Describe the movements of the gastrointestinal tract. Explain the law of the gut and the migrating motor complex.

Introduction

Electrical Basis — Slow Waves

• Basic electrical rhythm (BER) — spontaneous, rhythmic partial depolarisations of gut smooth muscle
• Generated by the interstitial cells of Cajal — the pacemaker of the gut
• Slow waves themselves do not cause contraction; contraction occurs only when spike potentials ride on their crest

Enteric Nervous System — the "little Brain"

• Contains about 100 million neurones — as many as the spinal cord
• Can function independently of the CNS; extrinsic nerves only modulate it

Types of Movement

Law of the Gut (myenteric Reflex)

Bolus distends the gut → Behind the bolus — circular muscle contracts (ACh, substance P) → Ahead of the bolus — circular muscle relaxes (VIP, NO) → Longitudinal muscle ahead contracts → Bolus propelled orally to aborally

• Stated by Bayliss and Starling
• Peristalsis is therefore polarised — it travels one way only

Migrating Motor Complex

• Occurs in the fasting state only; abolished by a meal
• Cycles every 90–120 minutes
• Mediated by motilin
• Function — the "intestinal housekeeper"; sweeps residual debris and bacteria onward, preventing bacterial overgrowth
• Erythromycin is a motilin agonist — used as a prokinetic

Gastric Emptying and Defaecation

• Gastric emptying — promoted by gastrin and distension; delayed by fat (the most potent), acid, hypertonicity and protein in the duodenum via the enterogastric reflex and CCK
• Order of emptying — carbohydrate > protein > fat
• Defaecation reflex — rectal distension → sacral cord (S2–S4) → contraction of rectum, relaxation of internal sphincter; external sphincter is under voluntary control via the pudendal nerve

Control of Motility

Important gastrointestinal reflexes

• Gastrocolic reflex — food in stomach → ↑ colonic motility → the urge to defaecate after a meal
• Enterogastric reflex — duodenal distension or acid → ↓ gastric emptying
• Gastroileal reflex — ↑ gastric activity → ↑ ileal emptying into the colon

Applied Aspects

• Achalasia cardia — loss of myenteric neurones at the lower oesophageal sphincter → failure to relax → dysphagia and dilated oesophagus
• Hirschsprung disease — congenital absence of ganglion cells in the distal colon → that segment cannot relax → functional obstruction with proximal megacolon
• Paralytic ileus — after abdominal surgery or peritonitis; motility stops → distension and absent bowel sounds
• Dumping syndrome — after gastrectomy, rapid emptying of hypertonic chyme → fluid shift into the gut → palpitations, sweating, faintness