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Gastrointestinal System – Motility

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Gastrointestinal System – Motility GASTROINTESTINAL SYSTEM – MOTILITY Learning Objectives 1. Describe the anatomy of the muscle layers in the wall of the gastrointestinal tract. 2. Explain gastrointestinal motility and pacemaker function. 3. Explain the patterns of motility in the fed and fasted states. GENERAL ANATOMY Enteric nervous system (Auerbach’s plexus) Smooth muscle in the wall of the GI tract is single unit type. These cells are electrically coupled by gap junctions. ELECTRICAL SLOW WAVES = PACEMAKERS • Not all electrical slow waves reach threshold. • At threshold, voltage gated calcium channels open triggering an action potential. Contraction in the intestine follows an action potential. In the stomach all electrical slow waves lead to contraction. SEGMENTATION: MOTILITY IN THE FED STATE “SLOSHING” back and forth! Alternating contraction is bordered by relaxation. PERISTALSIS: MOTILITY IN THE FED STATE PROGRESSIVE MOVEMENT Outer muscle layer contracts; Inner muscle layer relaxes MIGRATING MOTOR COMPLEX IN FASTING •MIGRATING MOTOR COMPLEX (MMC) sweeps the GI tract to prevent over growth of bacteria. “GOOD HOUSEKEEPING ROLE” • MMC is initiated by the hormone, motilin, which is secreted by an empty stomach as well by the duodenum. • MMC usually starts in the stomach but can initiate in the small intestine (duodenum and jejunum). The frequency of contractions differs from the peristalsis associated with feeding. SPHINCTERS • Tonic inhibition of the smooth muscle (inner circular) layer of muscularis externa when there is no motility within the tract. • Regulated relaxation by turning off tonic inhibition to open the sphincter. • Regulated opening in response to Migrating Motor Complex (MMC) between meals. MASS MOVEMENTS • Waves of contraction that propel semi-solid feces into distal colon, and then into rectum and anal canal. Involves the inner circular layer of the muscularis externa. • Occur 1-3 times per day. • External anal sphincter is under voluntary control. • Defecation expels fecal material. It occurs with the relaxation of the anal sphincter and an increase in intra- abdominal pressure. VOMITING • Protective function against ingestion of toxins. Also triggered by either delayed emptying or rapid emptying (dumping syndrome) of the stomach. • Expulsion of chyme from stomach usually but can include duodenal contents. • Controlled by Vomiting Center located in medulla of brain stem. • Respiratory and abdominal muscles are used in vomiting, not smooth muscle of the gastrointestinal tract wall. GENERAL CONCEPTS 1. Movement involves the coordinated activities of the two outer muscle masses in the wall of the gastrointestinal tract. 2. Movement and activities are coordinated by the enteric nervous system with modulation from the CNS. 3. Pacemakers cells generate spontaneous electrical activity called electrical slow waves. An action potential can fire at the peak of a slow wave; contraction follows. 4. In the fed state two types of motility occur, segmentation (or mixing) and peristalsis. In the fasted state, the migrating motor complex sweeps the tract clean. 5. Tonic contractions occur at the sphincters and when there is no motility within the tract. .
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