Migrating motor complex: phases, control and fasting

The migrating motor complex (MMC) is a repeating cycle of contractions in the fasting gut, that is, between meals, when the stomach and small bowel are empty. It appears to clear residual contents, for example undigested material.

The cycle is divided into phases. Phase I is a long quiet period with little contractile activity. In phase II the contractions are irregular and increase in frequency. Phase III is a short burst: it lasts minutes and is the most intense and regular activity of the cycle. It starts in the antrum of the stomach or in the duodenum and travels towards the colon, that is, distally or aborally. This is the main clearing front: it pushes undigested residue onwards. Phase IV is the winding-down period after the burst, as activity declines and merges into the next phase I.

When you eat, the MMC stops. The gut switches to the fed pattern, in which segmentation mixes chyme (the partly digested food mixture) with enzymes, while short peristaltic runs carry it onwards. Once digestion ends and the gut is empty again, the cycle resumes.

Motilin, a hormone from the lining of the duodenum, is classically associated with a phase III that begins in the stomach in humans. Serotonin can also induce phase III, but one that starts in the duodenum. The stomach depends more on the vagus nerve, whereas the small bowel is regulated mainly by sensory neurons within the enteric nervous system, the nerve circuits that belong to the gut itself. A truncal vagotomy cuts the main trunks of the vagus nerve that go to the stomach and gut, so the stomach's cycle is disrupted more, and the small-bowel cycle is relatively preserved.