Peristalsis is the organized propulsion of material over variable distances within the intestinal lumen. The muscle layers of the intestine behave in a stereotypical pattern during peristaltic propulsion (Fig. 26.20). This pattern is determined by the integrated circuits of the ENS. During peristalsis, the longitudinal muscle layer in the segment ahead of the advancing intraluminal contents contracts while the circular muscle layer simultaneously relaxes. The intestinal tube behaves like a cylinder with constant surface area. The shortening of the longitudinal axis of the cylinder is accompanied by a widening of the cross-sectional diameter. The simultaneous shortening of the longitudinal muscle and relaxation of the circular muscle results in expansion of the lumen, which prepares a receiving segment for the forward-moving intraluminal contents during peristalsis.
The second component of stereotyped peristaltic behavior is contraction of the circular muscle in the segment behind the advancing intraluminal contents. The longitudi-
Relaxation of Contraction of longitudinal muscle; longitudinal muscle;
contraction of circular muscle inhibition of circular muscle
nal muscle layer in this segment relaxes simultaneously with contraction of the circular muscle, resulting in the conversion of this region to a propulsive segment that propels the luminal contents ahead, into the receiving segment. Intestinal segments ahead of the advancing front become receiving segments and then propulsive segments in succession as the peristaltic complex of propulsive and receiving segments travels along the intestine.
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