Meissner plexus

**Location:** Submucosal layer of the gut wall, between the muscularis mucosae and the circular muscle layer. Best developed in the small intestine and colon. Contrast with the Auerbach (myenteric) plexus, which lies between the circular and longitudinal layers of the muscularis externa.

**Function:** Controls mucosal function: secretion (glands, fluid/electrolyte transport), absorption, local blood flow, and sensory input from the mucosa (chemoreceptors, mechanoreceptors). Helps coordinate local reflexes. By contrast, Auerbach's plexus mainly controls motility (peristalsis, segmentation).

**Innervation and neurotransmitters:** - Parasympathetic preganglionic fibers (vagus and pelvic splanchnic nerves, S2-S4) synapse on enteric neurons (nicotinic ACh). Postganglionic enteric neurons release ACh (stimulates secretion) and VIP (stimulates secretion and causes vasodilation). - Sympathetic postganglionic fibers (norepinephrine) inhibit secretion and cause vasoconstriction. - The enteric nervous system can function independently of CNS input (reflex arcs).

**Clinical significance:** - **Hirschsprung disease (congenital aganglionic megacolon):** Failure of neural crest cell migration (caudal to rostral) leads to absence of both Meissner and Auerbach plexuses in the distal colon, usually starting at the rectum. This causes a tonically contracted aganglionic segment with proximal dilation, delayed passage of meconium (>48 hours), and chronic constipation. Associated with RET mutations (also seen in MEN2A/2B) and with Down syndrome. Diagnosis: rectal suction biopsy showing absent ganglion cells in the submucosa and increased acetylcholinesterase staining of nerve fibers. Barium enema shows a transition zone. Treatment: surgical resection of the aganglionic segment. - **Achalasia:** Involves loss of myenteric (Auerbach) neurons, not Meissner. This is a common distractor. - **Chagas disease:** Destruction of enteric ganglia (Trypanosoma cruzi) causes megaesophagus and megacolon. - **Diabetic autonomic neuropathy** can impair enteric function (gastroparesis, diarrhea or constipation).

**High-yield mnemonic:** "Meissner = Mucosa/Submucosa (secretion)"; "Auerbach = Around (between muscle layers), motility."

**Embryology:** Enteric neurons derive from neural crest cells (vagal and sacral crest) that migrate into the gut wall.