myenteric plexus

**Location:** Lies between the inner circular and outer longitudinal smooth muscle layers of the muscularis externa, extending from esophagus to anus. Mnemonic: "Auerbach = Between muscle layers" (Meissner = submucosa; "Meissner's = Mucosa-side").

**Function:** Part of the enteric nervous system (ENS), the "little brain of the gut." Primarily regulates motor activity: coordinates peristalsis and segmentation by controlling smooth muscle tone and contraction of both muscle layers. It also influences secretion and blood flow to a lesser degree (those are chiefly submucosal/Meissner plexus functions).

**Neurotransmitters:** - Excitatory (contraction): acetylcholine and substance P (via muscarinic M3 receptors on smooth muscle). - Inhibitory (relaxation): VIP and nitric oxide (NO). These mediate the relaxation of sphincters and the "descending relaxation" part of the peristaltic reflex. - Peristaltic reflex: Oral contraction and anal relaxation of the bowel segment around a bolus.

**Extrinsic modulation:** Parasympathetic (vagus and pelvic splanchnic nerves) synapses on myenteric neurons, increasing motility. Sympathetic input (from the prevertebral ganglia) decreases motility. The ENS can function autonomously without CNS input.

**Embryology:** Derived from neural crest cells that migrate craniocaudally along the gut (vagal neural crest primarily). Failure of migration leads to aganglionosis.

**Clinical correlations (high-yield):** 1. **Hirschsprung disease (congenital aganglionic megacolon):** Absent ganglion cells in both the myenteric and submucosal plexuses of the distal colon (typically rectosigmoid) due to failed neural crest migration. Associated with RET mutations (also seen in MEN2A) and Down syndrome. Presents with failure to pass meconium in the first 48 hours, chronic constipation, and a transition zone on barium enema. The aganglionic segment is tonically contracted, with proximal dilation. Rectal suction biopsy shows absent ganglion cells and hypertrophied nerve fibers with increased acetylcholinesterase staining. Treatment is surgical resection of the aganglionic segment. Complications include enterocolitis and toxic megacolon. 2. **Achalasia:** Loss of inhibitory (NO/VIP) myenteric neurons in the esophagus causes failure of LES relaxation and absent peristalsis. Barium swallow shows a "bird's beak." Manometry shows incomplete LES relaxation and increased LES pressure. Secondary achalasia occurs in **Chagas disease** (*Trypanosoma cruzi* destroys myenteric ganglia), causing megaesophagus and megacolon. Also associated with esophageal cancer risk (squamous cell). 3. **Diabetic gastroparesis and other dysmotility:** Autonomic/enteric neuropathy. 4. **Paraneoplastic achalasia/pseudo-obstruction:** Anti-Hu (ANNA-1) antibodies, classically with small cell lung cancer. 5. **Intestinal pseudo-obstruction:** Impaired motility without mechanical obstruction.

**Distinguish from the submucosal (Meissner) plexus:** Located in the submucosa; mainly controls secretion, local blood flow, and absorption (the sensing of the luminal environment). It is absent in the esophagus and sparse in the stomach. It is also absent in Hirschsprung disease.

**Histology pearls:** Ganglion cells appear as clusters of large neurons with glial support cells between the two muscle layers. Interstitial cells of Cajal (the GI pacemaker cells that generate slow waves) are closely associated with the myenteric plexus.