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- Understand how the motility functions of the small intestine and colon contribute to the integrated response to a meal
- Describe the muscle layers and their connections to the enteric nervous system that subserve intestinal motility
- Identify the sphincters that control movement of intestinal contents between segments, or out of the body
- Define the motility patterns that characterize movements of the small and large intestines under fed and fasted conditions and their control mechanisms
- Distinguish between mixing patterns and those that propel contents along the length of the intestine
- Describe reflexes that coordinate the motility functions of the small intestine and colon with the function of the stomach
- Understand the process whereby undigestible residues of the meal are eliminated from the body
- Understand the pathophysiology of disease states where intestinal motility is abnormal
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The ability of the walls of the small and large intestines to contract and relax allows for the movement of intestinal contents from one site to another. Specific motility patterns subserve the functions of each intestinal segment. In addition, specialized muscle regions, or sphincters, retard the passage of intestinal contents in a controlled fashion at specific sites.
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Role and Significance in the Small Intestine
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As we have learned from previous chapters, the primary role of the small intestine is to digest the various components of the meal and to absorb the resulting nutrients into the bloodstream or lymphatic system. The motility patterns observed in the small intestine are profoundly altered by eating. The duration of such changes depends on the caloric load and the type of nutrients ingested, with lipids having the most durable effect. During the fed state, many of the motility patterns in the small intestine are designed not to propel the intestinal contents aborally, but rather to mix the contents with the various digestive secretions and prolong their exposure to the absorptive epithelium. The muscle layers of the small intestine interact to provide for “two steps forward and one step back,” retaining the intestinal contents long enough to provide for efficient extraction of most or all useful substances. In general, therefore, the motility functions of the small intestine control the rate of nutrient absorption. The speed with which the contents are propelled also varies along the length of the small intestine. Movement is fastest in the duodenum and jejunum, providing for rapid mixing and propulsion of the contents both orally and aborally. Motility then slows in the ileum, providing additional time for the absorption of slowly permeable nutrients, and particularly, lipids. Then, once the meal is digested and absorbed, the small intestine converts to the migrating motor complex (MMC) we also discussed for the stomach, a pattern of relative quiescence punctuated by propulsive motility patterns that expel undigested residues through the small intestine and into the colon.
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Role and Significance in the Colon
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The functions of the colon are quite distinct from those ...