Which mechanism is responsible for the 'gastrocolic reflex' that produces the urge to defaecate after a meal?

• A Osmoreceptors in the duodenum detect hyperosmotic chyme and reflexly stimulate colonic motor activity via sympathetic fibres
• B Caloric density of the meal directly stimulates colonic smooth muscle via absorbed fatty acid signals
• C Serotonin from enterochromaffin cells in the stomach reflexly inhibits the ileal brake to allow rapid colonic filling
• D Gastric distension activates long vagal and intrinsic enteric reflex arcs that increase colonic peristaltic waves, mediated in part by cholecystokinin and gastrin ✓

Explanation

The gastrocolic reflex is triggered by gastric distension (mechanical) and nutrient entry into the small bowel. It is mediated by both extrinsic vagal reflexes and intrinsic enteric nervous system connections. Hormones — particularly gastrin (from antral G cells) and cholecystokinin (from duodenal I cells) — amplify colonic mass movements (peristaltic rushes). This explains the post-prandial urgency, especially in patients with irritable bowel syndrome where the reflex is exaggerated. Sympathetic pathways generally inhibit motility; the ileal brake is a feedback inhibition triggered by ileal fat, distinct from the gastrocolic reflex.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.