Porth's Essentials of Pathophysiology, 4e

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Gastrointestinal and Hepatobiliary Function

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Neural Control of Gastrointestinal Motility The motility of the GI tract can be modulated by input from two sets of nerves, the intrinsic and extrinsic ner- vous systems. The intrinsic nervous system, called the enteric nervous system , has cell bodies that are con- tained within the wall of the GI tract. The extrinsic ner- vous system, which has nerves with cell bodies that are located outside the digestive tract, is part of the auto- nomic nervous system (ANS). In addition, a number of peptides, including neurotransmitters and GI hormones, assist in regulating GI motility. Enteric Nervous System Innervation The GI tract has a nervous system all its own called the enteric nervous system which lies entirely within the wall of the GI tract, beginning in the esophagus and continu- ing all the way to the anus. The enteric nervous system is composed of two plexuses: an outer myenteric (Auerbach) plexus and an inner submucosal (Meissner) plexus. These two plexuses are networks of nerve fibers and ganglion cell bodies. Interneurons in the plexuses connect afferent sen- sory fibers, efferent motor neurons, and secretory cells to form reflex circuits that are located within the GI tract wall. The myenteric plexus consists mainly of a linear chain of interconnecting neurons that is located between the circu- lar and longitudinal muscle layers of the muscular externa. Because it lies between the two muscle layers and extends all the way down the GI tract, it is concerned mainly with motility along the length of the gut. The submucosal plexus , which lies between the submucosal and mucosal layers of the wall, is mainly concerned with controlling the function of each segment of the GI tract. It integrates sig- nals received from the mucosal layer into local control of motility, intestinal secretions, and absorption of nutrients. The activity of the neurons in the myenteric and submucosal plexuses is regulated by local influ- ences, input from the ANS, and interconnecting fibers that transmit information between the two plexuses. Mechanoreceptors monitor the stretch and distention of the GI tract wall, and chemoreceptors monitor the chemical composition (i.e., osmolarity, pH, and digestive products of protein and fat metabolism) of its contents. These receptors can communicate directly with gangli- onic cells in the intramural plexuses or with visceral afferent fibers that influence ANS control of GI function. Autonomic Innervation The autonomic innervation of the GI system is mediated by both the sympathetic and parasympathetic nervous systems (see Chapter 34, Fig. 34-23). In general, stimu- lation of the parasympathetic nervous system causes a general increase in activity of the entire enteric nervous system, whereas sympathetic stimulation inhibits activ- ity causing many effects. Parasympathetic innervation to the stomach, small intestine, cecum, ascending colon, and transverse colon occurs through the vagus nerve. The remainder of the colon is innervated by parasympathetic fibers that exit the sacral segments of the spinal cord by way of the

SUMMARY CONCEPTS (continued)

Gastrointestinal Innervation and Motility The motility of the GI tract propels food and fluids along its length, from mouth to anus, in a manner that facilitates digestion and absorption. The movements of the GI tract can be either rhythmic or tonic. The rhyth- mic movements move food forward and keep the GI contents mixed. Rhythmic movements are found in the esophagus, antrum of the stomach, and small intestine. The tonic movements consist of a constant level of con- traction or tone without regular periods of relaxation. They are found in the lower esophagus, the upper region of the stomach, the ileocecal valve, and the internal anal sphincter. All of the contractile tissue in the GI tract is smooth muscle, except for that in the mouth and pharynx, the upper third of the esophagus, and the external anal sphincter. Although the smooth muscle found in each region of the GI tract exhibits structural and functional differences, certain basic properties are common to all of the muscle cells. For example, all of the smooth mus- cle of the GI tract is unitary smooth muscle, in which the cells are electrically coupled by low-resistance pathways so that electrical signals initiating muscle contractions can move rapidly from one fiber to the next. Like the self-excitable cardiac muscle cells in the heart, some smooth muscle cells throughout the GI tract function as pacemaker cells. These cells display rhyth- mic, spontaneous oscillations in membrane potentials, called slow waves , ranging in frequency from about 3 per minute in the stomach to 12 per minute in the duodenum. Slow waves are generated by a thin layer of interstitial cells located between the longitudinal and circular muscle layers. mucosal layer that produces mucus, secretes digestive enzymes, and absorbs the breakdown products of digestion; an underlying submucosal layer that contains blood vessels, lymph vessels, and nerves that control the secretory activity of the mucosal glands; a layer of circular and longitudinal smooth muscle fibers that mix and propel the gut contents along its length; and an outer serosal or adventitial layer that forms the peritoneum (serosa) or attaches structures of the GI tract to the body wall (adventitia). of the small intestine—is where most of the digestive and absorptive processes occur.The lower segment—the cecum, colon, and rectum of the large intestine—serves as a storage channel for the efficient elimination of waste. ■■ Throughout its length, except for the mouth, pharynx, and upper esophagus, the wall of the digestive tract is composed of four layers: an inner