Give an account of gastrointestinal motility and its control.

Outline:

·        Role of motility in digestion

·        Intrinsic innervation

·        Types of movement

·        Stomach:

- receptive relaxation

- gastric contractions

- gastric emptying and regulation

·        Small intestine:

- intestinal slow waves

- reflexes

- migrating motor complex

Essay:

            Digestion involves both mechanical and chemical breakdown of ingested material. For these processes to be carried out optimally, the contents must be held long enough for breakdown and mixing to occur and gastrointestinal contents must be moved analward to make room for more food and to remove wastes. The motor activities of the gastrointestinal (GI) tract are regulated by myogenic mechanisms arising from the intrinsic properties of smooth muscle. Neurotransmitters and hormones act mainly to modulate this basic muscular activity.

            The smooth muscle cells of the GI tract are long and slender and are electrically connected to one another through gap junctions, forming a latticework of smooth muscle bundles enabling it to function as a syncytium. They are subjected to a continual but slow electrical activity. Slow waves are slow, undulating changes in the resting membrane potential. Its frequency varies from 3 per minute in the stomach to 12 per minute in the duodenum. Slow waves are generated by interstitial cells located in a thin layer between the longitudinal and circular layers of the muscularis external. The amplitude of the slow waves can be modulated by activity of the intrinsic and extrinsic nerves and by hormones and paracrine substances. If the peak of the slow wave exceeds the threshold potential, action potentials are triggered which enhances the contractile force of the muscle.

            The basic propulsive movement of the GI tract is peristalsis which cause food to move forward along the tract at an appropriate rate for digestion and absorption. The initiation of peristalsis begins with the appearance of a contractile ring around the gut and then moves forward and spreads along the tube, moving forward any material in front of it. At the same time, the gut relaxes downstream (receptive relaxation) towards the anus, thus allowing the food to be propelled easily analward.

            The stomach serves as a reservoir for the large volume of food that can be ingested at a single meal until the food can be processed in the duodenum. The presence of food in the stomach increases its contractile activity, mixing food with gastric secretions and grinding it into smaller particles to form chyme. The gastric contents are delivered to the duodenum at a rate that affords optimal mixing with the pancreatic-biliary secretions.

            When food is passed from the esophagus to the stomach, the contractile activity of the fundus is inhibited, enabling it to easily accomodate 1-2L of food. Stretch receptors in the stomach detect the presence of food and initiate a vagovagal reflex from the stomach to the brain stem and then back to the stomach. The postganglionic vagal fibers release VIP or nitric oxide which decreases the rate of vagal firing and hence the tone in the muscular wall of stomach.

            During the cephalic and gastric phases of digestion, the release of acetylcholine from postganglionic parasympathetic nerve endings and gastrin from the G cells stimulates gastric contractility. These contractions increase in force and velocity as they approach the gastroduodenal junction and therefore the major mixing activity occurs in the antrum of the stomach. As each peristaltic wave reaches the pylorus, the pyloric sphincter snaps shut, so that the stomach empties in small squirts. Hence, gastric emptying occurs when the chyme is decomposed into small enough pieces to fit through the pyloric sphincter. The rapid contraction of the antrum also propels the chyme back into the antrum; this movement, called retropulsion, is effective at mixing and breaking down gastric contents.

            The rate at which the stomach empties is regulated by signals from both the stomach and the duodenum. Increased food volume in the stomach promotes increased emptying from the stomach. Local excitatory reflexes are initiated by expansions of the antrum. The duodenal and jejunal mucosa contain receptors that sense acidity, osmotic pressure, fat digestion products, peptides and amino acids. Gastric emptying is slowed by hypertonic solutions in the duodenum, a duodenal pH of below 3.5, and the presence of amino acids and fatty acids. As a result of these mechanisms, the rate at which chyme enter the small intestine do not exceed the rate at which the small intestine can process it. Acid is not dumped into the duodenum more rapidly than it can be neutralized by pancreatic and duodenal secretions.

            The contractile activity of the smooth muscles lining the small intestine serves to mix the chyme with the digestive juices and bile to facilitate digestion and absorption and to propel chyme from the duodenum to the colon. Segmentation is the most common type of intestinal contraction. During segmentation, about 2cm of the intestinal wall contracts, forcing the chyme back toward the stomach and toward the colon. When the muscle relaxes, the chyme returns to the area from which it was displaced. This back-and-forth movement enables the chyme to become thoroughly mixed with the digestive juices. Segmentation occurs throughout the digestive period, being generated continuously by the intestinal slow waves. The higher frequency of segmentation in the proximal intestine than in the distal intestine propels the chyme slowly toward the colon. The relative slow rate of net propulsion of chyme in the small intestine allows time for digestion and absorption.

            Peristalic activity of the small intestine increases after a meal and is caused by the beginning entry of chyme into the duodenum. The gastroenteric reflex is initiated by distention of the stomach and conducted through the myenteric plexus from the stomach down along the wall of the small intestine. Hormones such as CCK, gastrin, insulin and serotonin enhances intestinal motility while secretin and glucagon inhibit it. The emptying of chyme from the small intestine into the cecum is regulated by the ileocecal sphincter. When the cecum is distended, the contraction of the ileocecal sphincter is intensified and ileal peristalsis is inhibited delaying the emptying of additional chyme from the ileum.

            During the interdigestive period, any undigested food particles left in the stomach is removed by the migrating motor complex (MMC). The MMC is a peristaltic wave that repeats every 75 to 90 minutes from the esophagus and travels through the entire GI tract to the terminal ileum. The MMC sweeps the small bowel content clean and empty its contents in the colon.