GI Motility

GI Motility

Pacemaker Cells and Slow Waves

Interstitial cells of Cajal (ICCs) are the pacemaker cells of the GI tract. They generate rhythmic slow waves (oscillations in membrane potential). When a slow wave reaches threshold (modified by enteric/autonomic nervous input and hormones), it triggers an action potential → smooth muscle contraction. Slow wave frequency determines the maximum contraction frequency: ~3/min in stomach, ~12/min in duodenum, ~8/min in ileum.

Motility Patterns

Peristalsis: propulsive; coordinated contraction proximal to and relaxation distal to a bolus. Involves both circular (contracts orad, relaxes aborad) and longitudinal muscle (shortens segment). Driven by the myenteric plexus (law of the intestine).

Segmentation: mixing; rhythmic, localised contractions of circular muscle alternate between contracting and relaxing — does NOT propel content net forward. Maximises contact with epithelium for digestion and absorption. Predominant pattern in small intestine postprandially.

Migrating Motor Complex (MMC): interdigestive (between meals); a wave of organised motility that sweeps the entire GI tract every ~90 minutes, clearing residual contents and bacteria. Initiated by motilin (released ~4–5 hours after a meal). Three phases: I (rest/quiescence), II (irregular activity), III (intense activity — "housekeeper"). Eating terminates the MMC and initiates postprandial segmentation.

Regional Motility

Mouth/Oropharynx: mastication (voluntary + reflex), salivation. Swallowing reflex: three stages — oral (voluntary propulsion to pharynx), pharyngeal (< 2 s, involuntary, trachea closed by epiglottis), oesophageal (peristaltic wave). Lower oesophageal sphincter (LOS) relaxes to admit bolus; tonic LOS contraction prevents reflux.

Stomach: proximal stomach (fundus + body) acts as a reservoir — receptive relaxation (vagally mediated) accommodates up to 1.5 L. Distal stomach (antrum) generates peristaltic contractions that grind solid food and propel it through the pylorus. Retropulsion (liquid squirts back from pylorus) aids mixing. Gastric emptying rate regulated by duodenal feedback (fat and acid delay emptying via CCK, enterogastric reflexes).

Large intestine: segmental contractions allow prolonged water and electrolyte absorption. Mass movements (3–4/day, typically postprandial — gastrocolic reflex) propel contents to the rectum. Defaecation reflex: rectal distension → internal anal sphincter relaxes (involuntary); external anal sphincter (voluntary, pudendal nerve) allows defaecation.