Motility of the Small Intestine Has Digestive and Interdigestive Phases
Motility of the small intestine occurs in two distinct phases: (1) during the digestive period after food intake and (2) during the interdigestive period when little food is present in the gut.
In the digestive phase there are two primary motility patterns: propulsive and nonpropulsive. The nonpropulsive pattern is referred to as segmentation. Segmentation results from localized contractions of circular muscle. Portions of small intestine, usually 3 to 4 cm long, contract tightly, dividing the gut into segments of constricted and dilated lumen. Within a few seconds, the constricted portions relax and new areas constrict (Figure 28-9). This action tends to “milk” gut contents back and forth within the small intestine, mixing them with digestive juices and circulating them over the absorptive mucosal surfaces. This type of motility does not contribute much to the net aboral propulsion of ingesta. In fact, segmentation tends to slow down the aboral movement of material because of closure of the intestinal lumen in the constricted segments.Propulsive activity during the digestive phase consists of peristaltic contractions that migrate down the gut in phase with the slow waves. Digestive-phase peristaltic contractions, in contrast to interdigestive-phase peristalsis, pass over short segments of intestine and then die out. Thus, ingesta are pushed down the gut for a short distance and then subjected to additional segmentation contractions and mixing activity.
The interdigestive phase of small-intestinal motility is characterized by waves of powerful peristaltic contractions that sweep over a large length of small intestine, sometimes traversing the entire organ. These waves arc referred to as the migrating motility complex (MMC) or, alternatively, the migrating myoelectric complex. The MMC begins in the duodenum as groups of slow waves that stimulate intense action potential and muscular contraction activity. The complex migrates down the intestine at the rate of the slow waves. Some of the MMCs die out before reaching the ileum, but some travel the entire length of the small intestine.
The MMC probably has a “housekeeping” function and serves to push undigested material out of the small intestine. The MMC may also be important in controlling the bacterial population in the upper gut. Normally the duodenum harbors a relatively small population of bacteria, and the population increases distally into the ileum, which has a moderately large number Ofbacterial organisms. The colon is heavily colonized by numerous species of bacteria. It is important for digestive function that this relative distribution of bacteria be maintained within the gut. The MMC may help to impede the migration of bacteria from the ileum to the duodenum.