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Chapter 14, F 09

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Chapter 14, F 09

  1. 1. Human Anatomy, Second Edition McKinley & O'Loughlin Chapter 14 Lecture Outline: Nervous Tissue
  2. 2. The Nervous System <ul><li>The primary communication and control system of the body . </li></ul><ul><li>Fast and homeostatic </li></ul><ul><li>It can be divided into structural and functional categories. </li></ul>14-
  3. 3. Organization of the Nervous System 14-
  4. 4. Structural Organization <ul><li>Central nervous system (CNS) </li></ul><ul><ul><li>brain and spinal cord </li></ul></ul><ul><li>Peripheral nervous system (PNS) </li></ul><ul><ul><li>cranial nerves (to and from the brain) </li></ul></ul><ul><ul><li>spinal nerves (to and from the spinal cord) </li></ul></ul><ul><ul><li>ganglia (clusters of neuron cell bodies located outside the CNS) </li></ul></ul>14-
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  6. 6. Functional Organization: Sensory and Motor Nervous Systems <ul><li>Three general functions </li></ul><ul><ul><li>Collecting information </li></ul></ul><ul><ul><li>Processing and evaluating information </li></ul></ul><ul><ul><li>Responding to information </li></ul></ul>14-
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  8. 8. Sensory Division <ul><li>Somatic sensory components are the general somatic senses—touch, pain, pressure, vibration, temperature, and proprioception . </li></ul><ul><li>Visceral sensory components transmit nerve impulses from blood vessels and viscera to the CNS. The visceral senses primarily include temperature and stretch (of the organ wall). </li></ul>14-
  9. 9. Motor Division <ul><li>The somatic motor component (somatic nervous system; SNS) conducts nerve impulses from the CNS to skeletal muscles. </li></ul><ul><ul><li>also known as the voluntary nervous system </li></ul></ul><ul><li>The autonomic motor component (autonomic nervous system; ANS) innervates internal organs, regulates smooth muscle, cardiac muscle, and glands. </li></ul><ul><ul><li>also known as the visceral motor system or involuntary nervous system </li></ul></ul>14-
  10. 10. Cytology of Nervous Tissue <ul><li>Two distinct cell types form nervous tissue. </li></ul><ul><ul><li>Neurons are excitable cells that initiate and transmit nerve impulses. Structural units of the nervous system. </li></ul></ul><ul><ul><li>Glial cells are nonexcitable cells that support and protect the neurons </li></ul></ul>14-
  11. 11. Neurons <ul><li>Neurons have a high metabolic rate. </li></ul><ul><li>Neurons have extreme longevity. </li></ul><ul><li>Neurons typically are non-mitotic. </li></ul>14-
  12. 12. New Neurons in Adults? <ul><li>A population of immature progenitor cells in the hippocampus known as neural stem cells can under certain circumstances mature into neurons </li></ul>14-
  13. 13. Neuron Structure <ul><li>Neurons come in all shapes and sizes , but all neurons share certain basic structural features. </li></ul><ul><ul><li>Some are tiny, less than 1 mm in the CNS </li></ul></ul><ul><ul><li>Some are some of the largest cells in the body </li></ul></ul><ul><li>A typical neuron has a cell body, dendrites, and axons. </li></ul>14-
  14. 14. Neuron Structure – Dendrites <ul><li>Dendrites tend to be shorter, smaller processes that branch off the cell body. </li></ul><ul><li>Some neurons have only one dendrite, while others have many (100’s often). </li></ul><ul><li>Dendrites conduct nerve impulses toward the cell body; they receive input and then transfer it to the cell body for processing. </li></ul><ul><li>The more dendrites a neuron has, the more nerve impulses that neuron can receive from other cells. </li></ul>14-
  15. 15. Neuron Structure – Cell Body <ul><li>The cell body serves as the neuron’s control center and is responsible for receiving, integrating, and sending nerve impulses. </li></ul><ul><li>Has most of the organelles found in other cells, but no centrioles (no mitosis or regeneration) </li></ul><ul><li>Chromatophilic substance, Nissl bodies - rough ER and ribosomes </li></ul><ul><li>Lipofuscin inclusions - probably lysosomal wastes. Yellowish-brown. Probably harmless. </li></ul><ul><li>Neurofibrils and intermediate filaments form cytoskeleton </li></ul>14-
  16. 16. Neuron Structure – Axon <ul><li>The larger, typically longer nerve cell process emanating from the cell body is the axon, sometimes called a nerve fiber. </li></ul><ul><li>Most neurons have one axon, but no more than one. May have collaterals. </li></ul><ul><li>The axon transmits a nerve impulse away from the cell body toward another cell. </li></ul><ul><li>May be short, absent, or long (3-4 feet) </li></ul><ul><li>Axon originates at the axon hillock and then tapers. The nerve impulse, action potential, starts at the first part, the initial segment. </li></ul><ul><li>Branches a lot at the end usually, axon terminals, telodendria. May be over 10,000 . Ends at synaptic knobs (end bulbs, etc.) </li></ul>14-
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  18. 18.
  19. 19. Neuron Classification <ul><li>Neurons vary widely in morphology and location. </li></ul><ul><li>They can be classified according to either their structure or their function. </li></ul>14-
  20. 20. Structural Classification <ul><li>By the number of processes extending from the cell body . </li></ul><ul><ul><li>Unipolar </li></ul></ul><ul><ul><ul><li>PNS mainly: sensory neurons </li></ul></ul></ul><ul><ul><li>Bipolar </li></ul></ul><ul><ul><ul><li>Rare: sense organs’ receptor cells in the eye, ear, and nose </li></ul></ul></ul><ul><ul><li>Multipolar </li></ul></ul><ul><ul><ul><li>Most common in humans </li></ul></ul></ul><ul><ul><ul><li>Major type in CNS </li></ul></ul></ul><ul><ul><ul><li>Most motor neurons and association neurons </li></ul></ul></ul>14-
  21. 21.
  22. 22. Functional Classification <ul><li>Sensory neurons </li></ul><ul><li>Interneurons or association neurons </li></ul><ul><li>Motor neurons </li></ul>14-
  23. 23. Interneurons <ul><li>Interneurons, or association neurons, lie entirely within the CNS and are multipolar . </li></ul><ul><li>They receive nerve impulses from many other neurons and carry out the integrative function of the nervous system. </li></ul><ul><li>Thus, interneurons facilitate communication between sensory and motor neurons . </li></ul><ul><li>Thousands of types </li></ul><ul><ul><li>Purkinje cells in the cerebellum </li></ul></ul><ul><ul><li>Renshaw cells in the spinal cord </li></ul></ul><ul><ul><li>Pyramidal cells in the cerebral cortex </li></ul></ul>14-
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  25. 25. Functions of Glial Cells <ul><li>Form a structural network. </li></ul><ul><li>Replace damaged neurons. </li></ul><ul><li>Assist neuronal development. </li></ul>14-
  26. 26. Glial Cells (Neuroglia) <ul><li>Occur within both the CNS and the PNS. </li></ul><ul><ul><li>4 of the 6 types are within the CNS. </li></ul></ul><ul><li>Smaller and capable of mitosis. </li></ul><ul><li>Do not transmit nerve impulses. </li></ul><ul><li>Physically protect and help nourish neurons, and provide an organized, supporting framework for all the nervous tissue. </li></ul><ul><li>Far outnumber neurons (about 10x). </li></ul><ul><li>Account for roughly half the volume of the nervous system. </li></ul><ul><li>Almost 1/2 of brain tumors are gliomas. </li></ul>14-
  27. 27.
  28. 28. <ul><li>Astrocytes exhibit a starlike shape due to projections from their surface. </li></ul><ul><li>Astrocytes are the most abundant and largest glial cells in the CNS, and they constitute over 90% of the tissue in some areas of the brain. </li></ul><ul><ul><li>Help from the blood-brain barrier (BBB) that strictly controls substances entering the nervous tissue in the brain from the bloodstream. </li></ul></ul><ul><ul><li>Regulate tissue fluid composition (ionic balance). </li></ul></ul><ul><ul><li>Forming a structural network. </li></ul></ul><ul><ul><li>Replacing damaged neurons. </li></ul></ul><ul><ul><li>Assisting neuronal development. </li></ul></ul>Glial Cells of the CNS 14-
  29. 29.
  30. 30. Glial Cells of the CNS <ul><li>Ependymal cells line the ventricles and spinal cord and produce and move CSF </li></ul>14-
  31. 31. Glial Cells of the CNS <ul><li>Microglial cells are brain macrophages </li></ul><ul><li>Rarest and smallest of the glial cells </li></ul>14-
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  33. 33. Glial Cells of the CNS <ul><li>Oligodendrocytes myelinate fibers in the CNS </li></ul><ul><li>One oligodendrocyte may help myelinate 60 axons </li></ul><ul><li>Cell body not involved in wrapping (like an octopus) </li></ul><ul><li>No neurilemma </li></ul><ul><li>Large nodes of Ranvier </li></ul>14-
  34. 34. Glial Cells of the PNS <ul><li>Satellite cells are found in the ganglia around the cell bodies. </li></ul><ul><li>Physically separate cell bodies from the surrounding interstitial fluid and regulate exchanges </li></ul>14-
  35. 35.
  36. 36. Glial Cells of the PNS <ul><li>Neurolemmocytes or Schwann cells myelinate in the PNS </li></ul><ul><li>Have neurilemma around myelin sheath </li></ul><ul><li>Many Schwann cells/axon </li></ul><ul><li>Smaller nodes of Ranvier (neurofibril nodes) </li></ul>14-
  37. 37. Myelination of Axons <ul><li>Main activity of axons is nerve impulse conduction. </li></ul><ul><li>Nerve impulse (action potential) is the rapid movement of an electrical charge along a neuron’s plasma membrane. </li></ul><ul><ul><li>A voltage (potential) change that moves along the plasma membrane </li></ul></ul><ul><ul><li>The ability to travel along an axon is affected by myelination </li></ul></ul>14-
  38. 38. Myelination <ul><li>Myelination is the process by which part of an axon is wrapped with a myelin sheath, a protective fatty coating that gives it glossy-white appearance ( white matter ). </li></ul><ul><li>The myelin sheath supports, protects, and insulates an axon. </li></ul><ul><li>No change in voltage can occur across the membrane in the insulated portion of an axon. </li></ul>14-
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  40. 40.
  41. 41.
  42. 42. Nerve Impulse Conduction <ul><li>In a myelinated axon, the nerve impulse “jumps” from neurofibril node to neurofibril node and is called saltatory conduction . </li></ul><ul><li>In an unmyelinated axon, the nerve impulse must travel the entire length of the axon and is called continuous conduction and is slower. Pain stimuli carried this way. </li></ul><ul><li>A myelinated axon produces a faster nerve impulse and it requires less energy. </li></ul>14-
  43. 43. Development of the Myelin Sheath <ul><li>Starts during late fetal development and first year </li></ul><ul><li>Increases to maturity </li></ul><ul><li>Increase in conduction rate with increase myelination </li></ul><ul><li>Explains why infants are slower and less coordinated in their responses </li></ul>14-
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  45. 45. Regeneration of PNS Axons <ul><li>PNS axons are vulnerable to cuts, crushing injuries, and other trauma. </li></ul><ul><li>A damaged axon can regenerate, however, if at least some neurilemma remains. </li></ul><ul><li>PNS axon regeneration depends upon three factors. </li></ul><ul><ul><li>the amount of damage </li></ul></ul><ul><ul><li>neurolemmocyte secretion of nerve growth factors to stimulate outgrowth of severed axons </li></ul></ul><ul><ul><li>the distance between the site of the damaged axon and the effector organ </li></ul></ul>14-
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  47. 47.
  48. 48. Regeneration of CNS Axons <ul><li>Very limited </li></ul><ul><li>Oligodendrocoytes do not release a nerve growth factor </li></ul><ul><li>Large number of axons crowded together </li></ul><ul><li>Astrocytes and connective tissue coverings may form some scar tissue that obstructs axon regrowth </li></ul><ul><li>No neurilemma to act as a regeneration tube </li></ul>14-
  49. 49. Demyelination <ul><li>Progressive destruction of myelin sheath is accompanied by inflammation, axon damage, and scarring of neural tissue </li></ul><ul><li>Usually results in a gradual loss of sensation and motor control </li></ul>14-
  50. 50. Causes of Demyelination <ul><li>Heavy metal poisoning </li></ul><ul><li>Diptheria </li></ul><ul><li>Multiple sclerosis </li></ul><ul><li>Guillain-Barre syndrome </li></ul>14-
  51. 51. Nerves <ul><li>A nerve is a bundle of parallel axons . </li></ul><ul><li>Like a muscle, a nerve has three successive connective tissue wrappings. </li></ul><ul><ul><li>endoneurium - a delicate layer of loose connective tissue </li></ul></ul><ul><ul><li>perineurium - a cellular and fibrous connective tissue layer that wraps groups of axons into bundles called fascicles </li></ul></ul><ul><ul><li>epineurium - a superficial connective tissue covering </li></ul></ul><ul><ul><ul><li>A thick layer of dense irregular fibrous connective tissue encloses the entire nerve, providing both support and protection </li></ul></ul></ul>14-
  52. 52.
  53. 53.
  54. 54. Nerves <ul><li>Nerves are a component of the peripheral nervous system. </li></ul><ul><li>Sensory (afferent) nerves convey sensory information to the CNS . </li></ul><ul><li>Motor (efferent) nerves convey motor impulses from the CNS to the muscles and glands. </li></ul><ul><li>Axons terminate as they contact other neurons, muscle cells, or gland cells. </li></ul><ul><li>An axon transmits a nerve impulse at a specialized junction called a synapse . </li></ul>14-
  55. 55. Nerve Regeneration and Spinal Cord Injuries <ul><li>Spinal injuries at the neck used to be fatal because of respiratory failure </li></ul><ul><li>Much better survival now from early steroid use and antibiotics </li></ul><ul><li>Reconnection and partial restoration of function of severed cords in rats </li></ul><ul><ul><li>Olfactory nerve bridge spans injured area and acts as a guide (not in humans yet) </li></ul></ul><ul><ul><li>Neural stem cells may be able to regenerate CNS axons </li></ul></ul><ul><li>Christopher Reeve, paralyzed below the second cervical vertebra, made tremendous progress and raised funds and awareness </li></ul>14-
  56. 56. Synapses <ul><li>Presynaptic neurons transmit nerve impulses along their axonal membranes toward a synapse. </li></ul><ul><li>Postsynaptic neurons conduct nerve impulses through their dendritic and cell body membranes away from the synapse. </li></ul><ul><li>Axons may establish synaptic contacts with any portion of the surface of another neuron, except those regions that are myelinated. </li></ul>14-
  57. 57.
  58. 58.
  59. 59. Synaptic Communication 14-
  60. 60. Electrical Synapses <ul><li>Electrical synapses are not very common in mammals. </li></ul><ul><li>In humans, these synapses occur primarily between smooth muscle cells where quick, uniform innervation is essential. </li></ul><ul><li>Electrical synapses are also located in cardiac muscle and the developing embryo. </li></ul><ul><li>Use gap junctions . </li></ul><ul><li>Advantages </li></ul><ul><ul><li>Faster communication </li></ul></ul><ul><ul><li>Can synchronize the activity of a group of neurons or muscle fibers </li></ul></ul>14-
  61. 61.
  62. 62. Chemical Synapses <ul><li>The most numerous type of synapse is the chemical synapse. </li></ul><ul><li>It facilitates most of the interactions between neurons and all communications between neurons and effectors. </li></ul><ul><li>At these junctions, the presynaptic membrane releases a signaling molecule called a neurotransmitter , such as acetylcholine (ACh), into the synaptic cleft. </li></ul><ul><li>Other types of neurons use other neurotransmitters. </li></ul><ul><li>Synaptic delay of about 0.5 msec </li></ul>14-
  63. 63.
  64. 64. Neurotransmitters <ul><li>Are released only from the plasma membrane of the presynaptic cell. </li></ul><ul><li>It then binds to receptor proteins found only on the plasma membrane of the postsynaptic cell. </li></ul><ul><li>A unidirectional flow of information and communication takes place. </li></ul><ul><li>Two factors influence the rate of conduction of the impulse: the axon’s diameter and the presence (or absence) of a myelin sheath. </li></ul>14-
  65. 65. Graded Potentials <ul><li>Graded potentials occur on the dendrites and cell body (no polarity reversal, a local depolarization) </li></ul><ul><li>Spreads from the receptive zone to the axon hillock (the trigger zone). It decreases in strength as it travels. </li></ul><ul><li>If sufficient graded potentials at the initial segment, then it will initiate an action potential (all or none) if the voltage threshold is exceeded. </li></ul>14-
  66. 66. Synapses <ul><li>The typical CNS neuron receives input from 1,000 to 10,000 synapses! </li></ul><ul><li>Synapses may be excitatory and/or inhibitory. </li></ul><ul><li>Integration of inputs on post-synaptic neuron can result in: </li></ul><ul><ul><li>1 or more nerve impulses if the threshold is reached or surpassed </li></ul></ul><ul><ul><li>No impulse if inhibitory effects are greater than the excitatory effects </li></ul></ul><ul><ul><li>Facilitated: if subthreshold stimulation. More easily can generate an impulse. </li></ul></ul>14-
  67. 67. Synapses <ul><li>The neurotransmitter affects the post-synaptic neuron as long as it remains in the synaptic cleft. </li></ul><ul><li>Three ways to remove </li></ul><ul><ul><li>Diffusion </li></ul></ul><ul><ul><li>Destroyed by enzymes </li></ul></ul><ul><ul><li>Reuptake into presynaptic neuron or transport to neighboring neuron </li></ul></ul><ul><ul><ul><li>Example, Prozac is a selective serotonin reuptake inhibitor (SSRI) </li></ul></ul></ul>14-
  68. 68. Synapses <ul><li>Synapses are essential for homeostasis by transmitting certain impulses and inhibiting others. </li></ul><ul><li>Often brain and psychiatric disorders result from disruption of synaptic communication. </li></ul><ul><li>Also site for most drugs that affect brain, therapeutic and addictive </li></ul>14-
  69. 69. Neural Integration and Neuronal Pools <ul><li>Billions of interneurons within the CNS are grouped in complex patterns called neuronal pools (or neuronal circuits or pathways). </li></ul><ul><li>Neuronal pools are defined based upon function , not anatomy, into four types of circuits: </li></ul><ul><ul><li>converging </li></ul></ul><ul><ul><li>diverging </li></ul></ul><ul><ul><li>reverberating </li></ul></ul><ul><ul><li>parallel-after-discharge </li></ul></ul><ul><li>A pool may be localized, or its neurons may be distributed in several different regions of the CNS. </li></ul>14-
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  71. 71. Nervous System Disorders <ul><li>Amyotrophic lateral sclerosis </li></ul><ul><li>Multiple sclerosis </li></ul><ul><li>Parkinson disease </li></ul><ul><li>Guillain-Barre syndrome </li></ul>14-
  72. 72. Neural Tube Defects <ul><li>Serious developmental deformities of the brain, spinal cord, and meninges </li></ul><ul><li>Anencephaly </li></ul><ul><li>Spina bifida </li></ul>14-
  73. 73. Development of the Nervous System <ul><li>Begins in the third week in the embryo from the ectoderm. </li></ul>14-

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