects. However, its molecular pase activation. In the process, the enzymes form amechanism is much more complex than that in verte- wheel-like structure with seven spokes known as anbrates. apoptosome. Smac/DIABLO binds to several inhibit- The final common pathway bringing about apopto- ing proteins, lifting the inhibition of caspase-9 and thussis is activation of caspases, a group of cysteine pro- increasing apoptotic activity.teases. Many of these have been characterized to date inmammals; 11 have been found in humans. They exist Molecular Medicinein cells as inactive proenzymes until activated by thecellular machinery. The net result is DNA fragmenta- Fundamental research on molecular aspects of genetics,tion, cytoplasmic and chromatin condensation, and regulation of gene expression, and protein synthesis haseventually membrane bleb formation, with cell breakup been paying off in clinical medicine at a rapidly acceler-and removal of the debris by phagocytes. ating rate.
  • 40. THE GENERAL & CELLULAR BASIS OF MEDICAL PHYSIOLOGY / 27 The p53 protein produced by this gene triggers apopto- sis. It is also a nuclear transcription factor that appears to increase production of a 21-kDa protein that blocks MD1 MD2 Membrane two cell cycle enzymes, slowing the cycle and permit- ting repair of mutations and other defects in DNA. The p53 gene is mutated in up to 50% of human cancer pa- ABC1 ABC2 tients, with the production of p53 proteins that fail to Cytoplasm slow the cell cycle and permit other mutations in DNA S to persist. The accumulated mutations eventually cause ATP ATP cancer. Gene therapy is still in its infancy, but various inge-Figure 1–23. General structure of eukaryotic ABC nious approaches to the problem of getting genes intotransporter proteins that move ions, other substances, cells are now being developed. One that is already inand proteins across membranes. ABC1 and ABC2, ATP- clinical trials for some diseases involves removal of cellsbinding domains; MD1 and MD2, membrane domains; from the patient with the disease, transfection of theS, substrate. (Modified from Kuchler K, Thorner J: Secre- cells with normal genes in vitro, and reinjection of thetion of peptides and proteins lacking hydrophobic signal cells into the patient as an autotransplant. Another issequences: The role of adenosine triphosphate-driven insertion of appropriate genes into relatively benignmembrane translocators. Endocr Rev 1992;13:499.) viruses that are then administered to patients to carry the genes to the cells they invade. One early dividend was an understanding of the Golgi Apparatus & Vesicularmechanisms by which antibiotics exert their effects. Al-most all act by inhibiting protein synthesis at one or an- Transport in Cellsother of the steps described above. Antiviral drugs act in The Golgi apparatus is a collection of membrane-en-a similar way; for example, acyclovir and ganciclovir act closed sacs (cisterns) that are stacked like dinner platesby inhibiting DNA polymerase. Some of these drugs (Figure 1–4). There are usually about six sacs in eachhave this effect primarily in bacteria, but others inhibit apparatus, but there may be more. One or more Golgiprotein synthesis in the cells of other animals, including apparatuses are present in all eukaryotic cells, usuallymammals. This fact makes antibiotics of great value for near the nucleus. The Golgi apparatus is a polarizedresearch as well as for treatment of infections. structure, with cis and trans sides (Figure 1–24). Mem- Single genetic abnormalities that cause over branous vesicles containing newly synthesized proteins600 human diseases have now been identified. Many of bud off from the granular endoplasmic reticulum andthe diseases are rare, but others are more common and fuse with the cistern on the cis side of the apparatus.some cause conditions that are severe and eventually The proteins are then passed via other vesicles to thefatal. Examples include the defectively regulated Cl– middle cisterns and finally to the cistern on the transchannel in cystic fibrosis (see above and Chapter 34) side, from which vesicles branch off into the cyto-and the unstable trinucleotide repeats in various parts plasm. From the trans Golgi, vesicles shuttle to theof the genome that cause Huntington’s disease, the lysosomes and to the cell exterior via a constitutive andfragile X syndrome, and several other neurologic dis- a nonconstitutive pathway, both involving exocytosiseases (see Chapter 12). Abnormalities in mitochondrial (see below). Conversely, vesicles are pinched off fromDNA can also cause human diseases such as Leber’s the cell membrane by endocytosis (see below) and passhereditary optic neuropathy and some forms of car- to endosomes. From there, they are recycled. Exocyto-diomyopathy. Not surprisingly, genetic aspects of can- sis and endocytosis in nerve endings are special cases ofcer are probably receiving the greatest current attention. vesicle transport and are discussed in Chapter 4.Some cancers are caused by oncogenes, genes that are The vesicles carry out their voyages by a combina-carried in the genomes of cancer cells and are responsi- tion of common mechanisms along with special mecha-ble for producing their malignant properties. These nisms that determine where inside the cell they go. Agenes are derived by somatic mutation from closely re- prominent feature is the presence of proteins calledlated proto-oncogenes, which are normal genes that SNAREs (for soluble N-ethylmalemite-sensitive factorcontrol growth. Over 100 oncogenes have been de- attachment receptor). The v- (for vesicle) SNAREs onscribed. Another group of genes produce proteins that vesicle membranes interact in a lock-and-key fashionsuppress tumors, and more than 10 of these tumor with t- (for target) SNAREs. The vesicles are ticketedsuppressor genes have been described. The most stud- for specific loci (eg, Golgi sacs, cell membranes) by par-ied of these is the p53 gene on human chromosome 17. ticular molecules such as mannose-6-phospate.
  • 41. 28 / CHAPTER 1 ER Golgi apparatus Secretory granules Regulated secretion Constitutive secretion Recycling Endocytosis Nucleus Lysosome Late endosome Early endosomeFigure 1–24. Pathways involved in protein processing in cells. In the cell, the initial glycosylation of proteins oc- sion then breaks down, leaving the contents of the vesi-curs with the attachment of preformed oligosaccharides cle outside and the cell membrane intact. This is thein the endoplasmic reticulum, but these oligosaccha- Ca2+-dependent process of exocytosis (Figure 1–25).rides are altered to a variety of different carbohydrate Note that secretion from the cell occurs via twomoieties in the Golgi apparatus. pathways (Figure 1–24). In the nonconstitutive path- way, proteins from the Golgi apparatus initially enterQuality Control secretory granules, where processing of prohormones to the mature hormones occurs before exocytosis. TheThe processes involved in protein synthesis, folding, other pathway, the constitutive pathway, involves theand migration to the various parts of the cell are so prompt transport of proteins to the cell membrane incomplex that it is remarkable that more errors and ab- vesicles, with little or no processing or storage. Thenormalities do not occur. The fact that these processes nonconstitutive pathway is sometimes called the regu-work as well as they do is because of mechanisms at lated pathway, but this term is misleading because theeach level that are responsible for “quality control.” output of proteins by the constitutive pathway is alsoDamaged DNA is detected and repaired or bypassed. regulated.The various RNAs are also checked during the transla-tion process. Finally, when the protein chains are in theendoplasmic reticulum and Golgi apparatus, defective Endocytosisstructure is detected and the abnormal proteins are de-graded in lysosomes and proteasomes. The net result is Endocytosis is the reverse of exocytosis. There are vari-a remarkable accuracy in the production of the proteins ous types. Phagocytosis (“cell eating”) is the process byneeded for normal body function. which bacteria, dead tissue, or other bits of microscopic material are engulfed by cells such as the polymor- phonuclear leukocytes of the blood. The materialTRANSPORT ACROSS CELL MEMBRANES makes contact with the cell membrane, which then in-Transport across cell membranes is accomplished pri- vaginates. The invagination is pinched off, leaving themarily by exocytosis, endocytosis, movement through engulfed material in the membrane-enclosed vacuoleion channels, and primary and secondary active trans- and the cell membrane intact. Pinocytosis (“cell drink-port. ing”) is essentially the same process, the difference being that the substances ingested are in solution andExocytosis not visible under the microscope. Endocytosis can be constitutive or clathrin-medi-Vesicles containing material for export are ticketed to ated. Constitutive endocytosis is not a specializedthe cell membrane (Figure 1–24), where they bond via process, whereas clathrin-mediated endocytosis oc-the v-SNARE/t-SNARE arrangement. The area of fu- curs at membrane indentations where the protein
  • 42. THE GENERAL & CELLULAR BASIS OF MEDICAL PHYSIOLOGY / 29Figure 1–25. Exocytosis and endocytosis. Note that in exocytosis the cytoplasmic sides of two membranes fuse,whereas in endocytosis two noncytoplasmic sides fuse. (Reproduced, with permission, from Alberts B et al: Molecular Bi-ology of the Cell, 4th ed. Garland Science, 2002.)clathrin accumulates. Clathrin molecules have the 17). It also plays a major role in synaptic function (seeshape of triskelions, with three legs radiating from a Chapter 4).central hub (Figure 1–26). As endocytosis progresses, It is apparent that exocytosis adds to the totalthe clathrin molecules form a geometric array that sur- amount of membrane surrounding the cell, and ifrounds the endocytotic vesicle. At the neck of the vesi- membrane were not removed elsewhere at an equiva-cle, a guanosine triphosphatase protein called dynamin lent rate, the cell would enlarge. However, removal ofis involved, either directly or indirectly, in pinching off cell membrane occurs by endocytosis, and such exocy-the vesicle; this protein has therefore been called a “pin- tosis–endocytosis coupling maintains the surface area ofchase.” Once the complete vesicle is formed, the the cell at its normal size.clathrin falls off and the three-legged proteins recycle toform another vesicle. The vesicle fuses with and dumps Rafts & Caveolaeits contents into an early endosome (Figure 1–24).From the early endosome, a new vesicle can bud off Some areas of the cell membrane are especially rich inand return to the cell membrane (see Figure 4–4). Al- cholesterol and sphingolipids and have been calledternatively, the early endosome can become a late en- rafts. These rafts are probably the precursors of flask-dosome and fuse with a lysosome (Figure 1–24) in shaped membrane depressions called caveolae (littlewhich the contents are digested by the lysosomal pro- caves) when their walls become infiltrated with a pro-teases. tein called caveolin that resembles clathrin. Three iso- Clathrin-mediated endocytosis is responsible for the forms of caveolin (caveolins-1, -2, and -3) have beeninternalization of many receptors and the ligands identified. There is considerable debate about the func-bound to them—including, for example, nerve growth tions of rafts and caveolae, with evidence that they arefactor and low-density lipoproteins (LDL; see Chapter involved in cholesterol regulation and transcytosis (see
  • 43. 30 / CHAPTER 1 Golgi have coat proteins I and II (COPI and COPII). Certain amino acid sequences or attached groups on the transported proteins ticket the proteins for particu- lar locations. For example, the amino acid sequence Asn–Pro–any amino acid–Tyr tickets transport from the cell surface to the endosomes, and mannose-6-phos- phate groups ticket transfer from the Golgi to man- nose-6-phosphate receptors (MPR) on the lysosomes. Various small guanosine triphosphate (GTP)-bind- ing proteins of the Rab family (see below) are associated with the various types of vesicles. They appear to guideFigure 1–26. Clathrin molecule on the surface of an and facilitate orderly attachments of these vesicles. Hu-endocytotic vesicle. Note the characteristic triskelion mans have 60 Rab proteins and 35 SNARE proteins.shape and the fact that with other clathrin molecules itforms a net supporting the vesicle. Distribution of Ions & Other Substances Across Cell Membranesbelow). However, mice in which the gene for caveolin- The unique properties of the cell membranes are re-1 is knocked out are relatively healthy, with only some sponsible for the differences in the composition of in-poorly understood blood vessel and pulmonary abnor- tracellular and interstitial fluid. Specific values for onemalities. mammalian tissue are shown in Table 1–2. Average val- ues for humans are shown in Figure 1–27.Coats & Vesicle Transport Membrane PermeabilityIt now appears that all vesicles involved in transporthave protein coats. In humans, 53 coat complex sub- & Membrane Transport Proteinsunits have been identified. Vesicles that transport pro- An important technique that has permitted major ad-teins from the trans Golgi to lysosomes have AP- vances in our knowledge about transport proteins is1 clathrin coats, and endocytotic vesicles that transport patch clamping. A micropipette is placed on the mem-to endosomes have AP-2 clathrin coats. Vesicles that brane of a cell and forms a tight seal to the membrane.transport between the endoplasmic reticulum and the The patch of membrane under the pipette tip usually Extracellular fluid Intracellular fluid 200 Plasma Interstitial fluid Misc. 150 phosphates Cell membrane K+ meq/L H2O Capillaries Na+ Cl− 100 Cl− Na+ Na+ 50 HCO3 − Pr − K+ Pr − K+ HCO3 − HCO3 − 0 Cl−Figure 1–27. Electrolyte composition of human body fluids. Note that the values are in meq/L of water, not ofbody fluid. (Reproduced, with permission, from Johnson LR [editor]: Essential Medical Physiology. Raven Press, 1992.)
  • 44. THE GENERAL & CELLULAR BASIS OF MEDICAL PHYSIOLOGY / 31contains only a few transport proteins, and they can be at which they transport ions can be varied; that is, theystudied in detail (Figure 1–28). The cell can be left in- are gated. Some are gated by alterations in membranetact (cell-attached patch clamp). Alternatively, the potential (voltage-gated), whereas others are opened orpatch can be pulled loose from the cell, forming an in- closed when they bind a ligand (ligand-gated). The lig-side-out patch. A third alternative is to suck out the and is often external (eg, a neurotransmitter or a hor-patch with the micropipette still attached to the rest of mone). However, it can also be internal; intracellularthe cell membrane, providing direct access to the inte- Ca2+, cAMP, lipids, or one of the G proteins producedrior of the cell (whole cell recording). in cells (see below) can bind directly to channels and Small, nonpolar molecules (including O2 and N2) activate them. Some channels are also opened by me-and small uncharged polar molecules such as CO2 dif- chanical stretch, and these mechanosensitive channelsfuse across the lipid membranes of cells. However, the play an important role in cell movement. A typical volt-membranes have very limited permeability to other age-gated channel is the Na+ channel, and a typical lig-substances. Instead, they cross the membranes by endo- and-gated channel is the acetylcholine receptor (seecytosis and exocytosis and by passage through highly Chapter 4).specific transport proteins, transmembrane proteins Other transport proteins are carriers that bind ionsthat form channels for ions or transport substances such and other molecules and then change their configura-as glucose, urea, and amino acids. The limited perme- tion, moving the bound molecule from one side of theability applies even to water, with simple diffusion cell membrane to the other. Molecules move from areasbeing supplemented throughout the body with various of high concentration to areas of low concentrationwater channels (aquaporins; see Chapters 14 and 38). (down their chemical gradient), and cations move toFor reference, the sizes of ions and other biologically negatively charged areas whereas anions move to posi-important substances are summarized in Table 1–5. tively charged areas (down their electrical gradient). Some transport proteins are simple aqueous ion When carrier proteins move substances in the directionchannels, though many of these have special features of their chemical or electrical gradients, no energy inputthat make them effective for a given substance such as is required and the process is called facilitated diffu-Ca2+ or, in the case of aquaporins, for water. Many sion. A typical example is glucose transport by the glu-transport proteins are continuously open, but the rate cose transporter, which moves glucose down its concen- A Cell-attached patch B Inside-out patch C Whole-cell patch Electrode Pipette Cell membrane Closed Closed pA pA ms ms Open OpenFigure 1–28. Types of patch clamps used to study activity of ion channels across a cell membrane. In A and B, thechanges in membrane current with time are also shown. (Modified from Ackerman MJ, Clapham DE: Ion channels: Basicscience and clinical disease. N Engl J Med 1997;336:1575.)
  • 45. 32 / CHAPTER 1Table 1–5. Size of hydrated ions and other Ion Channelssubstances of biologic interest. There are ion channels for K+, Na+, Ca2+, and Cl–, and each exists in multiple forms with diverse properties. Atomic or Most are made up of identical or very similar subunits. Molecular Radius Figure 1–29 show the multiunit structure of various Substance Weight (nm) channels in diagrammatic cross-section. Cl− 35 0.12 Most K+ channels are tetramers, with each of the four subunits forming part of the pore through which + K 39 0.12 K+ ions pass. Structural analysis of a bacterial voltage- H2O 18 0.12 gated K+ channel indicates that each of the four sub- units have a paddle-like extension containing four 2+ Ca 40 0.15 charges. When the channel is closed, these extensions Na + 23 0.18 are near the negatively charged interior of the cell (Fig- ure 1–30). When the membrane potential is reduced, Urea 60 0.23 the paddles containing the charges bend through the + membrane to its exterior surface, causing the channel to Li 7 0.24 open. The bacterial K+ channel is very similar to the Glucose 180 0.38 voltage-gated K+ channels in a wide variety of species, including mammals. In the acetylcholine ion channel Sucrose 342 0.48 and other ligand-gated cation or anion channels, five Inulin 5000 0.75 subunits make up the pore. Members of the CLC fam- ily of Cl– channels are dimers, but they have two pores, Albumin 69,000 7.50 one in each subunit. Finally, aquaporins are tetramersData from Moore EW: Physiology of Intestinal Water and with a water pore in each of the subunits. Recently, aElectrolyte Absorption. American Gastroenterological Asso- number of ion channels with intrinsic enzyme activityciation, 1976. have been cloned. More than 30 different voltage-gated or cyclic nu- cleotide-gated Na+ and Ca2+ channels of this type havetration gradient from the ECF to the cytoplasm of the been described. A Ca2+ channel and a Na+ channel arecell (see Chapter 19). Other carriers transport sub- shown in extended diagrammatic form in Figure 1–31.stances against their electrical and chemical gradients. The toxins tetrodotoxin (TTX) and saxitoxin (STX)This form of transport requires energy and is called ac- bind to the Na+ channels and block them. The numbertive transport. In animal cells, the energy is providedalmost exclusively by hydrolysis of ATP. Not surpris-ingly, therefore, the carrier molecules are ATPases, en-zymes that catalyze the hydrolysis of ATP. One of theseATPases is sodium–potassium-activated adenosine A B C Dtriphosphatase (Na+–K+ ATPase), which is also knownas the Na+–K+ pump. There are also H+–K+ ATPases inthe gastric mucosa (see Chapter 26) and the renaltubules (see Chapter 38). Ca2+ ATPase pumps Ca2+ outof cells. Proton ATPases acidify many intracellular or-ganelles, including parts of the Golgi complex and lyso-somes. Figure 1–29. Different ways in which ion channels Some of the transport proteins are called uniports, form pores. Many K+ channels are tetramers (A), withbecause they transport only one substance. Others arecalled symports, because transport requires the binding each protein subunit forming part of the channel. In lig-of more than one substance to the transport protein and-gated cation and anion channels (B) such as theand the substances are transported across the mem- acetylcholine receptor, five identical or very similar sub-brane together. An example is the symport in the in- units form the channel. Cl– channels from the CLC fam-testinal mucosa that is responsible for the cotransport ily are dimers (C), with an intracellular pore in each sub-by facilitated diffusion of Na+ and glucose from the in- unit. Aquaporin water channels (D) are tetramers withtestinal lumen into mucosal cells (see Chapter 25). an intracellular channel in each subunit. (Reproduced,Other transporters are called antiports because they ex- with permission, from Jentsch TJ: Chloride channels arechange one substance for another. different. Nature 2002;415:276.)
  • 46. THE GENERAL & CELLULAR BASIS OF MEDICAL PHYSIOLOGY / 33 Closed amples include the γ-aminobutyric acid A (GABAA) ECF and glycine receptors in the central nervous system + + + + + + (CNS) (see Chapter 4). The CFTR receptor that is mu- tated in cystic fibrosis is also a Cl– channel. Ion channel + + + + mutations cause a variety of channelopathies—diseases that mostly affect muscle and brain tissue and produce – – – – – – episodic paralyses or convulsions. Cytoplasm Na+–K+ ATPase – – – – – – As noted above, Na+–K+ ATPase catalyzes the hydroly- + + sis of ATP to adenosine diphosphate (ADP) and uses + + the energy to extrude three Na+ from the cell and take two K+ into the cell for each molecule of ATP hy- + + + + + + drolyzed. It is an electrogenic pump in that it moves three positive charges out of the cell for each two that it Open moves in, and it is therefore said to have a coupling ratio of 3:2. It is found in all parts of the body. Its ac-Figure 1–30. Opening of a voltage-sensitive K+ chan- tivity is inhibited by ouabain and related digitalis glyco-nel in a bacterium. Positively charged “flaps” move sides used in the treatment of heart failure. It is a het-through the membrane to the extracellular surface erodimer made up of an α subunit with a molecularwhen the membrane potential is reduced. A similar weight of approximately 100,000 and a β subunit withmechanism may well operate in eukaryotes, including a molecular weight of approximately 55,000. Both ex-mammals. (Modified and reproduced, with permission, tend through the cell membrane (Figure 1–32). Separa-from Jiang Y et al: The principle of a gating charge move- tion of the subunits eliminates activity. However, the βment in a voltage-dependent K+ channel. Nature subunit is a glycoprotein, whereas Na+ and K+ transport2003;43:42.) occur through the α subunit. The β subunit has a sin- gle membrane-spanning domain and three extracellular glycosylation sites, all of which appear to have attachedand distribution of the Na+ channels can be determined carbohydrate residues. These residues account for oneby tagging them with labeled TTX or STX and analyz- third of its molecular weight. The α subunit probablying the distribution of the label. spans the cell membrane 10 times, with the amino and Another family of Na+ channels with a different carboxyl terminals both located intracellularly. Thisstructure has been found in the apical membranes of subunit has intracellular Na+- and ATP-binding sitesepithelial cells in the kidneys, colon, lungs, and brain. and a phosphorylation site; it also has extracellularThose epithelial sodium channels (ENaCs) are made binding sites for K+ and ouabain. The endogenous lig-up of three subunits encoded by three different genes. and of the ouabain-binding sight is unsettled, but itEach of the subunits probably spans the membrane may be endogenously produced ouabain (see Chaptertwice, and the amino terminal and carboxyl terminal 24).When Na+ binds to the α subunit, ATP also bindsare located inside the cell. The α subunit transports and is converted to ADP, with a phosphate being trans-Na+, whereas the β and γ subunits do not. However, ferred to Asp 376, the phosphorylation site. This causesthe addition of the β and γ subunits increases Na+ a change in the configuration of the protein, extrudingtransport through the α subunit. ENaCs are inhibited Na+ into the ECF. K+ then binds extracellularly, de-by the diuretic amiloride, which binds to the α subunit, phosphorylating the α subunit, which returns to itsand they used to be called amiloride-inhibitable Na+ previous conformation, releasing K+ into the cytoplasm.channels. The ENaCs in the kidney play an important The α and β subunits are heterogeneous, with α1,role in the regulation of ECF volume by aldosterone α2, and α3 subunits and β1, β2, and β3 subunits de-(see Chapter 38). ENaC knockout mice are born alive scribed so far. The α1 isoform is found in the mem-but promptly die because they cannot pump Na+ and branes of most cells, whereas α2 is present in muscle,hence water out of their lungs. heart, adipose tissue, and brain, and α3 is present in Humans have several types of Cl– channels. The heart and brain. The β1 subunit is widely distributedCLC dimeric channels (Figure 1–29) are found in but is absent in certain astrocytes, vestibular cells of theplants, bacteria, and animals, and there are nine differ- inner ear, and glycolytic fast-twitch muscles. The fast-ent CLC genes in humans. Other Cl– channels have the twitch muscles contain only β2 subunits. The differentsame pentameric form as the acetylcholine receptor; ex- α and β subunit structures of Na+–K+ ATPase in vari-
  • 47. 34 / CHAPTER 1 Rat brain Na+ channel I II III IV Outside 1 2 3 4 5 6 1 2 3 4 5 6 1 2 3 4 5 6 1 2 3 4 5 6 Inside C N Rabbit skeletal muscle Ca2+ channel I II III IV 1 2 3 4 5 6 1 2 3 4 5 6 1 2 3 4 5 6 1 2 3 4 5 6Figure 1–31. Diagrammatic representation of the structure of the principal subunits of two ion channels. SomeNa+ and Ca2+ channels have additional subunits. The Arabic numbers identify the α-helical domains that cross thecell membrane. H5 domain not shown. (After Catterall WK. Modified and reproduced from Hall ZW: An Introduction toMolecular Neurobiology. Sinauer, 1992.)ous tissues probably represents specialization for spe- ond messengers produced in cells, including cAMP andcific tissue functions. diacylglycerol (DAG; see below); the magnitude and di- rection of the observed effects vary with the experimen-Regulation of Na+–K+ ATPase Activity tal conditions. Thyroid hormones increase pump activ- ity by a genomic action to increase the formation ofThe amount of Na+ normally found in cells is not Na+–K+ ATPase molecules. Aldosterone also increasesenough to saturate the pump, so if the Na+ increases, the number of pumps, although this effect is probablymore is pumped out. Pump activity is affected by sec- secondary (see Chapters 20 and 38). Dopamine in the
  • 48. THE GENERAL & CELLULAR BASIS OF MEDICAL PHYSIOLOGY / 35 Active transport 2K+ 2K+ Ouabain Ouabain β ECF 3 3Na+ 3Na+ Na+ 2 ATP ADP + Pi Cl− Na+ Ca2+ Na+ Cotransport Countertransport Cytoplasm Na+ 15 meq/L K+, 2Cl− 1 α K+ 150 − Na+ 5 H+ Cl− 7 − 4 Sugars K+ K or amino 3Na+ acids Cl− H+ − − − −Figure 1–32. Na+–K+ ATPase. The intracellular portion Vm = −70 mVof the α subunit has a Na+-binding site (1), a phospho- + + + +rylation site (4), and an ATP-binding site (5). The extra- Na+cellular portion has a K+-binding site (2) and a ouabain- Na+ 140 meq/Lbinding site (3). (From Horisberger J-D et al: K+ 4 − Cl− 105 −Structure–function relationship of Na–K-ATPase. Annu RevPhysiol 1991;53:565. Reproduced, with permission, from Figure 1–33. Composite diagram of main secondarythe Annual Review of Physiology, vol. 53. Copyright © 1991 effects of active transport of Na+ and K+. Na+–K+ ATPaseby Annual Reviews Inc.) converts the chemical energy of ATP hydrolysis into maintenance of an inward gradient for Na+ and an out- ward gradient for K+. The energy of the gradients is used for countertransport, cotransport, and mainte-kidney inhibits the pump by phosphorylating it, caus- nance of the membrane potential. (Reproduced, withing a natriuresis. Insulin increases pump activity, prob- permission, from Skou JC: The Na–K pump. News Physiolably by a variety of different mechanisms. Sci 1992;7:95.)Secondary Active Transport Transport Across EpitheliaIn many situations, the active transport of Na+ is cou-pled to the transport of other substances (secondary In the gastrointestinal tract, the pulmonary airways, theactive transport). For example, the luminal mem- renal tubules, and other structures, substances enter onebranes of mucosal cells in the small intestine contain a side of a cell and exit another, producing movement ofsymport that transports glucose into the cell only if Na+ the substance from one side of the epithelium to thebinds to the protein and is transported into the cell at other. For transepithelial transport to occur, the cellsthe same time. From the cells, the glucose enters the need to be bound by tight junctions and, obviously,blood. The electrochemical gradient for Na+ is main- have different ion channels and transport proteins intained by the active transport of Na+ out of the mucosal different parts of their membranes. Most of the in-cell into ECF (see Chapter 25). Other examples are stances of secondary active transport cited in the pre-shown in Figure 1–33. In the heart, Na+–K+ ATPase ceding paragraph involve transepithelial movement ofindirectly affects Ca2+ transport. An antiport in the ions and other molecules.membranes of cardiac muscle cells normally exchangesintracellular Ca2+ for extracellular Na+. The role of this THE CAPILLARY WALLantiport in the production of the positively inotropic Filtrationeffect of ouabain and digitalis is discussed in Chapter 3. Active transport of Na+ and K+ is one of the major The capillary wall separating plasma from interstitialenergy-using processes in the body. On the average, it fluid is different from the cell membranes separating in-accounts for about 24% of the energy utilized by cells, terstitial fluid from intracellular fluid because the pres-and in neurons it accounts for 70%. Thus, it accounts sure difference across it makes filtration a significantfor a large part of the basal metabolism. factor in producing movement of water and solute. By
  • 49. 36 / CHAPTER 1definition, filtration is the process by which fluid is ocytosis on the interstitial side of the cells. The trans-forced through a membrane or other barrier because of port mechanism makes use of coated vesicles that ap-a difference in pressure on the two sides. pear to be coated with caveolin and is called transcyto- sis, vesicular transport, or cytopempsis.Oncotic PressureThe structure of the capillary wall varies from one vas- INTERCELLULAR COMMUNICATIONcular bed to another (see Chapter 30). However, inskeletal muscle and many other organs, water and rela- Cells communicate with one another via chemical mes-tively small solutes are the only substances that cross the sengers. Within a given tissue, some messengers movewall with ease. The apertures in the junctions between from cell to cell via gap junctions (see above) withoutthe endothelial cells are too small to permit plasma pro- entering the ECF. In addition, cells are affected byteins and other colloids to pass through in significant chemical messengers secreted into the ECF. Thesequantities. The colloids have a high molecular weight chemical messengers bind to protein receptors on thebut are present in large amounts. Small amounts cross surface of the cell or, in some instances, in the cyto-the capillary wall by vesicular transport (see below), but plasm or the nucleus, triggering sequences of intracellu-their effect is slight. Therefore, the capillary wall be- lar changes that produce their physiologic effects. Threehaves like a membrane impermeable to colloids, and general types of intercellular communication are medi-these exert an osmotic pressure of about 25 mm Hg. ated by messengers in the ECF: (1) neural communi-The colloid osmotic pressure due to the plasma colloids cation, in which neurotransmitters are released atis called the oncotic pressure. Filtration across the cap- synaptic junctions from nerve cells and act across a nar-illary membrane as a result of the hydrostatic pressure row synaptic cleft on a postsynaptic cell (see Chapterhead in the vascular system is opposed by the oncotic 4); (2) endocrine communication, in which hormonespressure. The way the balance between the hydrostatic and growth factors reach cells via the circulating bloodand oncotic pressures controls exchanges across the cap- (see Chapters 18–24); and (3) paracrine communica-illary wall is considered in detail in Chapter 30. tion, in which the products of cells diffuse in the ECF to affect neighboring cells that may be some distanceTranscytosis away (Figure 1–34). In addition, cells secrete chemical messengers that in some situations bind to receptors onVesicles are present in the cytoplasm of endothelial the same cell, that is, the cell that secreted the messen-cells, and tagged protein molecules injected into the ger (autocrine communication). The chemical mes-bloodstream have been found in the vesicles and in the sengers include amines, amino acids, steroids, polypep-interstitium. This indicates that small amounts of pro- tides, and in some instances lipids, purine nucleotides,tein are transported out of capillaries across endothelial and pyrimidine nucleotides. It is worth noting that incells by endocytosis on the capillary side followed by ex- various parts of the body, the same chemical messenger PARACRINE AND GAP JUNCTIONS SYNAPTIC AUTOCRINE ENDOCRINE A P Message transmission Directly from cell Across synaptic By diffusion in By circulating to cell cleft interstitial fluid body fluids Local or general Local Local Locally diffuse General Specificity depends on Anatomic location Anatomic location Receptors Receptors and receptorsFigure 1–34. Intercellular communication by chemical mediators. A, autocrine; P, paracrine.
  • 50. THE GENERAL & CELLULAR BASIS OF MEDICAL PHYSIOLOGY / 37can function as a neurotransmitter, a paracrine media- tors in the membrane. Some receptors are recycled aftertor, a hormone secreted by neurons into the blood internalization, whereas others are replaced by de novo(neural hormone), and a hormone secreted by gland synthesis in the cell. Another type of down-regulation iscells into the blood. desensitization, in which receptors are chemically mod- An additional form of intercellular communication ified in ways that make them less responsive (see Chap-is called juxtacrine communication. Some cells ex- ter 4).press multiple repeats of growth factors such as trans-forming growth factor alpha (TGF␣) extracellularlyon transmembrane proteins that provide an anchor to Mechanisms by Which Chemicalthe cell. Other cells have TGFα receptors. Conse- Messengers Actquently, TGFα anchored to a cell can bind to a TGFαreceptor on another cell, linking the two. This could be The principal mechanisms by which chemical messen-important in producing local foci of growth in tissues. gers exert their intracellular effects are summarized in Table 1–6. Ligands such as acetylcholine bind directly to ion channels in the cell membrane, changing their conductance. Thyroid and steroid hormones, 1,25-di-Radioimmunoassay hydroxycholecalciferol, and retinoids enter cells and actAntibodies to the polypeptides and proteins are readily on one or another member of a family of structurallyproduced, and, by using special techniques, it is possi- related cytoplasmic or nuclear receptors. The activatedble to make antibodies to the other chemical messen- receptor binds to DNA and increases transcription ofgers as well. The antibodies can be used to measure the selected mRNAs. Many other ligands in the ECF bindmessengers in body fluids and in tissue extracts by ra- to receptors on the surface of cells, and many of themdioimmunoassay. This technique depends on the fact trigger the release of intracellular mediators such asthat the naturally occurring, unlabeled ligand and cAMP, IP3, and DAG (see below) that initiate changesadded radioactive ligand compete to bind to an anti- in cell function. Consequently, the extracellular ligandsbody to the ligand. The greater the amount of unla- are called “first messengers” and the intracellular me-beled ligand in the specimen being analyzed, the more diators are called “second messengers.”it competes and the smaller the amount of radioactive Second messengers bring about many short-termligand that binds to the antibody. Radioimmunoassays changes in cell function by altering enzyme function,are extensively used in research and in clinical medi- triggering exocytosis, and so on, but they also alter tran-cine. scription of various genes. They do this in part by acti- vating transcription factors already present in the cell, and these activated factors induce the transcription ofReceptors for Hormones, immediate-early genes (Figure 1–20). The transcription factors that are the products of the immediate-earlyNeurotransmitters, & Other Ligands genes then activate other genes which produce moreMany of the receptors for chemical messengers have long-term effects.now been isolated and characterized. These proteins are When activated, many of the membrane receptorsnot static components of the cell, but their numbers in- initiate release of second messengers or other intracellu-crease and decrease in response to various stimuli, and lar events via GTP-binding proteins (G proteins; seetheir properties change with changes in physiologic below). The second messengers generally activate pro-conditions. When a hormone or neurotransmitter is tein kinases, enzymes that catalyze the phosphoryla-present in excess, the number of active receptors gener- tion of tyrosine or serine and threonine residues in pro-ally decreases (down-regulation), whereas in the pres- teins. More than 300 protein kinases have beenence of a deficiency of the chemical messenger, there is described. Some of the principal ones that are impor-an increase in the number of active receptors (up-regu- tant in mammals are summarized in Table 1–7. Addi-lation). Angiotensin II in its actions on the adrenal cor- tion of phosphate groups changes the configuration oftex is an exception; it increases rather than decreases the the proteins, altering their functions and consequentlynumber of its receptors in the adrenal. In the case of re- the functions of the cell. In some instances (eg, the in-ceptors in the membrane, receptor-mediated endocyto- sulin receptor) the intracellular portions of the recep-sis is responsible for down-regulation in some instances; tors themselves are protein kinases, and in others, theyligands bind to their receptors, and the ligand–receptor phosphorylate themselves (autophosphorylation). Ob-complexes move laterally in the membrane to coated viously, phosphatases are also important, since re-pits, where they are taken into the cell by endocytosis moval of a phosphate group inactivates some transport(internalization). This decreases the number of recep- proteins or enzymes whereas it activates others.
  • 51. 38 / CHAPTER 1Table 1–6. Principal mechanisms by which chemical messengers in the ECFa bring about changesin cell function. Mechanism Examples Open or close ion channels in cell membrane Acetylcholine on nicotinic cholinergic receptor; norepi- nephrine on K+ channel in the heart Act via cytoplasmic or nuclear receptors to increase Thyroid hormones, retinoic acid, steroid hormones transcription of selected mRNAs Activate phospholipase C with intracellular production of DAG, Angiotensin II, norepinephrine via α1-adrenergic IP3, and other inositol phosphates receptor, vasopressin via V1 receptor Activate or inhibit adenylyl cyclase, causing increased or Norepinephrine via β1-adrenergic receptor (increased decreased intracellular production of cAMP cAMP); norepinephrine via α2-adrenergic receptor (decreased cAMP) Increase cGMP in cell ANP; NO (EDRF) Increase tyrosine kinase activity of cytoplasmic portions of Insulin, EGF, PDGF, M-CSF transmembrane receptors Increase serine or threonine kinase activity TGFβ, MAPKsaFor abbreviations, see Appendix. Stimulation of Transcription When thyroid and steroid hormones, 1,25-dihydroxyc- holecalciferol, and retinoids bind to their receptors in-Table 1–7. Principal protein kinases. side cells, the conformation of the receptor protein is changed and a DNA-binding domain is exposed Phosphorylate serine and/or threonine residues (Figure 1–35). The receptor–hormone complex moves Calmodulin-dependent to DNA, where it binds to enhancer elements in the untranslated 5′-flanking portions of certain genes. The Myosin light-chain kinase Phosphorylase kinase Ca2+/calmodulin kinase I Cytoplasm Ca2+/calmodulin kinase II Nucleus Ca2+/calmodulin kinase III Binding to enhancer- Gene Calcium-phospholipid-dependent like element in DNA R Pre-mRNA H Protein kinase C (seven subspecies) Transformation of mRNA Cyclic nucleotide-dependent receptor to expose DNA-binding domain cAMPa-dependent kinase (protein kinase A; two mRNA subspecies) Binding to receptor Protein located in nucleus cGMP-dependent kinase or in cytoplasm Response Phosphorylate tyrosine residues H Insulin receptor, EGF receptor, PDGF receptor, and M-CSF receptor have tyrosine kinase activity Figure 1–35. Mechanism of action of steroid and thy-aFor abbreviations, see Appendix. roid hormones. H, hormone; R, receptor.
  • 52. THE GENERAL & CELLULAR BASIS OF MEDICAL PHYSIOLOGY / 39estrogen and the triiodothyronine (T3) receptors bind and β) have been identified; the α estrogen receptorhormones in the nucleus. The T3 receptors also bind and the β T3 receptor are shown in the figure. All thesethyroxine (T4), but with less affinity. The glucocorti- receptors are part of a superfamily of receptors thatcoid receptor is located mainly in the cytoplasm but have in common a highly conserved cysteine-richmigrates promptly to the nucleus as soon as it binds its DNA-binding domain; a ligand-binding domain at orligand. The initial location of the other receptors that near the carboxyl terminal of the receptor; and a rela-act in this fashion is unsettled. In any case, binding of tively variable, poorly conserved amino terminal region.the receptor–hormone complex to DNA increases the When a ligand binds to one of them, it becomes a tran-transcription of mRNAs encoded by the gene to which scription factor and binds to DNA via zinc fingers.it binds. The mRNAs are translated in the ribosomes, Other receptors in the family include the receptors forwith the production of increased quantities of proteins progesterone, androgen, and 1,25-dihydroxycholecal-that alter cell function. ciferol. Many other factors that regulate genes act via At least for the glucocorticoid, estrogen, and proges- receptors of this type in species ranging from fruit fliesterone receptors, the receptor is bound to the heat shock to humans, and over 70 members of this receptor su-protein Hsp90 and other proteins in the absence of the perfamily have been described. Ligands are now knownsteroid, and it appears that the heat shock protein covers for about half of these, but the remaining half are or-the DNA-binding domain. When the steroid binds to phan receptors, for which the ligands are still unidenti-the receptor, the conformation change releases the heat fied. Retinoic acid, which is a derivative of retinol (vita-shock protein, exposing the DNA-binding domain. min A), has an extensive role in fetal development, and Heat shock proteins are a group of intracellular pro- there are three retinoic acid receptors (α, β, and γ) en-teins whose amounts increase when cells are exposed to coded by two families of retinoic acid receptors, RARheat and other stresses, and they help the cells survive a and RXR. T3 receptors form homodimers before bind-variety of stresses. Consequently, it is probably more ing to DNA, but heterodimers with retinoic receptorsappropriate to call them stress proteins. also form and bind, and their actions are complex (see Chapter 18).Structure of Receptors Rapid Actions of SteroidsThe structures of the human glucocorticoid and miner-alocorticoid receptors are shown in Figure 1–36. Two Some of the actions of steroids are much more rapidestrogen receptors (α and β) and two T3 receptors (α than those known to be mediated via binding to DNA. Examples include the rapid increase in the Ca2+ concen- tration in sperm heads that is produced by progesterone and prompt steroid-induced alteration in the functions DNA Ligand of various neurons. This has led to the hypothesis that N Cys Cortisol C there are nongenomic actions of steroids which are 1 421 486 528 777 mediated by putative membrane receptors and second messengers inside the cells. Molecular biologic evidence 1 602 670 734 984 points to the existence of these receptors, though de-N Cys Aldosterone C tailed information about them is still lacking. Steroids also bind to GABAA receptors, facilitating their action (see Chapter 4). 1 185 250 595 N Cys α Estrogen C Intracellular Ca2+ Ca2+ regulates a very large number of physiologic 1 102 169 456 processes that are as diverse as proliferation, neural sig- N Cys β T3 C naling, learning, contraction, secretion, and fertiliza- tion, so regulation of intracellular Ca2+ is of great im- portance. The free Ca2+ concentration in the cytoplasmFigure 1–36. Structure of human glucocorticoid, min- at rest is maintained at about 100 nmol/L. Theeralocorticoid, α-estrogen, and β-T3 receptors. Note Ca2+ concentration in the interstitial fluid is aboutthat each receptor has a cysteine-rich DNA-binding do- 12,000 times the cytoplasmic concentration (ie,main and a ligand-binding domain at or near the car- 1,200,000 nmol/L), so there is a marked inwardly di-boxyl terminal, with considerable variability in the rected concentration gradient as well as an inwardly di-amino terminal part of the protein. The numbers iden- rected electrical gradient. Much of the intracellulartify amino acid residues. Ca2+ is bound by the endoplasmic reticulum and other
  • 53. 40 / CHAPTER 1 intracellular Ca2+ supply and refills the endoplasmic reticulum. The exact identity of the SOCCs is still un- Ca2+ CaBP Effects known, and there is debate about the signal from the (volt) 2H+ endoplasmic reticulum that opens them. However, evi- ATP dence is accumulating that IP3 is responsible for both Ca2+ Ca2+ (lig) the internal release from the endoplasmic reticulum Ca2+ and the activation of the SOCCs. Ca2+ 3Na+(SOCC) Ca2+ Calcium-Binding Proteins Many different Ca2+-binding proteins have been de- scribed, including troponin, calmodulin, and cal- bindin. Troponin is the Ca2+-binding protein involved Mitochondrion Endoplasmic reticulum in contraction of skeletal muscle (see Chapter 3). Calmodulin contains 148 amino acid residues (FigureFigure 1–37. Ca2+ metabolism in mammalian cells. 1–38) and has four Ca2+-binding domains. It is unique in that amino acid residue 115 is trimethylated, and itCa2+ is stored in the endoplasmic reticulum and mito- is extensively conserved, being found in plants as well aschondria and can be released from them to replenish animals. When calmodulin binds Ca2+, it is capable ofcytoplasmic Ca2+. Calcium-binding proteins (CaBP) bind activating five different calmodulin-dependent kinasescytoplasmic Ca2+ and, when activated in this fashion, (Table 1–7). One of these is myosin light-chain ki-bring about a variety of physiologic effects. Ca2+ enters nase, which phosphorylates myosin. This brings aboutthe cells via voltage-gated (volt) and ligand-gated (lig) contraction in smooth muscle. Another is phosphory-Ca2+ channels and SOCCs. It is transported out of thecell by a Ca2+–H+ ATPase and an Na+– Ca2+ antiport. 70 90 T M M R V F L A F R A D E K E D Korganelles (Figure 1–37), and these organelles provide a F D M R Istore from which Ca2+ can be mobilized via ligand- Ca I K E N Ca E L D Rgated channels to increase the concentration of free V D A G T T E E Y A H ICa2+ in the cytoplasm. Increased cytoplasmic Ca2+ E G 80 S 100 M V N 60binds to and activates calcium-binding proteins, and I (Me)3 N G L T 110 M D Q L E A Ethese in turn activate a number of protein kinases. T E Ca2+ enters cells through many different Ca2+ chan- 40 N P L T 120nels. Some of these are ligand-gated and others are volt- S L G Q D E E V Dage-gated. Stretch-activated channels appear to exist as M R Ewell. The voltage-gated Ca2+ channels are often divided V T I G D G M T T T E Iinto T (transient) or L (long-lasting) types depending G K COOH V D 130 1 R Kon whether they do or do not inactivate during main- L E Ca N F E A N Ca A Etained depolarization. D E K 10 A T E 140 M Ca2+ is pumped out of cells in exchange for two H+ 20 E K A I F L S F Q Q Vby a Ca2+–H+ ATPase, and it is transported out of cells Eby an antiport driven by the Na+ gradient that ex- Echanges three Na+ for each Ca2+. T L Many second messengers act by increasing the cyto- Qplasmic Ca2+ concentration. The increase is produced Dby releasing Ca2+ from intracellular stores—primarily A NH Acthe endoplasmic reticulum—or by increasing the entryof Ca2+ into cells, or by both mechanisms. IP3 is the Figure 1–38. Structure of calmodulin from bovinemajor second messenger that causes Ca2+ release from brain. Single-letter abbreviations are used for thethe endoplasmic reticulum. In many tissues, transient amino acid residues (see Table 17–2). Note the four cal-release of Ca2+ from internal stores into the cytoplasm cium domains (dark residues) flanked on either side bytriggers opening of a population of Ca2+ channels in stretches of α helix. (Reproduced, with permission, fromthe cell membrane (store-operated Ca2+ channels; Cheung WY: Calmodulin: An overview. Fed ProcSOCCs). The resulting Ca2+ influx replenishes the total 1982;41:2253.)
  • 54. THE GENERAL & CELLULAR BASIS OF MEDICAL PHYSIOLOGY / 41lase kinase, which activates phosphorylase (see Chapter catalytic units that catalyze the intracellular formation17). Ca2+/calmodulin kinases I and II are concerned of second messengers or couple the receptors directly towith synaptic function, and Ca2+/calmodulin kinase III ion channels. These G proteins are made up of threeis concerned with protein synthesis. Another calmod- subunits designated α, β, and γ (Figure 1–39). The αulin-activated protein is calcineurin, a phosphatase subunit is bound to GDP. When a ligand binds to a G-that inactivates Ca2+ channels by dephosphorylating coupled receptor, this GDP is exchanged for GTP andthem. It also plays a role in activating T cells and is in- the α subunit separates from the combined β and γhibited by some immunosuppressants (see Chapter 27). subunits. The separated α subunit brings about many biologic effects. The β and γ subunits do not separateMechanisms of Diversity of Ca2+ Actions from each other, and βγ also activates a variety of effec- tors. The intrinsic GTPase activity of the α subunitIt may seem difficult to understand how intracellular then converts GTP to GDP, and this leads to reassocia-Ca2+ can have so many varied effects as a second mes- tion of the α with the βγ subunit and termination of ef-senger. Part of the explanation is that Ca2+ may have fector activation.different effects at low and at high concentrations. The Heterotrimeric G proteins relay signals from overion may be in high concentration at the site of its re- 1000 receptors, and their effectors in the cells includelease from an organelle or a channel (Ca2+ sparks) and ion channels and enzymes. Examples are listed in Tableat a subsequent lower concentration after it diffuses 1–8. There are 16 α, 6 β, and 12 γ genes, so a largethroughout the cell. Some of the changes it produces number of subunits are produced, and they can com-can outlast the rise in intracellular Ca2+ concentration bine in various ways. They can be divided into fivebecause of the way it binds to some of the Ca2+-binding families, each with a relatively characteristic set of effec-proteins. In addition, once released, intracellular Ca2+ tors. The families are Gs, Gi, Gt, Gq, and G13.concentrations frequently oscillate at regular intervals, Many G proteins are modified by having specificand there is evidence that the frequency and, to a lesser lipids attached to them, ie, they are lipidated (Figureextent, the amplitude of those oscillations codes infor- 1–6). Trimeric G proteins may be myristolated, palmi-mation for effector mechanisms. Finally, increases in toylated, or prenylated. Small G proteins may be preny-intracellular Ca2+ concentration can spread from cell to lated.cell in waves, producing coordinated events such as therhythmic beating of cilia in epithelial tissue. NucleotideG Proteins exchangeA common way to translate a signal to a biologic effect Output Input GDP GTPinside cells is by way of nucleotide regulatory proteins(G proteins) that bind GTP. GTP is the guanosine GTPaseanalog of ATP (see Chapter 17). When the signal activityreaches a G protein, the protein exchanges GDP forGTP. The GTP–protein complex brings about the ef-fect. The inherent GTPase activity of the protein thenconverts GTP to GDP, restoring the resting state. TheGTPase activity is accelerated by a family of RGS (reg-ulators of G protein signaling) proteins that accelerate β γthe formation of GDP. GDP α GTP α β γ Small G proteins are involved in many cellularfunctions. Members of the Rab family of these proteinsregulate the rate of vesicle traffic between the endoplas- Effectorsmic reticulum, the Golgi apparatus, lysosomes, endo-somes, and the cell membrane. Another family of small Figure 1–39. Heterotrimeric G proteins. Top: Sum-GTP-binding proteins, the Rho/Rac family, mediates mary of overall reaction. Bottom: When the ligandinteractions between the cytoskeleton and cell mem- (square) binds to the serpentine receptor in the cellbrane, and a third family, the Ras family, regulates membrane, GTP replaces GDP on the α subunit. GTP-αgrowth by transmitting signals from the cell membrane separates from the βγ subunit and GTP-α and βγ bothto the nucleus. The members of these three families are activate various effectors, producing physiologic ef-related to the product of the ras proto-oncogene. fects. The intrinsic GTPase activity of GTP-α then con- Another family of G proteins, the larger het- verts GTP to GDP, and the α, β, and γ subunits reassoci-erotrimeric G proteins, couple cell surface receptors to ate.
  • 55. 42 / CHAPTER 1Table 1–8. Some of the ligands for receptors (inositol 1,4,5-triphosphate; IP3). When one of thesecoupled to heterotrimeric G proteins. ligands binds to its receptor, activation of the receptor produces activation of phospholipase C on the inner Class Ligand surface of the membrane via Gq. Phospholipase C (PLC) has at least eight isoforms, and the PLCβ1 and Neurotransmitters Epinephrine PLCβ2 forms are activated by G proteins. They catalyze Norepinephrine the hydrolysis of phosphatidylinositol 4,5-diphosphate Dopamine (PIP2) to form IP3 and diacylglycerol (DAG) (Figure 5-Hydroxytryptamine 1–41). Tyrosine kinase-linked receptors can also pro- Histamine duce IP3 and DAG by activating PLCγ1. The IP3 dif- Acetylcholine fuses to the endoplasmic reticulum, where it triggers Adenosine the release of Ca2+ into the cytoplasm (Figure 1–42). Opioids The IP3 receptor resembles the ryanodine receptor, Tachykinins Substance P which is the Ca2+ channel in the sarcoplasmic reticulum Neurokinin A of skeletal muscle (see Chapter 3), except that the IP3 Neuropeptide K receptor is half as large. DAG is also a second messen- ger; it stays in the cell membrane, where it activates one Other peptides Angiotensin II of the seven subspecies of protein kinase C (Table Arginine vasopressin 1–7). Examples of ligands that act via these second Oxytocin messengers are listed in Table 1–6. VIP, GRP, TRH, PTH Glycoprotein hormones TSH, FSH, LH, hCG Cyclic AMP Arachidonic acid derivatives Thromboxane A2 Another important second messenger is cyclic AMP (cAMP) (Figure 1–43). Some of the many ligands that Other Odorants act via this compound are listed in Table 1–6. Cyclic Tastants AMP is cyclic adenosine 3′,5′-monophosphate. It is Endothelins Platelet-activating factor formed from ATP by the action of the enzyme adeny- Cannabinoids lyl cyclase and converted to physiologically inactive 5′- Light AMP by the action of the enzyme phosphodiesterase. Cyclic AMP activates one of the cyclic nucleotide-de- pendent protein kinases (protein kinase A, PKA) that,Serpentine Receptors like protein kinase C, catalyzes the phosphorylation of proteins, changing their conformation and alteringAll the heterotrimeric G protein-coupled receptors that their activity. A typical example is the activation ofhave been characterized to date are proteins that span phosphorylase kinase in the liver by epinephrine viathe cell membrane seven times (serpentine receptors). cAMP and protein kinase A (see Figure 17–13). In ad-These receptors may be palmitoylated. A very large dition, the active catalytic subunit of PKA moves to thenumber have been cloned, and their functions are mul- nucleus and phosphorylates the cAMP-responsive ele-tiple and diverse. The structures of two of them are ment-binding protein (CREB). This transcriptionshown in Figure 1–40. In general, small ligands bind to factor then binds to DNA and alters transcription of athe amino acid residues in the membrane, whereas large number of genes.polypeptide and protein ligands bind to the extracellu- Cyclic AMP is metabolized by a phosphodiesterase.lar domains, which are bigger and better developed in This phosphodiesterase is inhibited by methylxanthinesthe receptors for polypeptides and proteins. It is gener- such as caffeine and theophylline; consequently, theseally amino acid residues in the third cytoplasmic loop, compounds augment hormonal and transmitter effectsthe loop nearest the carboxyl terminal, that interact mediated via cAMP.with the G proteins. Activation of Adenylyl CyclaseInositol Triphosphate & Diacylglycerol Five components are involved in the mechanism byas Second Messengers which ligands bring about changes in the intracellularThe link between membrane binding of a ligand that concentration of cAMP: a catalytic unit, adenylyl cy-acts via Ca2+ and the prompt increase in the cytoplas- clase, which catalyzes the conversion of ATP to cAMP;mic Ca2+ concentration is often inositol triphosphate stimulatory and inhibitory receptors; and stimulatory
  • 56. THE GENERAL & CELLULAR BASIS OF MEDICAL PHYSIOLOGY / 43 D P V H S G N T T L L F D S D N G P P G M NH2 R V V C T K N S F P V Y F N P G E T G N M NH2 H P S F D E V ␤2-Adrenergic receptor A Intradiskal C G I D Rhodopsin T P S Y E C Y H K Q surface C Y E Extracellular I D E T Y F G P T Q M T R C P D surface A C Y L V F G C G E H K D E E N F G A N Q F Y P A W N E L E M F H F G I G S W E T Q D K W T T N L I P H Y F V A D P W L G R N H G V M C R N Q K Q S F S N T P G M A L I N E F W Y W H Q A Y I V H E V Y F S M T Y L F A T W G V E S F F I Y I F M I L M S B A G F T S I D MQ I P A I A S V I N V I L L N L A A Y T T T F L G G E L P P A V I Y M F A V Q T I P A V I V P V V V L C L F S S I V I F F W L G M F L Q G F I A L A C A F V V A Y P F F A L A I V A L G M V T A S T L Q S S F Y V P L W C Y V N S L I M L V M F L W S L A L A M V H F I I L W C I K T S A F G N V L D I E T V I W P L V L T F A F N G F P D A V V L A W T F P L I L F A V Y N V L V I A C A L L C V I V M L I V M V F T G M I P L I Y I N F L A L N L I E R Y A V G M V I F F I V M I P V I Y T A I S T I A V D V M R V Y S I G L C R S T L Y L I Y V V V I A H C Y G I V M I M M A F R A R T P C R N K D F V N N Q K Q Y Y K R I A F L T F R N C K V L Q T K E L V E M N I N A K P K E A F F H E E V P G V V T A K K L C F K Q E R Q V F L Q H T M F L Q T V I T A K R Q L Q K K K L R S N F R T T K L T T L I V T S K T Cytoplasmic S P S L H F R G E S K D S C L Cytoplasmic surface E A A A Q Q Q E S A L F K Y Q C surface P S R R R C N L G Q V E Q D G R S G H G L R G S HOOC A P A V Q S T E T K S V T T S A E D D G L P N K S S K S A E G M Y D T K G N G N S S Y G N G Y A G C Q L G Q E K E S E R L C E D P P G T E S F V N C Q HOOC L P S D N T S C N R G Q S D L S L S P V T GFigure 1–40. Structure of the two serpentine receptors. The individual amino acid residues are identified by theirsingle-letter codes, and the dark-colored residues are sites of phosphorylation. The Y-shaped symbols identify glyco-sylation sites. Note the extracellular amino terminal, the intracellular carboxyl terminal, and, the seven membrane-spanning portions of each protein. (Reproduced, with permission, from Benovic JL et al: Light-dependent phosphoryla-tion of rhodopsin by β-adrenergic receptor kinase. Reprinted by permission from Nature 1986;321:869. Copyright © 1986by Macmillan Magazines Ltd.)and inhibitory G proteins that link the receptor to the with the intracellular concentration, and only smallcatalytic unit (Figure 1–44). Like the receptors, adeny- amounts of extracellular cAMP enter cells.lyl cyclase is a transmembrane protein, and it crosses Two bacterial toxins have important effects onthe membrane 12 times. Eight isoforms of this enzyme adenylyl cyclase that are mediated by G proteins. The Ahave been described, and, combined with the many dif- subunit of cholera toxin catalyzes the transfer of ADP-ferent forms of G proteins, this permits the cAMP ribose to an arginine residue in the middle of the α sub-pathway to be customized to specific tissue needs. unit of Gs. This inhibits its GTPase activity, producingWhen the appropriate ligand binds to a stimulatory re- prolonged stimulation of adenylyl cyclase (see Chapterceptor, a Gs α subunit activates one of the adenylyl cy- 25). Pertussis toxin catalyzes ADP-ribosylation of aclases. Conversely, when the appropriate ligand binds cysteine residue near the carboxyl terminal of the αto the inhibitory receptor, a Gi α subunit inhibits subunit of Gi. This inhibits the function of Gi. In addi-adenylyl cyclase. The receptors are specific, responding tion to the implications of these alterations in disease,at low threshold to only one or a select group of related both toxins are used for fundamental research on Gligands. However, heterotrimeric G proteins mediate protein function. The drug forskolin stimulates adeny-the stimulatory and inhibitory effects produced by lyl cyclase activity by a direct action on the enzyme.many different ligands. In addition, cross-talk occursbetween the phospholipase C system and the adenylyl Guanylyl Cyclasecyclase system, and several of the isoforms of adenylylcyclase are stimulated by calmodulin. Finally, the ef- Another cyclic nucleotide of physiologic importance isfects of protein kinase A and protein kinase C are very cyclic guanosine monophosphate (cyclic GMP;widespread. Given this complexity, how are specific re- cGMP). Cyclic GMP is important in vision in bothsponses to specific stimuli obtained? The answer lies in rods and cones. In addition, there are cGMP-regulatedpart in tethering of the G proteins, adenylyl cyclase, ion channels, and cGMP activates cGMP-dependentand the protein kinases to the cytoskeleton so that local kinase (Table 1–7), producing a number of physiologicmicrodomains are created. Some of this tethering is car- effects.ried out by lipid products (Figure 1–6). Guanylyl cyclases are a family of enzymes that cat- Some cAMP escapes from cells on stimulation by alyze the formation of cGMP. They exist in two formscertain hormones, but the amounts are small compared (Figure 1–45). One form has an extracellular amino
  • 57. 44 / CHAPTER 1 Phosphatidylinositol PIP PIP2 Diacylglycerol (PI) Phospholipase C P P P 1 1 1 P IP3 5 1 4 4 4 P P P 5 4 P P Inositol IP IP2 + CDP-diacylglycerol Phosphatidic acidFigure 1–41. Metabolism of phosphatidylinositol in cell membranes. Phosphatidylinositol is successively phos-phorylated to form phosphatidylinositol 4-phosphate (PIP), then phosphatidylinositol 4,5-diphosphate (PIP2).Phospholipase Cβ1 and β2 catalyze the breakdown of PIP2 to inositol 1,4,5-triphosphate (IP3) and diacylglycerol.Other inositol phosphates and phosphatidylinositol derivatives can also be formed. IP3 is dephosphorylated to inosi-tol, and diacylglycerol is metabolized to cytosine diphosphate (CDP)-diacylglycerol. CDP-diacylglycerol and inositolthen combine to form phosphatidylinositol, completing the cycle. (Modified from Berridge MJ: Inositol triphosphateand diacylglycerol as second messengers. Biochem J 1984;220:345.)terminal domain that is a receptor, a single transmem- and serine–threonine kinases. Two examples are shownbrane domain, and a cytoplasmic portion with tyrosine in Figure 1–45.kinase-like and guanylyl cyclase catalytic activity. Threesuch guanylyl cyclases have been characterized. Two are Growth Factorsreceptors for ANP (ANPR-A and ANPR-B; see Chap- Growth factors have become increasingly important inter 24), and a third binds an Escherichia coli enterotoxin many different aspects of physiology. They are polypep-and the gastrointestinal polypeptide guanylin (see tides and proteins that are conveniently divided intoChapter 26). The other form of guanylyl cyclase is solu- three groups. One group is made up of agents that fos-ble, contains heme, and is totally intracellular. There ter the multiplication or development of various typesappear to be several isoforms of the intracellular en- of cells; nerve growth factor (see Chapter 2), insulin-zyme. They are activated by nitric oxide (NO) and like growth factor I (IGF-I; see Chapter 22), activinsNO-containing compounds. NO has multiple func- and inhibins (see Chapter 23), and epidermal growthtions in many different parts of the body. factor (EGF) are examples. More than 20 have been de- scribed. The cytokines are a second group. These fac-Phosphatases tors are produced by macrophages and lymphocytesNumerous phosphatases that remove phosphate groups and are important in regulation of the immune systemfrom proteins are found in cells. Frequently these are (see Chapter 27). Again, more than 20 have been de-closely associated with or coupled to tyrosine kinases scribed. The third group is made up of the colony-stim-
  • 58. THE GENERAL & CELLULAR BASIS OF MEDICAL PHYSIOLOGY / 45 Stimulatory Stimulatory Adenylyl Inhibitory receptor receptor cyclase receptor ISF ISF PIP2 DAG β PLC PKC β β α γ α γ α γ GS Gi Gq, etc IP3 Phosphoproteins Tyrosine kinase Cytoplasm PDE Cytoplasm ATP CAMP 5 AMP CaBP Ca2+ Protein kinase A ER Physiologic Physiologic effects effects PhosphoproteinsFigure 1–42. Diagrammatic representation of releaseof inositol triphosphate (IP3) and diacylglycerol (DAG) Physiologic effectsas second messengers. Binding of ligand to G protein-coupled receptor activates phospholipase C (PLC) β1 or Figure 1–44. The cAMP system. Activation of adeny-β2. Alternatively, activation of receptors with intracellu- lyl cyclase catalyzes the conversion of ATP to cAMP.lar tyrosine kinase domains can activate PLCγ1. The re- Cyclic AMP activates protein kinase A, which phospho-sulting hydrolysis of phosphatidylinositol 4,5-diphos- rylates proteins, producing physiologic effects. Stimula-phate (PIP2) produces IP3, which releases Ca2+ from the tory ligands bind to stimulatory receptors and activateendoplasmic reticulum (ER), and DAG, which activates adenylyl cyclase via Gs. Inhibitory ligands inhibit adeny-protein kinase C (PKC). CaBP, Ca2+-binding proteins. ISF, lyl cyclase via inhibitory receptors and Gi. ISF, interstitialinterstitial fluid. fluid.ulating factors that regulate proliferation and matura- their ligands, and the intracellular tyrosine kinase do-tion of red and white blood cells. mains cross-phosphorylate each other. One of the Receptors for EGF, platelet-derived growth factor pathways activated by phosphorylation leads, through(PDGF), and many of the other factors that foster cell the product of the ras proto-oncogene and several mi-multiplication and growth have a single membrane- togen-activated protein (MAP) kinases, directly to thespanning domain with an intracellular tyrosine kinase production of transcription factors in the nucleus thatdomain (Figure 1–46). When ligand binds to the re- alter gene expression. This important direct path fromceptor, the tyrosine kinase domain autophosphorylates the cell surface to the nucleus is shown diagrammati-itself. Some of the receptors dimerize when they bind cally in Figure 1–46. Note that Ras is one of the small O O Adenine O H2 C O P O− Adenine O H2C O O Adenine O H2C O P OH O P O− C AC C PD C H H C H H C OH H H C H O P O− H H H C C H C C C C H O O O O O O O H H O P O− H H H Adenosine triphosphate cAMP 5-Adenosine monophosphate O− (ATP) (5-AMP)Figure 1–43. Formation and metabolism of cAMP. AC, adenylyl cyclase; PD, phosphodiesterase.
  • 59. 46 / CHAPTER 1 ANP NH2 Growth factor NH2 NH2 Receptor ST EGF PDGF Cell membraneISFM Inactive Ras Ras Active Ras RasC NH2 PTK PTK PTP NH2 GDP GTP NH2 T K Grb2 PTK SOS Raf PTP cyc PTP cyc cyc COOH COOH COOH COOH COOH MAP KK COOH COOH Guanylyl Tyrosine Tyrosine MAP K cyclases kinases phosphatasesFigure 1–45. Diagrammatic representation of guany- TFlyl cyclases, tyrosine kinases, and tyrosine phos-phatases. ANP, atrial natriuretic peptide; C, cytoplasm;cyc, guanylyl cyclase domain; EGF, epidermal growth Nucleusfactor; ISF, interstitial fluid; M, cell membrane; PDGF,platelet-derived growth factor; PTK, tyrosine kinase do-main; PTP, tyrosine phosphatase domain; ST, E coli en- Altered gene activityterotoxin. (Modified from Koesling D, Böhme E, Schultz G:Guanylyl cyclases, a growing family of signal transducingenzymes. FASEB J 1991;5:2785.) Figure 1–46. One of the direct pathways by which growth factors alter gene activity. TK, tyrosine kinase domain; Grb2, Ras activator controller; Sos, Ras activa-G proteins that requires binding to GTP for activa- tor; Ras, product of the ras gene; MAP K, mitogen-acti-tion. vated protein kinase; MAP KK, MAP kinase kinase; TF, Receptors for the cytokines and the colony-stimulat- transcription factors. There is cross talk between thising factors differ from the other growth factors in that pathway and the cAMP pathway, as well as cross talkmost of them do not have tyrosine kinase domains in with the IP3–DAG pathway.their cytoplasmic portions and some have little or nocytoplasmic tail. However, they initiate tyrosine kinaseactivity in the cytoplasm. In some instances, this in-volves binding to the associated transmembrane protein Another family of receptors binds transforminggp130 (see Chapter 27). In particular, they activate the growth factor β (TGFβ) and related polypeptides.so-called Janus tyrosine kinases (JAKs) in the cyto- These receptors have serine–threonine kinase activity,plasm (Figure 1–47). These in turn phosphorylate sig- and their effects are mediated by SMADs, intracellularnal transducer and activator of transcription (STAT) proteins that when phosphorylated move to the nu-proteins. The phosphorylated STATs form homo- and cleus, bind to DNA, and, with other factors, initiateheterodimers and move to the nucleus, where they act transcription of various genes.as transcription factors. There are four known mam- As noted above, integrins also initiate phosphoryla-malian JAKs and seven known STATs. The tion of proteins that enter the nucleus and alter geneJAK–STAT pathway is also activated by growth hor- transcription.mone (see Figure 22–4) and is another important direct Note that a common theme is activation of tran-path from the cell surface to the nucleus. However, it scription factors that without activation are “locked” inshould be emphasized that both the Ras and the the cytoplasm. Once activated, the transcription factorJAK–STAT pathways are complex and there is cross moves to the nucleus and alters gene transcription. Ad-talk between them and the phospholipase C and cAMP ditional examples include nuclear factors NF-AT (seepathways. Figure 27–13) and NF-κB (see Chapters 20 and 33).
  • 60. THE GENERAL & CELLULAR BASIS OF MEDICAL PHYSIOLOGY / 47 Ligand Finally, note that the whole subject of second mes- sengers and intracellular signaling has become im- A Receptor mensely complex, with multiple pathways and interac- ISF tions. It is only possible in a book such as this to list highlights and present general themes that will aid the JAK JAK Cytoplasm reader in understanding the rest of physiology. Receptor & G Protein Diseases STAT STAT Many diseases are being traced to mutations of the genes for receptors. For example, loss-of-function re- B ceptor mutations that cause disease have been reported Ligand for the 1,25-dihydroxycholecalciferol receptor (see Chapter 21) and the insulin receptor (see Chapter 19). Certain other diseases are caused by production of anti- JAK JAK bodies against receptors. Thus, antibodies against thy- P roid-stimulating hormone (TSH) receptors cause P P Graves’ disease (see Chapter 18), and antibodies against nicotinic acetylcholine receptors cause myasthenia STAT STAT gravis (see Chapter 4). An example of loss of function of a receptor is the type of nephrogenic diabetes insipidus that is due to C loss of the ability of mutated V2 vasopressin receptors to Ligand mediate concentration of the urine (see Chapters 14 and 38). Mutant receptors can gain as well as lose JAK JAK function. A gain-of-function mutation of the Ca2+ re- P P ceptor (see Chapter 21) causes excess inhibition of P P parathyroid hormone secretion and familial hypercal- STAT P STAT ciuric hypocalcemia. G proteins can also undergo loss- of-function or gain-of-function mutations that cause disease (Table 1–9). In one form of pseudohy- D poparathyroidism, a mutated Gs α fails to respond to Ligand parathyroid hormone, producing the symptoms of hy- poparathyroidism without any decline in circulating JAK JAK parathyroid hormone. Testotoxicosis is an interesting P P disease that combines gain and loss of function. In this condition, an activating mutation of Gs α causes excess P P testosterone secretion and prepubertal sexual matura- tion. However, this mutation is temperature-sensitive and is active only at the relatively low temperature of STAT the testes (33 °C; see Chapter 23). At 37 °C, the nor- P P mal temperature of the rest of the body, it is replaced Nucleus by loss of function, with the production of hy- poparathyroidism and decreased responsiveness to DNA TSH. A different activating mutation in Gs α is associ- ated with the rough-bordered areas of skin pigmenta-Figure 1–47. Signal transduction via the JAK–STAT tion and hypercortisolism in the McCune–Albrightpathway. A: Ligand binding leads to dimerization of re- syndrome. This mutation occurs during fetal develop-ceptor. B: Activation and tyrosine phosphorylation of ment, creating a mosaic of normal and abnormal cells.JAKs. C: JAKs phosphorylate STATs. D: STATs dimerize A third mutation in Gs α reduces its intrinsic GTPaseand move to nucleus, where they bind to response ele- activity. As a result, it is much more active than normal,ments on DNA. (Modified from Takeda K, Kishimoto T, and excess cAMP is produced. This causes hyperplasiaAkira S: STAT6: Its role in interleukin 4-mediated biological and eventually neoplasia in somatotrope cells of the an-functions. J Mol Med 1997;75:317.) terior pituitary. Forty percent of somatotrope tumors
  • 61. 48 / CHAPTER 1Table 1–9. Examples of abnormalities caused by loss- or gain-of-function mutations of heterotrimericG-protein-coupled receptors and G proteins. Site Type of Mutation DiseaseReceptor Cone opsins Loss Color blindness Rhodopsin Loss Congenital night blindness Two forms of retinitis pigmentosa V2 vasopressin Loss X-linked nephrogenic diabetes insipidus ACTH Loss Familial glucocorticoid deficiency LH Gain Familial male precocious puberty TSH Gain Familial nonautoimmune hyperthyroidism TSH Loss Familial hypothyroidism 2+ Ca Gain Familial hypercalciuric hypocalcemia Thromboxane A2 Loss Congenital bleeding Endothelin B Loss Hirschsprung diseaseG protein Gs α Loss Pseudohypothyroidism type 1a Gs α Gain/loss Testotoxicosis Gs α Gain (mosaic) McCune–Albright syndrome Gs α Gain Somatotroph adenomas with acromegaly Gi α Gain Ovarian and adrenocortical tumorsModified from Lem J: Diseases of G-protein-coupled signal transduction pathways: The mammalian visual system as amodel. Semin Neurosci 1998;9:232.causing acromegaly (see Chapter 22) have cells contain- amples, and a large part of physiology is concerned withing a somatic mutation of this type. regulatory mechanisms that act to maintain the con- stancy of the internal environment. Many of these reg- ulatory mechanisms operate on the principle of nega-HOMEOSTASIS tive feedback; deviations from a given normal set point are detected by a sensor, and signals from the sensorThe actual environment of the cells of the body is the trigger compensatory changes that continue until theinterstitial component of the ECF. Since normal cell set point is again reached.function depends on the constancy of this fluid, it isnot surprising that in multicellular animals, an im-mense number of regulatory mechanisms have evolved AGINGto maintain it. To describe “the various physiologic Aging is a general physiologic process that is as yetarrangements which serve to restore the normal state, poorly understood. In the United States, life expectancyonce it has been disturbed,” W.B. Cannon coined the has increased from 47 years in 1900 to about 75 yearsterm homeostasis. The buffering properties of the today. However, this increase is due for the most partbody fluids and the renal and respiratory adjustments to improved treatment and prevention of infectionsto the presence of excess acid or alkali are examples of and other causes of early death, so that more peoplehomeostatic mechanisms. There are countless other ex- survive into their 70s. In the meantime, the maximum
  • 62. THE GENERAL & CELLULAR BASIS OF MEDICAL PHYSIOLOGY / 49human life span of 100–110 years has increased little if 22) each has some salutary effects, but each also has un-at all. Aging affects cells and the systems made up of desirable side effects, and there is little if any evidencethem, as well as tissue components such as collagen, that they prolong life.and numerous theories have been advanced to explainthe phenomenon. One theory of aging holds that tissues age as a result REFERENCES FOR SECTION I:of random mutations in the DNA of somatic cells, with INTRODUCTIONconsequent introduction of cumulative abnormalities. Albert B et al: Molecular Biology of the Cell, 4th ed. Garland Sci-Others hold that cumulative abnormalities are pro- ence, 2002.duced by increased cross-linkage of collagen and other Arking R: Aging: A biological perspective. Am Scientist 2003;91:proteins, possibly as the end result of the nonenzymatic 508.combination of glucose with amino groups on these Asbury CL, Fehr AM, Block SM: Kinesin moves by an asymmetricmolecules. A third theory envisions aging as the cumu- hand-over-hand mechanism. Science 2003;302:2130.lative result of damage to tissues by free radicals formed Berridge MJ, Bootman MD, Lipp P: Calcium—A life and deathin them. It is interesting in this regard that species with signal. Nature 1998;395:645.longer life spans produce more superoxide dismutase, Blackhorn EH: Telomere states and cell fates. Nature 2000;408:53.an enzyme that inactivates oxygen-free radicals (see Cannon WB: The Wisdom of the Body. Norton, 1932.Chapter 27). Coleman DE: TRP channels as cellular sensors. Nature 2003;426: Evidence in favor of cumulative DNA abnormalities 517.is the recent demonstration that in Werner’s syn- Derynck R, Zhang YE: Smad-dependent and smad-independentdrome, a condition in which humans age at a markedly pathways in TGF-β signaling. Nature 2003:425:577.accelerated rate, the genetic abnormality is mutation of DiMauro S, Schon EA: Mitochondrial respiratory-chain diseases.a gene coding for a DNA helicase, one of the enzymes N Engl J Med 2003;348:2656.that helps split the DNA strands before replication. Downward J: The ins and outs of signaling. Nature 2001;411:759.This abnormality would be expected to produce unusu- Farfel Z, Bourne HR, Iiri T: The expanding spectrum of G protein diseases. N Engl J Med 1999;340:1012.ally rapid accumulation of chromosomal damage. Micethat lack one of the components of telomerase age Göhrlich D, Kutay V: Transport between the cell nucleus and the cytoplasm. Annu Rev Cell Dev Biology 1999;15:607.rapidly and have many of the defects that are character- Hill MM, Adrian C, Martin SJ: Portrait of a killer: The mitochon-istic of Werner’s syndrome in humans. In addition, as drial apoptosome emerges from the shadows. Mol Intervhuman cells age, there is a large accumulation of point 2003;3:19.mutations in the portion of their mitochondrial DNA Huntley SM (editor): Frontiers in cell biology: Quality control.that controls its reproduction. This could lead to defec- (special section) Science 1999;286:1881.tive energy production or, possibly, increases in free Jentsch TJ et al: Molecular structure and physiological function ofradicals in cells. chloride channels. Physiol Rev 2002;82:503. It is now established that in experimental animals, a Kaznetsov G, Nigam SJ: Folding of secretory and membrane pro-chronically decreased caloric intake prolongs life, and teins. N Engl J Med 1998;339:1688.this could be true in humans as well. One possible ex- Kersten S, Desvergne B, Wahil W: Roles of PPAPs in health andplanation for this effect of caloric restriction is de- disease. Nature 2000;405:421.creased metabolism, with decreased formation of pro- Kliewer SA, Lehman JM, Wilson TM: Orphan receptors: Shiftingtein cross-links and decreased production of free endocrinology into reverse. Science 1999;284:757.radicals. It may be relevant in this regard that in yeasts, Lamberts SWJ, Van den Beld AW, Van der Lely A-J: The en-worms, and flies, mutations in the homologs of one of docrinology of aging. Science 1997;278:419.the mammalian insulin pathways causes a dramatic pro- Nath D (editor): Cytoskeleton (special section) Nature 2003;422:longation of their life span. However, the exact cause of 739.the lengthened life span produced by caloric restriction Pawson T, Nash P: Assembly of cell regulatory systems through protein interaction domains. Science 2003;300:445.remains to be determined. Ray LB, Gough NR: Orienteering strategies for a signaling maze. In aging humans, declines occur in the circulating Science 2002;296:1632.levels of some sex hormones, the adrenal androgen de- Rebbechi MJ, Pentyala SN: Structure, function, and control ofhydroepiandrosterone and its sulfate, and growth hor- phosphoinositol-specific phospholipase C. Physiol Rev 2000;mone. Replacement therapy with estrogens and proges- 80:1291.terone in women (see Chapter 23) decreases the Rothman JE, Wieland FT: Protein sorting by transport vesicles.incidence of osteoporosis. Replacement therapy with Science 1996;272:227.testosterone (see Chapter 23), dehydroepiandrosterone Russell JM: Sodium–potassium–chloride cotransport. Physiol Rev(see Chapter 20), and growth hormone (see Chapter 2000;20:211.
  • 63. 50 / CHAPTER 1Schmid, R: Stem cells: A dramatic new therapeutic tool. J Gas- White TW: Nonredundant gap junction functions. News Physiol troenterol Hepatol 2002;19:636. Sci 2003;18:95.Scriver CR et al (editors): The Metabolic and Molecular Bases of In- Yellen G: The voltage-gated potassium channel and their relatives. herited Disease, 8th ed. McGraw-Hill, 2001. Nature 2002;419:35.Steel GJ et al: Coordinated activation of HSP 70 chaperones. Sci- ence 2004;303:98.Strehler E, Zacharias DA: Role of alternative splicing on generation of diversity among plasma membrane calcium pumps. Phys- iol Rev 2001;81:21.
  • 64. SECTION II Physiology of Nerve & Muscle Cells Excitable Tissue: Nerve 2INTRODUCTION vesicles in which the synaptic transmitters secreted by the nerves are stored (see Chapter 4).The human central nervous system (CNS) contains The axons of many neurons are myelinated, ie, theyabout 1011 (100 billion) neurons. It also contains acquire a sheath of myelin, a protein–lipid complex10–50 times this number of glial cells. It is a complex that is wrapped around the axon (Figure 2–3). Outsideorgan; it has been calculated that 40% of the human the CNS, the myelin is produced by Schwann cells,genes participate, at least to a degree, in its formation. glia-like cells found along the axon. Myelin forms whenThe neurons, the basic building blocks of the nervous a Schwann cell wraps its membrane around an axon upsystem, have evolved from primitive neuroeffector cells to 100 times. The myelin is then compacted when thethat respond to various stimuli by contracting. In more extracellular portions of a membrane protein calledcomplex animals, contraction has become the special- protein zero (P0) lock to the extracellular portions of P0ized function of muscle cells, whereas integration and in the apposing membrane. Various mutations in thetransmission of nerve impulses have become the spe- gene for P0 cause peripheral neuropathies; 29 differentcialized functions of neurons. This chapter is concerned mutations have been described that cause symptomswith the ways these neurons are excited and the way ranging from mild to severe. The myelin sheath en-they integrate and transmit impulses. velops the axon except at its ending and at the nodes of Ranvier, periodic 1-µm constrictions that are aboutNERVE CELLS 1 mm apart. The insulating function of myelin is dis- cussed below. Not all mammalian neurons are myeli-Morphology nated; some are unmyelinated, ie, are simply sur-Neurons in the mammalian central nervous system rounded by Schwann cells without the wrapping of thecome in many different shapes and sizes (Figure 2–1). Schwann cell membrane around the axon that producesHowever, most have the same parts as the typical spinal myelin. Most neurons in invertebrates are unmyeli-motor neuron illustrated in Figure 2–2. This cell has nated.five to seven processes called dendrites that extend out- In the CNS of mammals, most neurons are myeli-ward from the cell body and arborize extensively. Par- nated, but the cells that form the myelin are oligoden-ticularly in the cerebral and cerebellar cortex, the den- drogliocytes rather than Schwann cells (Figure 2–3).drites have small knobby projections called dendritic Unlike the Schwann cell, which forms the myelin be-spines. A typical neuron also has a long fibrous axon tween two nodes of Ranvier on a single neuron, oligo-that originates from a somewhat thickened area of the dendrogliocytes send off multiple processes that formcell body, the axon hillock. The first portion of the myelin on many neighboring axons. In multiple sclero-axon is called the initial segment. The axon divides sis, a crippling autoimmune disease, patchy destructioninto terminal branches, each ending in a number of of myelin occurs in the CNS. The loss of myelin is as-synaptic knobs. The knobs are also called terminal sociated with delayed or blocked conduction in the de-buttons or axon telodendria. They contain granules or myelinated axons. 51
  • 65. 52 / CHAPTER 2 CORTICAL NEURONS Cerebrum Cerebellum Optic lobes CENTRAL INTERNEURONS CELL Secondary BODY sensory cells Short axon types CENTRAL EFFECTOR NEURONS Motor neurons Autonomic neurons Hypophysial neurons Neurohypophysis PERIPHERAL EFFECTOR NEURONS Autonomic ganglia PERIPHERAL BIPOLAR NEURONS Invertebrate ganglia CELL Optic (1) (2) (3) BODY Auditory (1) Vestibular Olfactory (2) Cutaneous (3) SENSORY FIELDS EFFECTOR FIELDSFigure 2–1. Some of the types of neurons in the mammalian nervous system. (Reproduced, with permission, fromBodian D: Introductory survey of neurons. Cold Spring Harbor Symp Quant Biol 1952;17:1.) Cell body (soma) Initial segment of axon Node of Ranvier Schwann cell Axon hillock Nucleus Terminal buttons DendritesFigure 2–2. Motor neuron with myelinated axon.
  • 66. EXCITABLE TISSUE: NERVE / 53 Schwann cell Receptor zone: Graded electrogenesis Site of origin of Axon conducted impulses Axon: All or none transmission Axons Oligodendrogliocyte Nerve endings: Secretion ofFigure 2–3. Top: Relation of Schwann cells to axons synaptic transmitterin peripheral nerves. On the left is an unmyelinatedaxon, and on the right is a myelinated axon. Note that Figure 2–4. Functional organization of neurons. Non-the cell membrane of the Schwann cell has wrapped it- conducted local potentials are integrated in the recep-self around and around the axon. tor zone, and action potentials are initiated at a siteBottom: Myelination of axons in the central nervous close to the receptor zone (arrow). The action poten-system by oligodendrogliocytes. One oligodendroglio- tials are conducted along the axon to the nerve end-cyte sends processes to up to 40 axons. ings, where they cause release of synaptic transmitters. within the axon (eg, auditory neurons) or attached to The dimensions of some neurons are truly remark- the side of the axon (eg, cutaneous neurons; see Figureable. For spinal neurons supplying the muscles of the 2–1). Its location makes no difference as far as the re-foot, for example, it has been calculated that if the cell ceptor function of the dendritic zone and the transmis-body were the size of a tennis ball, the dendrites of the sion function of the axon are concerned.cell would fill a large room and the axon would be up It should be noted that the size and complexity ofto 1. 6 km (almost a mile) long although only 13 mm the dendritic trees on neurons varies markedly (Figure(half an inch) in diameter. 2–1; see also Figures 11–1 and 12–14). In addition to The conventional terminology used for the parts of integrated passive electrical activity, propagated actiona neuron works well enough for spinal motor neurons potentials appear to be generated in dendrites in someand interneurons, but there are problems in terms of special situations. Furthermore, new research suggests“dendrites” and “axons” when it is applied to other that dendrites have more complex functions. This topictypes of neurons found in the nervous system. From a is discussed in greater detail in Chapter 4.functional point of view (see below and Chapters 4 and5), neurons generally have four important zones: (1) a Protein Synthesis & Axoplasmic Transportreceptor, or dendritic zone, where multiple local poten-tial changes generated by synaptic connections are inte- Nerve cells are secretory cells, but they differ from othergrated (Figure 2–4); (2) a site where propagated action secretory cells in that the secretory zone is generally atpotentials are generated (the initial segment in spinal the end of the axon, far removed from the cell body.motor neurons, the initial node of Ranvier in cutaneous The apparatus for protein synthesis is located for thesensory neurons); (3) an axonal process that transmits most part in the cell body, with transport of proteinspropagated impulses to the nerve endings; and (4) the and polypeptides to the axonal ending by axoplasmicnerve endings, where action potentials cause the release flow. Thus, the cell body maintains the functional andof synaptic transmitters. The cell body is often located anatomic integrity of the axon; if the axon is cut, theat the dendritic zone end of the axon, but it can be part distal to the cut degenerates (wallerian degenera-
  • 67. 54 / CHAPTER 2tion). Anterograde transport occurs along micro- plifiers and the cathode ray oscilloscope. Modern am-tubules. The molecular motors involved are discussed plifiers magnify potential changes 1000 times or more,in Chapter 1. Fast transport occurs at about 400 mm/d, and the cathode ray oscilloscope provides an almost in-and slow anterograde transport occurs at 0. ertia-less and almost instantaneously responding “lever”5–10 mm/d. Retrograde transport in the opposite di- for recording electrical events.rection also occurs along microtubules at about200 mm/d. Synaptic vesicles recycle in the membrane, The Cathode Ray Oscilloscopebut some used vesicles are carried back to the cell bodyand deposited in lysosomes. Some of the material taken The cathode ray oscilloscope (CRO) is used to mea-up at the ending by endocytosis, including nerve sure the electrical events in living tissue. In the CRO,growth factor (see below) and various viruses, is also electrons emitted from a cathode are directed into a fo-transported back to the cell body. cused beam that strikes the face of the glass tube in A potentially important exception to these princi- which the cathode is located (Figure 2–5). The face isples seems to occur in some dendrites. In them, single coated with one of a number of substances (phosphors)strands of mRNA transported from the cell body make that emit light when struck by electrons. A verticalcontact with appropriate ribosomes, and protein syn- metal plate is placed on either side of the electronthesis appears to create local protein domains (see beam. When a voltage is applied across these plates, theChapter 4). negatively charged electrons are drawn toward the posi- tively charged plate and repelled by the negatively charged plate. If the voltage applied to the verticalEXCITATION & CONDUCTION plates (X plates) is increased slowly and then reducedNerve cells have a low threshold for excitation. The suddenly and increased again, the beam moves steadilystimulus may be electrical, chemical, or mechanical. toward the positive plate, snaps back to its former posi-Two types of physicochemical disturbances are pro- tion, and moves toward the positive plate again. Appli-duced: local, nonpropagated potentials called, depend- cation of a “saw-tooth voltage” of this type thus causesing on their location, synaptic, generator, or electro- the beam to sweep across the face of the tube, and thetonic potentials; and propagated disturbances, the speed of the sweep is proportionate to the rate of rise ofaction potentials (or nerve impulses). These are the the applied voltage.only electrical responses of neurons and other excitable Another set of plates (Y plates) is arranged horizon-tissues, and they are the main language of the nervous tally, with one plate above and one below the beam.system. They are due to changes in the conduction of Voltages applied to these plates deflect the beam up andions across the cell membrane that are produced by al-terations in ion channels. The impulse is normally transmitted (conducted)along the axon to its termination. Nerves are not “tele- Y plates X platesphone wires” that transmit impulses passively; conduc-tion of nerve impulses, although rapid, is much slowerthan that of electricity. Nerve tissue is in fact a rela- Cathodetively poor passive conductor, and it would take a po-tential of many volts to produce a signal of a fraction ofa volt at the other end of a meter-long axon in the ab- Powersence of active processes in the nerve. Conduction is an Sweepactive, self-propagating process, and the impulse moves Screen Electron generatoralong the nerve at a constant amplitude and velocity. beamThe process is often compared to what happens when amatch is applied to one end of a train of gunpowder; by Amplifierigniting the powder particles immediately in front of it,the flame moves steadily down the train to its end. The electrical events in neurons are rapid, being Electrodesmeasured in milliseconds (ms); and the potentialchanges are small, being measured in millivolts (mV). NerveIn addition to development of microelectrodes with atip diameter of less than 1 µm, the principal advances Figure 2–5. Cathode ray oscilloscope. Simplified dia-that made detailed study of the electrical activity in gram of the principal connections when arranged tonerves possible were the development of electronic am- record potential changes in a nerve.
  • 68. EXCITABLE TISSUE: NERVE / 55down as it sweeps across the face of the tube, and the +35magnitude of the vertical deflection is proportionate to Overshootthe potential difference between the horizontal plates.When these plates are connected to electrodes on anerve, any changes in potential occurring in the nerve 0are recorded as vertical deflections of the beam as it Firingmoves across the tube. level mV After-depolarizationRecording From Single Neurons StimulusMammalian neurons are relatively small, but giant un- −55 artifactmyelinated nerve cells exist in a number of invertebrate After-hyperpolarizationspecies. Such cells are found, for example, in crabs −70(Carcinus), cuttlefish (Sepia), and squid (Loligo). The Latent periodfundamental properties of neurons were first deter- Timemined in these species and then found to be similar in CROmammals. The neck region of the muscular mantle ofthe squid contains single axons up to 1 mm in diame- Stimulatorter. The fundamental properties of these long axons aresimilar to those of mammalian axons. + −Resting Membrane Potential Axon MicroelectrodeWhen two electrodes are connected through a suitable inside axonamplifier to a CRO and placed on the surface of a sin-gle axon, no potential difference is observed. However, Figure 2–6. Action potential in a neuron recordedif one electrode is inserted into the interior of the cell, a with one electrode inside the cell.constant potential difference is observed, with the in-side negative relative to the outside of the cell at rest. known, the speed of conduction in the axon can be cal-This resting membrane potential is found in almost culated. For example, assume that the distance betweenall cells. Its genesis is discussed in Chapter 1. In neu- the cathodal stimulating electrode and the exterior elec-rons, it is usually about –70 mV. trode in Figure 2–6 is 4 cm. The cathode is normally the stimulating electrode (see below). If the latent pe-Latent Period riod is 2 ms, the speed of conduction is 4 cm/2 ms, or 20 m/s.If the axon is stimulated and a conducted impulse oc-curs, a characteristic series of potential changes known Action Potentialas the action potential is observed as the impulsepasses the external electrode (Figure 2–6). It is The first manifestation of the approaching action po-monophasic because one electrode is inside the cell. tential is a beginning depolarization of the membrane. When the stimulus is applied, the stimulus artifact, After an initial 15 mV of depolarization, the rate of de-a brief irregular deflection of the baseline, occurs. This polarization increases. The point at which this changeartifact is due to current leakage from the stimulating in rate occurs is called the firing level or sometimes theelectrodes to the recording electrodes. It usually occurs threshold. Thereafter, the tracing on the oscilloscopedespite careful shielding, but it is of value because it rapidly reaches and overshoots the isopotential (zeromarks on the cathode ray screen the point at which the potential) line to approximately +35 mV. It then re-stimulus was applied. verses and falls rapidly toward the resting level. When The stimulus artifact is followed by an isopotential repolarization is about 70% completed, the rate of re-interval (latent period) that ends with the start of the polarization decreases and the tracing approaches theaction potential and corresponds to the time it takes the resting level more slowly. The sharp rise and rapid fallimpulse to travel along the axon from the site of stimu- are the spike potential of the neuron, and the slowerlation to the recording electrodes. Its duration is pro- fall at the end of the process is the after-depolariza-portionate to the distance between the stimulating and tion. After reaching the previous resting level, the trac-recording electrodes and inversely proportionate to the ing overshoots slightly in the hyperpolarizing directionspeed of conduction. If the duration of the latent pe- to form the small but prolonged after-hyperpolariza-riod and the distance between the electrodes are tion.
  • 69. 56 / CHAPTER 2 The proportions of the tracing in Figure 2–6 are in- Once threshold intensity is reached, a full-fledgedtentionally distorted to illustrate the various compo- action potential is produced. Further increases in thenents of the action potential. A tracing with the com- intensity of a stimulus produce no increment or otherponents plotted on exact temporal and magnitude change in the action potential as long as the other ex-scales for a mammalian neuron is shown in Figure 2–7. perimental conditions remain constant. The action po-Note that the rise of the action is so rapid that it fails to tential fails to occur if the stimulus is subthreshold inshow clearly the change in depolarization rate at the fir- magnitude, and it occurs with a constant amplitudeing level, and also that the after-hyperpolarization is and form regardless of the strength of the stimulus ifonly about 1–2 mV in amplitude although it lasts the stimulus is at or above threshold intensity. The ac-about 40 ms. The duration of the after-depolarization is tion potential is therefore “all or none” in character andabout 4 ms in this instance. It is shorter and less promi- is said to obey the all-or-none law.nent in many other neurons. Changes may occur in theafter-polarizations without changes in the rest of the ac- Electrotonic Potentials, Localtion potential. For example, if the nerve has been con- Response, & Firing Levelducting repetitively for a long time, the after-hyperpo-larization is usually quite large. Although subthreshold stimuli do not produce an ac- tion potential, they do have an effect on the membrane“All-or-None” Law potential. This can be demonstrated by placing record- ing electrodes within a few millimeters of a stimulatingIf an axon is arranged for recording as shown in Figure electrode and applying subthreshold stimuli of fixed2–6, with the recording electrodes at an appreciable dis- duration. Application of such currents with a cathodetance from the stimulating electrodes, it is possible to leads to a localized depolarizing potential change thatdetermine the minimal intensity of stimulating current rises sharply and decays exponentially with time. The(threshold intensity) that, acting for a given duration, magnitude of this response drops off rapidly as the dis-will just produce an action potential. The threshold in- tance between the stimulating and recording electrodestensity varies with the duration; with weak stimuli it is is increased. Conversely, an anodal current produces along, and with strong stimuli it is short. The relation hyperpolarizing potential change of similar duration.between the strength and the duration of a threshold These potential changes are called electrotonic poten-stimulus is called the strength–duration curve. Slowly tials, those produced at a cathode being catelectro-rising currents fail to fire the nerve because the nerve tonic and those at an anode anelectrotonic.adapts to the applied stimulus, a process called accom- The anelectronic potential is proportionate to themodation. applied anodal current. The catelectronic potential is roughly proportionate at low applied cathodal current, but as the strength of the current is increased, the re- +35 sponse is greater due to the increasing addition of a local response of the membrane (Figure 2–8). Finally, Spike at 7–15 mV of depolarization, the firing level, runaway depolarization, and a spike potential result. 0 Changes in Excitability During Electrotonic Potentials mV &