A mixing pattern of motility replaces the MMC when the small intestine is in the digestive state following ingestion of a meal. The mixing movements are sometimes called segmenting movements or segmentation, as a result of their appearance on X-ray films of the small intestine. Peristaltic contractions, which propagate for only short distances, account for the segmentation appearance. Circular muscle contractions in short propulsive segments are separated on either end by relaxed receiving segments (Fig. 26.34). Each propulsive segment jets the chyme in both directions into the relaxed receiving segments where stirring and mixing occur. This happens continuously at closely spaced sites along the entire length of the small intestine. The intervals of time between mixing contractions are the same as for electrical slow waves or are multiples of the shortest slow-wave interval in the particular region of intestine. A higher frequency of electrical slow waves and associated contractions in more proximal regions and the peristaltic nature of the mixing movements result in a net aboral propulsion of the luminal contents over time.
The Role of the Vagus Nerves and ENS. The mixing pattern of small intestinal motility is programmed by the ENS. Signals transmitted by vagal efferent nerves to the ENS interrupt the MMC and initiate mixing motility during ingestion of a meal. After the vagus nerves are cut, a larger quantity of ingested food is necessary to interrupt the interdigestive motor pattern, and interruption of the MMCs is often incomplete. Evidence of vagal commands for the mixing pattern has been obtained in animals with cooling cuffs placed surgically around each vagus nerve. During the digestive state, cooling and blockade of im pulse transmission in the nerves result in an interruption of the pattern of mixing movements. When the vagus nerves are blocked during the digestive state, MMCs reappear in the intestine but not in the stomach. With warming of the nerves and release of the neural blockade, the mixing motility pattern returns.
kMixing movements. The segmentation pattern of motility is characteristic of the digestive state. Propulsive segments separated by receiving segments occur randomly at many sites along the small intestine. Mixing of the luminal contents occurs in the receiving segments. Receiving segments convert to propulsive segments, while propulsive segments become receiving segments.
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