Physiology of GIT: Gastro intestinal wall including many layers : - Serosa

1. Physiology of GIT: Gastro intestinal wall including many layers : - Serosa -Longtudinal muscle layer& Circular m. layer - Submucosa - Mucosa , a layer of smooth muscle lies in the deeper layer of mucosa called (muscularismucoasa). The motor function of the gut performed by different layer of smooth muscle..The smooth muscle fibers arranged in bundles , they extends longitudinally down the intestinal tract in the longitudinal m. layer, & arranged around the gut in the circular m. layer .Within each bundle the muscle fiber electrically connected with one another through large number of gab junction that allow low resistance movements of ions from one cell to the next. therefore electrical signals can travel readily from one fiber to the next within each bundle but more rapidly along the length of the bundle than sideways.

2. Each bundle of smooth separated from the next by loose connective tissue but the bundle fuse With one another at many points , therefore each muscle layer function as syncytium , that is when an action potential is elicited anywhere within the muscle mass , it generally travels in all directions in the muscle.the distance that it travels depends on the excitability.

3. Autonomic control of GIT: • The parasympathetic supply of GIT is divided into cranial & sacral divisions, except for few parasympathetic fibers to the mouth & pharangeal regions , the cranial parasympathetic transmitted almost entirely in the vagus nerve, these fibers provide innervation to the esophagus ,stomach pancreas & somewhat less to the intestine down through the 1st half of large intestine • ,the sacral parasympathetic originate in the S2, S3 &S4 segment of spinal cord . • The parasympathetic neurone of parasympathetic system located in the myenteric &submucosalplexuse, stimulation of parasympathetic nerve generally cause increase in activity of enteric nervouse system. • The sympathetic fiber of the of the GIT originate from segment T-5 & L-2 of spinall cord .

4. Hormonal control of GIT motility: 1- cholecystokiniin:(cck): secreted by the mucosa of duodenum& jejunum in response to the presence of some substances in the intestinal contents. it has a potent effect in increasing the contractility of GB, also it inhibit the stomach motility. 2- secretin secreted by S cell in the mucosa of duodenum in response to gastric acid juice , it has inhibitory effect on most of GIT. 3- gastric inhibitory peptide:secreted by the mucosa of upper S.I. ,it has mild effect in decreasing motor activity of stomach .

5. Transport & mixing of food in the GIT: Mastication (chewing): The teeth are designed for chewing , the anterior (incisors) provide strong cutting action & the posterior (molars)provide granding action . Most of chewing process caused by chewing reflex which can be explained as that : the presence of bolus of food in the mouth initiates reflex inhibition to the muscle of mastication which allow the lower jaw to drop, the drop in turn initiates a stretch reflex of the jaw muscle that leads to rebound contraction & , this automatically

6. raises the jaw to cause closure of the teeth, but it also compress the bolus again against the lining of the mouth, which inhibit the jaw muscle once again allowing the jaw to drop &rebound again

7. Chewing is important for digestion of all food but it especially important for most fruit ,raw vegetables because these have undigested cellulose membrane around their nutrient portions that must be broken before the food can be used , the digestive enzyme act only on the surface of food particles,the rate of digestion is highly depenndent on the total surface area exposed to the intestinal secretion .

8. Swallowing (deglutition): 1- voluntary stage When the food is ready for swallowing it is voluntary squeezed or rolled posteriorly in to the pharynx by pressure of tongue upward & bachward against the palate. 2- pharangeal stage of swallowing (involuntary) When the bolus of food enter the pharynx it stimulate the swallowing receptors arround the opening of pharynx & send signals to the brain stem to initiate swallowing 3- pharangeal stage of swallowing .

9. Function of lower esophageal sphincter (gasrtoesophageal sphincter GOS) :at the lower 2 -5 cm of esophagus above its junction with stomach , the circular muscle function as sphincter , this sphincter remain tonicallyconsrticted in contrast to the mid part of esophagus which remain relaxed . when peristaltic wave pass down the esophagus ,receptive relaxation relax the LOS ,& allow propulsion of swallowed food to the stommach.if LOS faild to relax result in condition known as achalassia.