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

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.

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.

Movements of the GIT

Propulsive movement (peristalsis). Mixing movement.

Movements of the GIT There are 2 types of movements: 1- Propulsive movement or peristalsis. It occurs at any site from the esophagus downward. Accumulation of food in any portion causes distension of that part & stimulation of myenteric plexus & so contraction of the circular Ms starts 2-3 cm above the distension, then the ring of contraction moves downward to push the food down.

2- Mixing movement. It occurs when there is peristalsis but the sphincter is closed, or due to local contractions of some parts of GIT. This movement leads to mixing of the luminal contents with the secretions of the GIT to digest the contents and to make contact of the content with GIT mucosa for absorption.

All the absorbed nutrients reaches the liver (except fat, which is absorbed into intestinal lymphatics). In the liver ½ - 2/3 of the nutrients are stored. Also any bacteria or harmful substances that enters the circulation from the GIT are removed by the Kuppfer cells.

Transport & mixing of food The time which food stays in different parts of GIT is very important & any part of this time is properly controlled by several neural & hormonal factors so that the passage of food should not be too rapid nor too slow.

The intrinsic desire for food is called hunger, while the type of food which person seeks is called the appetite.

Mastication (Chewing) Chewing is important to break large food particles into small ones & mix them with the saliva, so facilitating easy swallowing & further digestion. Most of the muscles of chewing are innervated by the motor branches of the 5th cranial nerve.

Swallowing (Deglutition) 1- The voluntary stage. 2- The pharyngeal stage. 3- The esophageal phase.

Swallowing (Deglutition) 1- The voluntary stage. The tongue pushes the food upward & backward against the palate while the lips are closed, after that swallowing becomes involuntary. 2- The pharyngeal stage. a- The soft palate moves upward to close the posterior nares preventing reflux of food to the nose.

b- The palatopharyngeal folds are approximated so that only well chewed food can pass in between them. c- The vocal cords are strongly approximated, the larynx is pulled up & the epiglottis closes the larynx, all these occur to prevent the passage of food to the larynx. d- The upward movement of the larynx pull & enlarges the opening of the esophagus to receive the bolus of food. e- The pharyngoesophageal sphincter is relaxed, with rapid & strong peristaltic waves start in the upper part of the pharynx & spreads down to the middle & lower part then to the esophagus.

Pharyngeal phase The sensory stimuli from the posterior portion of the mouth & pharynx passes via the trigeminal & glossopharyngeal Ns to the swallowing center in the medulla oblongata & pons, then efferent impulses return via the 5th , 9th, 10th & 12th cranial Ns. Trigem. 5, glossoph. 9, vagus 10, hypoglosal 12

3- The esophageal phase. The esophagus have 2 types of peristaltic movements; primary & secondary. The primary is continuation of the peristaltic waves begins in the pharynx, it is facilitated by the gravity in upright position.

If the primary peristalsis fails to push the food to the stomach, then the secondary peristaltic waves result from distention of the esophagus by the retained food & they continue until all the food passes to the stomach. The Ms of the pharynx & upper esophagus are striated & they are supplied by the glossopharyngeal N. While the Ms of the lower 2/3rd of the esophagus are smooth & supplied by the vagus N in connection with the myenteric plexus.

The lower esophageal sphincter (Gastroesophageal sph.) 2 factors prevent reflux of gastric content into the esophagus: 1- contraction of the circular Ms of the lower esophageal end, which relaxes only when peristaltic waves passes down to the esophagus. 2- The esophagogastric angle, which acts as a valve-like mechanism to prevent reflux.

Motor functions of the stomach 1- Storage of food: When food enters the stomach, the stomach wall relaxes until it accumulates about 1.5 L. This is mediated by the vagovagal reflex i.e sensory stimuli arise from the stomach travel through the vagus to the brainstem, then inhibitory orders return through the vagus to the stomach to inhibit stomach contractions.

2- Mixing of food with gastric secretions until it becomes semifluid material called Chyme. When food reaches the stomach, the slow waves of stomach initiates weak peristaltic contractions that push the content of stomach toward the antrum, but these contractions are weak & cannot overcome the resistance of the pyloric sphincter, so the food is mixed with the secretions of gastric glands. Hunger contractions: These are strong peristaltic contractions that arise 12 - 24 hours after last meal, also they appear when the level of blood sugar is low, they cause pain in the pit of the stomach this pain is called hunger pangs.. Glucostat cells sensitive to rate of glucose utilization; in the hypothalamus

3- Emptying of the stomach: The stomach contractions becomes stronger so that they overcome the resistance of pyloric sphincter & with each contraction few milliliters of chyme pushed into the duodenum. Stretching of the wall of stomach stimulates myenteric plexus so that strong contractions will be elicited. In addition, the products of meat digestion stimulates the antral cells (G cells) to secrete gastrin which stimulates gastric motility.

Movements of the small intestine 1- Mixing movement (segmentation contraction): When the chyme distends the small intestine wall, many concentric contractions appear, then after these segments relax, other sets of contraction starts at new points.

3-5 hours to pass from the pylorus to the ileocecal valve. 2- Peristalsis: - Each peristaltic wave progress to only 3-5 cm, therefore, the chyme requires about 3-5 hours to pass from the pylorus to the ileocecal valve. - These peristaltic waves greatly increase after meal & they cause progression of the chyme & spread it to be well digested & absorbed. 2-3 times per min.

Peristaltic rush It is strong & powerful waves that wash down the contents of small intestine to the large intestine. It occurs when there is irritant chyme or when there is excessive distention of the small intestine. Rush = ينطلق- يهجم- يهرول

Migrating motor complex These are moderately powerful waves occur in fasting person, starting from the stomach sweeping any excess digestive secretions or other intra intestinal debris into the colon, thereby preventing their accumulation in the upper GIT.

Functions of the ileocaecal valve This valve is important to prevent regurgitation of the fecal material from the caecum into the ileum. The ileocecal sphinctor slightly resist the emptying of ileum, so, prolongs the stay of the chyme, thereby facilitates its absorption.