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Digestive system I. Alimentary tract Continuous provision –Water –Electrolytes –Nutrients Achieved by –Movement of food –Digestion Mechanical and chemical.

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Presentation on theme: "Digestive system I. Alimentary tract Continuous provision –Water –Electrolytes –Nutrients Achieved by –Movement of food –Digestion Mechanical and chemical."— Presentation transcript:

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

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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