The Migrating Motor Complex

The migrating motor complex is a distinct pattern of electromechanical activity observed in gastrointestinal smooth muscle during the periods between meals. It is thought to serve a "housekeeping" role and sweep residual undigested material through the digestive tube. As studied in dogs and man, the cycle recurs every 1.5 to 2 hours and consists of 4 phases:

1. A period of smooth muscle quiescence lasting 45 to 60 minutes, during which there are only rare action potentials and contractions.
2. A period of roughly 30 minutes in which peristaltic contractions occur and progressively increase in frequency. Peristalsis originates in the stomach and propagates through the small intestine.
3. The phase lasting 5 to 15 minutes in which rapid, evenly spaced peristaltic contractions occur. In contrast to the digestive period, the pylorus remains open during these peristaltic contractions, allowing many indigestible materials to pass into the small intestine.
4. A short period of transition between the barrage of contractions in phase 3 and the inactivity of phase 1.

An increase in gastric, biliary and pancreatic secretion is also seen in conjunction with the motor activity. These secretions probably aid in the cleansing activity of the migrating motor complex and assist in preventing a buildup of bacterial populations in the proximal segments of the digestive tube.

The periodic nature of the migrating motor complex is thought to be controlled from the central nervous system and may be implemented in part by the enteric hormone motilin. Like real housekeeping, the migrating motor complex is readily overridden by "more important" processes - for example, ingestion of food will abolish a migrating motor complex and restore a digestive pattern of motility.

Aside from its apparent importance in maintaining patency of the gastrointestinal lumen, the migrating motor complex has potentially important theraputic and social implications:

• Blood levels of drugs that are differentially absorbed in the stomach and small intestine may vary unpredictably depending on whether they are taken during digestive or interdigestive phases, when mechanisms of gastric emptying are different - this is reflected in the recommendation accompanying many drugs to ingest before or with meals.
• Migrating motor complexes can be noisy and are the cause of "growling." Fortunately, you can usually squelch a migrating motor complex by ingesting some food, allowing transition into a quiet, digestive pattern of motility. If food is not available (let's say you're undergoing a job interview or waiting in line with your date at a concert), you can at least explain the phenomenon and gain points for intellect.

Send comments to Richard.Bowen@colostate.edu