Migrating motor complex
Migrating motor complexes (or migrating myoelectric complex or migratory motor complex or migratory myoelectric complex or MMC) are waves of electrical activity that sweep through the intestines in a regular cycle during fasting. These motor complexes trigger peristaltic waves, which facilitate transportation of indigestible substances such as bone, fiber, and foreign bodies from the stomach, through the small intestine, past the ileocecal sphincter, and into the colon. MMC activity varies widely across individuals and within an individual when measured on different days. The MMC occurs every 90-230 minutes during the interdigestive phase (i.e., between meals) and is responsible for the rumbling experienced when hungry. It also serves to transport bacteria from the small intestine to the large intestine and to inhibit the migration of colonic bacteria into the terminal ileum and an impairment to the MMC typically results in small intestinal bacterial overgrowth.
- Phase I – A prolonged period of quiescence (40-60% of total time);
- Phase II – Increased frequency of action potentials and smooth muscle contractility (20-30% of total time);
- Phase III – A few minutes of peak electrical and mechanical activity (5-10 minutes);
- Phase IV – Declining activity which merges with the next Phase I.
Movements of the small bowel are believed to be controlled by the central and enteric nervous systems, intestinal muscles, and numerous peptides and hormones. For example, the MMC is thought to be partially regulated by motilin, which is initiated in the stomach as a response to vagal stimulation, and does not directly depend on extrinsic nerves. Additionally, gastrin, insulin, cholecystokinin, glucagon, and secretin have been reported to disrupt the MMC.
Eating interrupts the MMC. For example, one study found that a continental breakfast of 450 Kcal causes the MMC to disappear for 213 ± 48 minutes. The number of calories and nature of food determine the length of the disruption with fats causing a longer disruption than carbohydrates which in turn cause a longer disruption than protein.
Autoimmunity following infection by a pathogen producing CdtB, such as C. jejuni, may be the leading cause of MMC impairment. Narcotics are also known to impair the MMC. Stress has been shown to reduce MMC activity as well.
Drugs used to enhance gastrointestinal motility are generally referred to as prokinetics. Serotonin induces phase III of the MMC and so serotonin receptor agonists are commonly administered as prokinetics. Motilin administration causes phase III contractions and so motilin agonists are another common prokinetic.
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