migrating motor complex

**Definition:** The MMC is a cyclic pattern of smooth muscle activity in the fasting state that migrates from the stomach (or lower esophageal sphincter region) to the terminal ileum. Cycles repeat about every 90-120 min until the next meal.

**Four phases:** - Phase I: quiescence, with little or no contractile activity (about 40-60% of the cycle). - Phase II: irregular, intermittent contractions with increasing frequency. - Phase III: the "housekeeping" burst, with intense, regular, high-amplitude peristaltic contractions at the maximal slow-wave frequency (about 3/min in the stomach, 12/min in the duodenum). This phase lasts about 5-10 min and propagates distally. - Phase IV: brief transition back to phase I (often not clearly distinguished).

**Regulation:** - Motilin, released from M cells of the duodenum and jejunum, peaks in phase with phase III. It acts on motilin receptors on smooth muscle and enteric neurons. - Vagal and enteric nervous system input (cholinergic, with a role for nitric oxide) modulates the cycle. - Feeding abolishes the MMC and replaces it with the fed pattern (irregular segmentation and peristalsis). The fed state raises insulin, CCK, and gastrin, and suppresses motilin. - Somatostatin can induce phase III-like activity (octreotide is used for this in scleroderma-related dysmotility). Opioids suppress the MMC.

**Function:** - Clears residual undigested food, desquamated cells, and secretions from the stomach and small bowel. - Limits bacterial overgrowth by moving bacteria to the colon and keeping the small bowel relatively sterile. - Large indigestible particles that are too big to pass through the pylorus during the fed state (>2 mm) are emptied during phase III.

**Clinical significance:** - SIBO (small intestinal bacterial overgrowth): loss or impairment of the MMC predisposes to stasis and bacterial overgrowth. Causes include diabetic autonomic neuropathy, systemic sclerosis (scleroderma), pseudo-obstruction, and opioid use. Findings: bloating, diarrhea or steatorrhea (bacterial deconjugation of bile salts), and vitamin B12 deficiency (bacteria consume B12) with elevated folate (bacteria produce it). - Gastroparesis: abnormal MMC activity, especially with diabetes. - Prokinetics: erythromycin (and azithromycin), a motilin receptor agonist, induces phase III-like contractions and is used for gastroparesis, but tachyphylaxis occurs. Low-dose erythromycin can also be used before endoscopy in upper GI bleeding to clear blood. Octreotide stimulates the MMC and is used in scleroderma. Prucalopride is a 5-HT4 agonist used for dysmotility and constipation. - Bezoar formation (failure to clear indigestible material). - Mnemonic: "Motilin = Move it out" during fasting. Fasting triggers the MMC and feeding turns it off.

**High-yield board points:** - MMC occurs only in the fasting (interdigestive) state. - Motilin is the key hormone; its agonist is erythromycin. - Loss of the MMC leads to SIBO. - Interstitial cells of Cajal generate the slow waves that set the frequency of phase III contractions.