DigestivePhysiological process

Gastrointestinal motility

Also known as: peristalsis, segmentation, migrating motor complex, MMC, gastrocolic reflex, slow waves, mass movements

GI motility is the coordinated contraction of gut smooth muscle that mixes and propels contents. It includes peristalsis, segmentation, the migrating motor complex and colonic mass movements, and it is paced by interstitial cells of Cajal and run by the enteric nervous system.

Show on body map Wall of the Gastrointestinal Tract

Overview

Slow wavesStomach ≈3/min · duodenum ≈12/min · ileum ≈8–9/min

MMC cycle≈90–120 min (fasting)

TransitStomach 2–4 h · small bowel 3–5 h · colon 12–48 h

Electrical basis. Interstitial cells of Cajal produce slow waves, a basal electrical rhythm of membrane depolarisations. Their frequency falls along the gut: about 3/min in the stomach, about 12/min in the duodenum and about 8–9/min in the terminal ileum. Slow waves alone do not cause contraction. When excitatory input (ACh, stretch) pushes a slow wave above threshold, Ca²⁺ spike potentials fire and the muscle contracts. The slow-wave frequency therefore sets the maximum contraction rate, and the number of spikes sets the force.

Peristalsis follows the "law of the intestine" described by Bayliss and Starling. Distension stimulates intrinsic primary afferent neurons, partly through serotonin released from enterochromaffin cells. The myenteric circuit then produces contraction above the bolus (ACh, substance P) and relaxation below it (NO, VIP, ATP), so the bolus is pushed aborally. Peristalsis needs an intact myenteric plexus. Segmentation is non-propulsive, alternating ring contractions of the circular muscle in adjacent segments. It mixes chyme and keeps bringing it into contact with the mucosa, and it is the main pattern in the small intestine after a meal.

Interdigestive pattern. In the fasting state the migrating motor complex (MMC) recurs about every 90–120 min. Phase I is quiescence. Phase II brings irregular contractions. Phase III is a 5–10-minute burst of intense, regular contractions that travels from the stomach to the terminal ileum. Phase III acts as an "intestinal housekeeper", sweeping out undigested debris and bacteria. It is triggered by motilin and is accompanied by hunger rumbles. Eating abolishes it immediately.

Parts & subdivisions

Gastric accommodation and emptying
Receptive relaxation of the fundus (vagal NO and VIP) accommodates the meal without a rise in pressure. Antral peristalsis about 3 times a minute with retropulsion grinds food. Only particles of about 1–2 mm pass the pylorus. The enterogastric reflex, CCK, secretin and GIP slow emptying.
Peristalsis
Propulsive reflex of oral contraction and anal relaxation, coordinated by the myenteric plexus.
Segmentation
Mixing contractions in the fed small intestine. The proximal-to-distal fall in frequency gives a slow net aboral drift.
Migrating motor complex
Fasting "housekeeper" wave every 90–120 min, triggered by motilin. Erythromycin (a motilin agonist) is used as a prokinetic.
Colonic motility
Haustral (segmental) contractions churn and dry the faeces slowly. Mass movements are powerful propulsive waves, 1–3 times a day, that move faeces to the rectum. Retrograde activity in the proximal colon prolongs the time available for absorption.
Gastrocolic and gastroileal reflexes
Food in the stomach triggers colonic mass movements (hence the urge to defecate after breakfast) and relaxation of the ileocaecal sphincter. Both reflexes run through enteric, vagal and hormonal (gastrin, CCK) pathways.
Inhibitory reflexes
The enterogastric reflex (duodenal acid, fat or distension inhibits gastric motility) and the intestino-intestinal reflex (gross distension of one segment relaxes the rest). Peritoneal irritation causes reflex ileus.
Vomiting
A coordinated medullary reflex triggered by gut afferents (vagal 5-HT3), the chemoreceptor trigger zone (area postrema; D2, 5-HT3, NK1 receptors), the vestibular system and higher centres. Retroperistalsis is followed by forceful abdominal contraction against a closed glottis.

Function

Mixes food with secretions and keeps it in contact with the absorptive mucosa.
Moves contents aborally at a rate that allows digestion and absorption.
Clears residue and bacteria between meals.
Stores contents and expels them under control.

Regulation: Intrinsic: interstitial cells of Cajal and the enteric nervous system. Extrinsic: parasympathetic activity increases motility and relaxes sphincters; sympathetic activity reduces motility and contracts sphincters. Hormonal: gastrin, CCK, motilin and ghrelin stimulate; secretin, GIP, GLP-1 and PYY inhibit (ileal brake).

Clinical significance

Ileus and gastroparesis

Paralytic ileus is a functional, non-mechanical failure of propulsion. Causes include abdominal surgery, peritonitis, hypokalaemia, opioids and anticholinergics. Diabetic gastroparesis, caused by autonomic neuropathy and loss of interstitial cells of Cajal, produces early satiety, nausea and erratic glycaemic control. Prokinetics such as metoclopramide and erythromycin can help.

Small intestinal bacterial overgrowth

Loss of MMC activity (diabetes, systemic sclerosis, opioids), blind loops or loss of the ileocaecal valve let bacteria colonise the small bowel. Bloating, diarrhoea, B12 deficiency (bacteria consume it) and fat malabsorption from bile-acid deconjugation follow.

Irritable bowel syndrome

A disorder of gut–brain interaction with altered motility, visceral hypersensitivity and abnormal central processing. Recurrent abdominal pain is linked to defecation and altered stool form, with no structural disease.

Test yourself

The migrating motor complex (MMC) is best described as:

A fasting "housekeeper" pattern recurring every 90–120 min, initiated by motilin
A post-prandial mixing pattern triggered by CCK
Colonic mass movements triggered by gastric distension
Receptive relaxation of the gastric fundus

Answer: A fasting "housekeeper" pattern recurring every 90–120 min, initiated by motilin

In the fasting state, phase III contractions sweep from the stomach to the terminal ileum about every 90–120 min under the influence of motilin, clearing residue and bacteria. Eating abolishes it. Erythromycin, a motilin agonist, is used as a prokinetic.