GI Motility and Pacemakers
Overview
GI motility is the coordinated muscular activity that propels food from the oropharynx to the rectum. Its baseline rhythm is set by Interstitial Cells of Cajal (ICC), acting as the pacemaker cells, embedded within the myenteric plexus.
Slow Waves and Action Potentials
ICCs generate rhythmic slow waves (Basic Electrical Rhythm), which are subthreshold oscillations. Smooth muscle contraction occurs ONLY when a slow wave is depolarised to threshold, generating a calcium-driven action potential (spike potential) triggered by stretch or neural input.
- Stomach: ~3 waves/min (sets maximum contraction frequency).
- Duodenum: ~12 waves/min (fastest BER in the gut).
- Ileum: ~8-9 waves/min.
- Rectum: ~2-3 waves/min.
Enteric Nervous Plexuses
- Auerbach's Plexus (Myenteric): Between circular and longitudinal muscle. Controls peristalsis.
- Meissner's Plexus (Submucosal): Controls mucosal secretion and local blood flow.
WarningHirschsprung's Disease
Congenital absence of both myenteric and submucosal ganglia in a distal colonic segment (always including the internal anal sphincter). Permanent tonic contraction of the aganglionic segment causes functional large bowel obstruction and massive proximal dilatation (megacolon). Classic barium enema finding: narrow aganglionic segment tapering to a 'transition zone', then massively dilated proximal bowel.
High Yield Facts
LightbulbFRCR / MD Prep Pearl
Gastric emptying scintigraphy: Tc-99m labelled solid meal. Gastric half-time (T1/2) >90 min = gastroparesis. Aetiology: Diabetes (neuropathy), post-vagotomy, hypothyroidism, medications (opiates, anticholinergics). Erythromycin is a motilin agonist used to stimulate GI motility in gastroparesis.