Propulsive Movements
Peristalsis
in the Small Intestine. Chyme is propelledthrough the small intestine by peristaltic waves. These can occur in
any part of the small intestine, and they move toward the anus at a velocity of
0.5 to 2.0 cm/sec, faster in the proximal intestine and slower in the ter-minal
intestine. They normally are very weak and usually die out after traveling only
3 to 5 centimeters, very rarely farther than 10 centimeters, so that forward
movement of the chyme is very slow, so slow in fact that net movement along the small intestine normally averages only 1 cm/min. This means that 3 to
5 hours are required for passage of chyme from the pylorus to the ileocecal
valve.
Control
of Peristalsis by Nervous and Hormonal Signals. Peri-staltic activity of the
small intestine is greatly increased after a meal. This is caused partly by the
beginning entry of chyme into the duodenum causing stretch of the duodenal
wall, but also by a so-called gastroentericreflex
that is initiated by distention of the stomach andconducted principally
through the myenteric plexus from the stomach down along the wall of the small
intestine.
In addition to the nervous signals that may affect small intestinal
peristalsis, several hormonal factors also affect peristalsis. They include gastrin, CCK, insulin, motilin, and serotonin, all of which enhanceintestinal motility and are
secreted during various phases of food processing. Conversely, secretin and glucagon inhibit small intestinal motility. The physio-logic
importance of each of these hormonal factors for controlling motility is still
questionable.
The function of the peristaltic waves in the small intestine is not
only to cause progression of chyme toward the ileocecal valve but also to
spread out the chyme along the intestinal mucosa. As the chyme enters the
intestines from the stomach and elicits peri-stalsis, this immediately spreads
the chyme along the intestine; and this process intensifies as additional chyme
enters the duodenum. On reaching the ileoce-cal valve, the chyme is sometimes
blocked for several hours until the person eats another meal; at that time, a gastroileal reflex intensifies
peristalsis in the ileum and forces the remaining chyme through the ileocecal
valve into the cecum of the large intestine.
Propulsive
Effect of the Segmentation Movements. The seg-mentation movements, although lasting
for only a few seconds at a time, often also travel 1 centimeter or so in the
anal direction and during that time help propel the food down the intestine.
The difference between the segmentation and the peristaltic movements is not as
great as might be implied by their separation into these two classifications.
Peristaltic
Rush. Although peristalsis in the smallintestine is normally weak,
intense irritation of the intestinal mucosa, as occurs in some severe cases of infectious
diarrhea, can cause both powerful and rapid peristalsis, called the peristaltic rush. This is initiated
partly by nervous reflexes that involve the autonomic nervous system and brain
stem and partly by intrinsic enhancement of the myenteric plexus reflexes
within the gut wall itself.The powerful peristaltic contractions travel long
distances in the small intestine within minutes, sweeping the contents of the
intestine into the colon and thereby relieving the small intestine of
irri-tative chyme and excessive distention.
Movements
Caused by the Muscularis Mucosae and Muscle Fibers of
the Villi. Themuscularis mucosaecan
cause shortfolds to appear in the intestinal mucosa. In addition, individual
fibers from this muscle extend into the intes-tinal villi and cause them to
contract intermittently. The mucosal folds increase the surface area exposed to
the chyme, thereby increasing absorption. Also, contrac-tions of the
villi—shortening, elongating, and shorten-ing again—“milk” the villi, so that
lymph flows freely from the central lacteals of the villi into the lymphatic
system. These mucosal and villous contractions are ini-tiated mainly by local
nervous reflexes in the submu-cosal nerve plexus that occur in response to
chyme in the small intestine.
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