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Updated: 03/11/08 02:32 PM |
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| HOME | HEAL | EDUCATE | RESEARCH | DIRECTORY | OUTREACH |
Authors: D.R. Saunders, C.E. Rubin, and J.D. Ostrow
Download Chapter ♦ Download Lecture Slides & Notes
Download Chapter ♦ Download Lecture Slides & Notes
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D. Small Intestinal Motility |
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Duodenal contractions, 12 per minute Ileal contractions 8 per minute Cholinergic stimulates contractions Adrenergic inhibits |
The two major motor functions of the small intestine are mixing and propulsion. Segmentations are short (1 –2 cm) annular constrictions which promote a to-and-fro movement of chyme. The frequency of segmental contractions is based on the underlying basal electrical rhythm (BER). The duodenum can contract up to twelve times per minute while ileal contractions approach eight per minute. This decreasing gradient of BER frequency favors the aboral movement of chyme. Muscular contractions are affected by intrinsic and extrinsic factors. Vagal discharges, CCK, opioid peptides stimulate segmental contractions, while sympathetic discharges, a-adrenergic agonists, nitric oxide, VIP, and glucagon inhibit these contractions.
Less frequent than segmentation are propulsive contractions whose waves migrate short distances (10 cm) in an aboral direction. |
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Importance of unstirred water layer |
The integration of segmentation and propulsion enhances absorption by mixing luminal contents with digestive juices, and by increasing the contact time between chyme and absorptive cell surfaces. Intestinal motility also decreases the thickness of the unstirred layer of water and mucus which overlies absorptive cells so that nutrients and electrolytes can diffuse readily to brush border membranes.
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MMCs and housekeeping waves |
During the interdigestive, or fasting state there is a cycle of motor activity which migrates from the stomach to distal ileum. This migrating motor complex (MMC) consists of a quiescent 45-60 minute period (phase I) followed by a period of 30-45 min of irregular electrical and mechanical activity (phase II). Finally, there is a short phase III which lasts 5-10 min when each slow wave is associated with spike potentials and muscular contractions. Phase III of the MMC is believed to have a housekeeping function which clears the stomach and upper small intestine of chyme in preparation for the next meal.
Vagal stimulation and hormones such as motilin are thought to be responsible for the initiation of MMCs. MMCs are immediately interrupted when a meal is taken; the motor activity of the small intestine in the fed state resembles phase II of the MMC with localized segmenting contractions and propagating movements traveling only short distances. |
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