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Understanding the Functions of the Digestive System: Alimentary Tract Overview

Dive into the detailed workings of the digestive system's alimentary tract. Learn about smooth muscle structure, nerve regulation, nutrient absorption, and more. Written in English.

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Understanding the Functions of the Digestive System: Alimentary Tract Overview

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  1. Digestive system I

  2. Alimentary tract • Continuous provision • Water • Electrolytes • Nutrients • Achieved by • Movement of food • Digestion • Mechanical and chemical • Absorption • Transport

  3. Anatomical structures

  4. Smooth muscles within the GI tract • Layers • Longitudinal • Length-wise • Circular • Formation of syncitium • Each fiber within respective layer • Connected via gap junctions • Ion movement

  5. Contraction of GI smooth muscles • Continual, slow intrinsic electrical activity • Slow waves • Not action potentials • Too low • Generated by the interaction of interstitial cells of Cajal • Periodic openings of channels • Do not usually cause muscle contraction

  6. Contraction of GI smooth muscles • Continual, slow intrinsic electrical activity • Spike potentials • Action potentials • Generated when the resting potential goes over -40 mV • Greater the rise in resting potential, greater the frequency • Lasts longer than normal action potential (10-20 mSec) • Generated by the movement of calcium ions • Slower channels

  7. Changes in resting potentials • Depolarization • Stretching of muscle • Acetylcholine • Stimulation of parasympathetic nerves • GI hormones • Hyperpolarization • Epinephrine and norepinephrine • Stimulation of sympathetic nerves

  8. Role of calcium ions • Entrance to cells • Slow waves • No muscle contraction • Spike potentials • Tonic • Continuous but not associated with slow waves • Continuous repetitive spike potentials • Hormones and other factors • Continuous entry of calcium ions • Not associated with changes in membrane potential

  9. Enteric nervous system • Regulation • GI tract movement • GI tract secretion

  10. Movement Afferent Fibers (local and other reflexes) Secretion Local Blood flow

  11. Myenteric plexus • Mostly linear chain • Extends entire length of the GI tract • Controls muscle activity along the length of the GI tract • Tonic contraction/tone of the wall • Intensity • Rhythm (slight)

  12. Myenteric plexus • Movement of peristalic wave • Increased conduction velocity of excitatory wave • Inhibitory neurons • Secretion of inhibitory peptide • Inhibition of sphincters • Inhibits food movement

  13. Submucosal plexus • Local functions • Absorption • Secretion • Contraction

  14. Role of ANS • Parasympathetic • Cranial • Vagus • Esophagus, stomach, and pancreas • Sacral • Large intestine and anus • Defecation reflex • Excitation • Increased activity

  15. Sympathetic • T5 and L2 of spinal cord • Celiac and mesenteric ganglia • Essentially innervates entire GI tract • Excitation • Inhibition of activity • Smooth muscle • Neurons of enteric nervous system T5 L2

  16. Neurotransmitters • Aceylcholine • Excitation • Norepinephrine/epinephrine • Inhibition

  17. Afferent sensory nerve fibers • Activation • Irritation of mucosa • Distention • Chemicals • Inhibition or activation • Transmission of information to the CNS • Afferent vagus nerves (80 %)

  18. Role of enteric nervous system • Generation of reflexes • Integrated within the enteric nervous system • Local reflex • Loop between the prevertebral sympathetic ganglia and GI tract • Signals from lower portion of the GI tract to regulate activity of the upper GI tract or vise versa

  19. Loop between the spinal cord/brain stem and the GI tract • Vagus nerves from the stomach to the brainstem • Pain reflex (inhibitory) • Defecation reflex

  20. Movement within the GI tract • Propulsive movement • Peristalsis • Generated in response to GI tract distension • Requires active myenteric plexus • Formation of the contractile rings • Receptive relaxation • Polarized movement • Move in one direction

  21. Mixing movement • Inhibition of peristalisis forward movement • Sphincter • Churning of the content within the segment • Local intermittent constrictive contractions

  22. Splanchnic circulation • Flow of blood • Afferent flow • The GI tract • Pancreas • Spleen • Enters liver via the portal vein • Flow through liver sinusoids • Exits liver via hepatic veins • Vena cava

  23. Absorption of nutrients • Water soluble molecules • 75 % temporally stored in liver • Fats • Intestinal lymphatics • Enters circulation via thoracic duct

  24. Arterial supply to the GI tract • Mesenteric arteries (superior and inferior) • Intestines • Celiac artery • Stomach • Branches of arteries • Muscle bundles • Intestinal villi • Submucosal vessels

  25. Rate of flow • Proportional to activity levels • Active absorption increases flow by max. 8 X • Increased flow • Vasodilators • Decreased tissue oxygen concentrations

  26. Counter-current exchange of oxygen • Diffusion of oxygen from arterioles to venules without going through circulation • Bypassed oxygen is not available for tissue metabolism

  27. Neural regulation • Parasympathietic stimulation • Increased flow • Increased glandular secretions • Cause of increased flow • Sympathetic stimulation • Decreased flow • Vasoconstriction • Overcome by local vasodilators • Local ischemia • Allows re-direction of blood

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