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1 introduction to gastrointestinal physiology


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an overview to gastrointestinal physiology

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1 introduction to gastrointestinal physiology

  1. 1. Introduction to Gastrointestinal Physiology
  2. 2. What we will learn today ……. • Different parts of gastrointestinal system • Overall Function • Physiologic anatomy • Gastrointestinal smooth muscle • Electrical activity of GI muscle • Neural control
  3. 3. Functions of GI tract 1) Digestion and absorption 2) Excretion of waste materials 3) Fluid and electrolyte balance 4) Immunity 5) Intestinal bacterial flora
  4. 4. Each part is adapted to its specific functions: a. Simple passage of food: Esophagus b. Temporary storage of food: Stomach c. Digestion and absorption: Small intestine
  5. 5. Physiologic Anatomy of the Gastrointestinal Wall
  6. 6. Muscle fibers are electrically connected with one another through large numbers of gap junctions  allow low-resistance movement of ions from one muscle cell to the next
  7. 7. Electrical Activity of Gastrointestinal Smooth Muscle Smooth muscle of the GIT is excited by almost continual slow, intrinsic electrical activity along the membranes of the muscle fibers. Two basic types of electrical waves: (1)Slow waves and (2)Spikes, Voltage of the RMP of the gastrointestinal smooth muscle can be made to change to different levels
  8. 8. "Slow waves" of smooth muscle membrane potential Not action potentials Slow, undulating changes in RMP bet -65to 40 mV BER (Basal Electrical Rhythm)- slow wave which oscillate significantly Frequency ranges: 3/min in the body of the stomach, 12/min in the duodenum, and about 8 or 9/min in the terminal ileum.
  9. 9. Precise cause of the slow waves  not completely understood complex interactions among the smooth muscle cells and specialized cells, called the interstitial cells of cajal, (electrical pacemakers for smooth muscle cells) Unique ion channels periodically open and produce inward (pacemaker) currents may generate slow wave activity Do not cause muscle contraction
  10. 10. Spike potentials True AP Occur automatically when the RMP >-40 mV Higher the slow wave potential rises, the greater the freq of the spike potentials: usu bet 1 - 10 spikes per sec
  11. 11. Channels responsible for AP : • Large numbers of Ca ions + smaller numbers of Na ions (calcium- sodium channels) • Slower to open and close • Long duration of AP (10 - 20 ms) • Movement of large amounts of Ca ions to the interior of the muscle fiber during AP plays a special role in causing the intestinal muscle fibers to contract
  12. 12. Factors that depolarize the membrane (1)Stretching of the muscle, (2)Stimulation by Ach released from the endings of parasympathetic nerves, (3)Stimulation by several specific GIT hormones. Factors that hyperpolarize the membrane (1)Effect of NE or E on the fiber membrane and (2)Stimulation of the sympathetic nerves
  13. 13. Neural Control of Gastrointestinal Function Enteric nervous system Autonomic nervous system: parasympathetic and sympathetic
  14. 14. Enteric Nervous System Lies entirely in the wall of the gut esophagus to anus Number of neurons: about 100 million Control gastrointestinal movements and secretion
  15. 15. Composed of two plexuses: Outer plexus/ Myenteric plexus / Auerbach's plexus • Bet the long. and circ. muscle layers Inner plexus/Submucosal plexus / Meissner's plexus • lies in the submucosa Controls gastrointestinal movements Controls gastrointestinal secretion and local blood flow
  16. 16. Submucosal plexus • Control function within the inner wall of each minute segment of the intestine Control local intestinal secretion, local absorption, and local contraction of the submucosal muscle that causes various degrees of infolding of the gastrointestinal mucosa
  17. 17. Types of Neurotransmitters Secreted by Enteric Neurons (1)acetylcholine  excites (2) Norepinephrine inhibits (3) adenosine triphosphate, (4) serotonin, (5) dopamine, (6) cholecystokinin, (7) substance P, (8) vasoactive intestinal polypeptide, (9) somatostatin, (10) leu-enkephalin, (11) met-enkephalin, and (12) bombesin.
  18. 18. • Extrinsic sympathetic and parasympathetic fibers connect both the myenteric and submucosal plexuses • Stimulation by these greatly enhance or inhibit gastrointestinal functions Autonomic nervous system
  19. 19. Sensory nerve endings send afferent fibers to • Both plexuses of the enteric system, • Prevertebral ganglia of the sympathetic nervous system, • Spinal cord, and • Vagus nerves all the way to the brain stem Sensory nerves can elicit • Local reflexes within the gut wall itself • Other reflexes that are relayed to the gut from either the prevertebral ganglia or the basal regions of the brain
  20. 20. What we learned today ……. • Different parts of gastrointestinal system • Overall Function • Physiologic anatomy • Gastrointestinal smooth muscle • Electrical activity of GI muscle • Neural control