GI Motility and the Enteric Nervous System
Key concepts on smooth muscle organization, slow waves, enteric plexuses, and region-specific motor patterns from swallowing to gastric emptying.
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Questions Covered in This Set
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What are slow waves and what generates them?
Rhythmic oscillations of smooth muscle membrane potential (~−60 to −40 mV) generated by interstitial cells of Cajal (ICC). They don't cause contraction themselves — only if their peak crosses threshold do Ca²⁺ spikes fire and produce force.
What determines the strength of a GI smooth muscle contraction?
The number of action potentials (spikes) riding on the slow wave peak; slow waves set maximum frequency and direction, while neural/hormonal input determines whether contraction occurs at all.
Slow wave frequencies by region
Stomach ~3/min, duodenum ~12/min, ileum ~8/min, colon ~3–8/min — a descending gradient that biases net movement aborally.
Myenteric vs submucosal plexus
Myenteric (Auerbach's), between circular and longitudinal muscle, controls motility. Submucosal (Meissner's) controls secretion and local blood flow.
Describe the peristaltic reflex neurotransmitters
Behind the bolus: ACh and substance P → circular muscle contraction. Ahead of the bolus: VIP and nitric oxide → receptive relaxation. Result is a propulsive ring travelling aborally.
Extrinsic autonomic control of the gut
Parasympathetic (vagus to transverse colon, pelvic nerves below) is mostly excitatory. Sympathetic (prevertebral ganglia, noradrenaline) inhibits motility and contracts sphincters. Both modulate rather than replace the ENS.
What is Hirschsprung's disease?
Failed neural crest cell migration → absent myenteric ganglia in distal colon. Loss of inhibitory NO/VIP neurons gives a tonically contracted narrow aganglionic segment with massive proximal dilatation.
Three phases of swallowing
Oral (voluntary); pharyngeal (~1 s, hard-wired medullary reflex — soft palate elevates, larynx rises, epiglottis tilts, respiration inhibited, UES relaxes); oesophageal (peristaltic wave).
Primary vs secondary peristalsis in the oesophagus
Primary: the swallow-initiated wave travelling ~3–5 cm/s. Secondary: triggered by local distension from food left behind.
What is achalasia?
Loss of inhibitory myenteric neurons → LES fails to relax plus absent oesophageal peristalsis; classic 'bird's beak' appearance on barium swallow. (Excess LES relaxation instead causes GORD.)
What is receptive relaxation?
Vagovagal reflex, NO/VIP-mediated relaxation of the proximal stomach allowing it to accommodate a swallowed meal without a large rise in pressure.
Why is GI smooth muscle a functional syncytium?
Circular and longitudinal muscle cells are electrically coupled by gap junctions, so they behave as one sheet rather than thousands of independent fibres.
