Gut wall, sphincters and secretory cells

Target & receptor · M3 wall & glands α2 presynaptic β2 wall α1 sphincters

Parasympathetic input (vagus to the splenic flexure, pelvic splanchnics beyond it) increases motility and secretion and relaxes sphincters through enteric NO/VIP neurones; sympathetic input inhibits enteric cholinergic neurones, relaxes the wall and contracts sphincters. The enteric plexuses execute peristalsis; the autonomic inputs set the gain.

Traced from the start

Detail

Parasympathetic territory: Vagus: oesophagus to splenic flexure; pelvic splanchnic S2–S4: descending colon to anal canal
Parasympathetic actions: M3 contraction of wall, relaxation of sphincters via NO/VIP, gastric acid (M3 on parietal cells plus gastrin), pancreatic enzyme secretion
Sympathetic actions: α2 inhibition of enteric acetylcholine release, β2 wall relaxation, α1 contraction of pyloric, ileocaecal and internal anal sphincters, splanchnic vasoconstriction

When it goes wrong

Abdominal surgery, opioids and electrolyte disturbance raise sympathetic inhibitory tone on the enteric plexus

Postoperative ileus: silent distended abdomen, absent flatus, vomiting; managed with restriction of opioids, mobilisation and correction of potassium

Acute colonic pseudo-obstruction (Ogilvie syndrome) in an elderly immobile inpatient

Massive colonic dilatation without mechanical block; neostigmine (cholinesterase inhibitor) restores M3 drive and decompresses the colon — give with atropine and cardiac monitoring for bradycardia

Practise this structure

1 question in the bank tagged Autonomic Control.

A 46-year-old man attends the clinic nine months after cardiac transplantation. He feels well, has no chest pain and walks four kilometres most days. He takes n…

Foundations · middle twist