CorpusHuman Anatomy
DigestiveTissue

Muscularis externa

Tunica muscularis

Also known as: muscularis propria, muscular layer of gut

The muscularis externa is the main muscle coat of the gut. An inner circular and an outer longitudinal layer of smooth muscle, with the myenteric plexus between them, mix and propel the contents.

Show on body map Wall of the Gastrointestinal Tract

Overview

The inner circular layer narrows the lumen. Its contractions produce segmentation and the constricting part of peristalsis, and it thickens to form sphincters: pyloric, ileocaecal and internal anal. The outer longitudinal layer shortens the gut. The myenteric (Auerbach) plexus and a network of interstitial cells of Cajal (ICC) lie between the two.

There are regional variations. The upper third of the oesophagus has skeletal muscle. The stomach has an additional inner oblique layer for churning. In the colon the longitudinal layer is concentrated into the three taeniae coli. The external anal sphincter is skeletal muscle.

Visceral smooth muscle cells are electrically coupled by gap junctions and behave as a functional syncytium. ICC generate slow waves, rhythmic oscillations of membrane potential. These do not cause contraction on their own. When a slow wave is brought above threshold by ACh or stretch, L-type Ca²⁺ channels fire spike potentials, and contraction follows. The force increases with the number of spikes.

Location

Between the submucosa and the serosa or adventitia along the whole alimentary canal.

Parts & subdivisions

  1. Inner circular layer
    Constriction, segmentation, sphincters.
  2. Outer longitudinal layer
    Shortening. Forms the taeniae coli in the colon.
  3. Inner oblique layer (stomach)
    Extra innermost layer for churning and grinding.
  4. Myenteric (Auerbach) plexus
    Enteric ganglia between the layers that coordinate motility.
  5. Interstitial cells of Cajal
    Pacemaker cells generating slow waves (about 3/min in the stomach, 12/min in the duodenum, 8–9/min in the ileum).

Anatomy

Histology

Two thick smooth-muscle layers (inner circular, outer longitudinal) with ganglia of the myenteric plexus between them. There are three layers in the stomach, and skeletal muscle in the upper oesophagus.

Function

  • Mixing (segmentation, churning).
  • Propulsion (peristalsis, mass movements).
  • Sphincter control.
Physiology

Slow waves set the maximum contraction frequency. Spike potentials (Ca²⁺) trigger contraction. Parasympathetic ACh and stretch increase spiking. Noradrenaline and inhibitory enteric transmitters (NO, VIP) hyperpolarise the muscle and relax it.

Clinical significance

Gastrointestinal stromal tumour (GIST)

A mesenchymal tumour arising from interstitial cells of Cajal or their precursors, most often in the stomach. It has activating KIT (CD117) or PDGFRA mutations and responds to the tyrosine-kinase inhibitor imatinib.

Hypertrophic pyloric stenosis and achalasia

Hypertrophy of the pyloric circular muscle blocks gastric emptying in infants. In achalasia, loss of inhibitory myenteric neurons prevents the LES (circular muscle) from relaxing.

Compiled from standard anatomy and physiology references — see the Digestive System sources. For education only; not medical advice.