Gastro-intestinal Tract (GIT) The GIT provides the body with water, electrolytes & nutrients.

General aims To understand the nervous & hormonal regulation of GIT. movements of each part of GIT. secretions of each part of GIT. the importance of the liver functions in metabolism, storage & synthesis of ….. how digestion & absorption of nutrients occurs. the pathophysiology of some GI disorders.

Guyton 6 ed 2006, P.771

In general the wall of GIT composes of mucosa, submucosa, inner circular & outer longitudinal layer of smooth muscles & serosa. In some parts there are additional muscle layer between the mucosa & submucosa called the muscularis mucosae. The smooth M. of GIT act as a synsytium, they are connected by tight junctions allowing the spread of excitement from one M. fiber to another by movement of ions between the cells, leading to spread of AP through the GIT.

Electrical activity of GIT smooth M. The RMP of GIT SM is -50 to -60 mV, but the RMP is not constant & on recording the electrical activity of SM they show slow waves, which is due to activity of Na+ pump. These slow waves vary in different parts of GIT; In the stomach it is 3 times / min. Duodenum is 12 times / min. Terminal ileum is 8-9 times / min. When the membrane potential becomes more +ve the Ca++ channels open leading to spike potential which causes real contraction

Factors affect RMP to become more +ve leading to depolarization: 1- Stretch of M. fiber. 2- Ach. 3- parasympathetic stimulation. 4- GIT hormones. Factors affect the RMP to more –ve leading to hyperpolarization: 1- NE or Epinephrine. 2- Sympathetic stimulation.

Control of GIT functions There are 2 control systems; 1- The nervous system. 2- The hormones.

The neural control I) The enteric nervous system. The GIT has its own NS which can function independently. It consists of 2 plexus: a- The myenteric (Auerbach plexus); it composes of neurons in between the longitudinal & circular M. layer extending from the esophagus to the anus. It controls the movement of GIT, i.e M. contraction.

b- Submucosal (Meissners plexus); It composes of neurons in the submucosa. It controls the secretions of GIT. When Ach is secreted from the enteric neurons it stimulates both contraction & secretion. While if NE is secreted from the enteric neurons it will inhibit both contraction & secretion.

II) The extrinsic innervation ( autonomic NS). The parasympathetic fibers to GIT run through vagus N. which supply the esophagus, stomach, pancreas, small intestine & 1st half of large intestine. Also parasympathetic fibers from S2, S3 & S4 pass through the pelvic Ns to supply the 2nd half of large intestine down to the anus. Parasympathetic stimulation increases GIT motility & secretion because it stimulates the enteric NS to secrete Ach. The sympathetic innervation originates from T5 to L2 spinal segments. Sympathetic stimulation inhibits GIT motility & secretion.

The afferent sensory fibers arise in the wall of the GIT, either it terminates in the enteric NS or go through the vagus N to the brain or through the sacral Ns to the spinal cord or through the sympathetic Ns to the prevertebral sympathetic ganglia. These sensory fibers are stimulated by; 1- Distension of the wall. 2- Irritation of the mucosa. 3- Certain chemical substances in the gut.

Hormonal control Some polypeptides secreted by the mucosa either act in paracrine fashion or they may enter the circulation. These hormones have important role in regulation of GIT secretion & motility. These hormones fall into 2 families: 1- Gastrin family. The primary members are gastrin & cholecystokinin (CCK). 2- Secretin family. The primary members are secretin, VIP & GIP. VIP= Vasoactive Intestinal Peptide, GIP= Gastric Inhibitary Peptide.

Gastrin It is produced by the G cells of the glands in the antral portion of gastric mucosa. G cells & other enteroendocrine cells are called APUD cells (amine precursor uptake & decaboxylation) because they are of neural crest origin, they contain amines related to NE or serotonin.

The physiological actions of gastrin are; 1- Stimulation of gastric acid & pepsin secretion. 2- Stimulation of growth of mucosa of stomach, small intestine & large intestine. 3- Stimulation of gastric motility. 4- Contraction of the muscles at the gastroesophageal junction. 5- Stimulate insulin & glucagon secretion after protein meal (not CHO meal).

enterochromaffin- like cells (ECL cells),

Factors increasing gastrin secretion: 1- Gastric content ( specially proteins) 2- Rate of discharge of vagus. 3- Calcium & epinephrine. 4- distention of the stomach. Factors inhibiting gastrin secretion: 1- Acid in the antrum. 2- Secretin, GIP, VIP, glucagon & calcitonin.

Cholecystokinin-Pancreozymin CCK-PZ This hormone is secreted by the mucosa of the duodenum & jejunum. Its half-life is 5 min. It has 2 important functions: 1- Contraction of GB. 2- Increase secretion of pancreatic juice. Factors increase CCK secretion: 1- Contact of intestinal mucosa with products of protein digestion. 2- Presence of FA in the duodenum. Function: 3. inhibition of stomach contraction.

Secretin It is secreted by the glands in the mucosa of the upper portion of small intestine. Its half-life is 5 min. Its functions: 1- Increase secretion of bicarbonate by the pancreatic duct & biliary tracts. 2- Augments the action of CCK in producing pancreatic secretion. 3- Decrease gastric acid secretion. 4- Contraction of pyloric sphincter. Its secretion is increased by products of protein digestion & acids.

Gastric inhibitory peptide (GIP) It is secreted by the mucosa of duodenum & jejunum. Its secretion is stimulated by glucose & fat in the duodenum. Its functions: 1- Inhibits gastric acid secretion & motility. 2- Stimulation of insulin secretion.

Vasoactive intestinal peptite (VIP) It is found in the nerves of the GIT, brain & ANS. Its half-life is 2 min. Its functions: 1- Stimulation of intestinal secretion of electrolyte & water. 2- Relaxation of intestinal smooth M & sphincters. 3- Dilation of peripheral BVs. 4- Inhibition of gastric acid secretion. 5- It potentiates the action of Ach in salivary glands.

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