In most parts of the digestive tract, the muscularis externa consists of two concentric and relatively thick layers of smooth muscle. The cells in the inner layer form a tight spiral, described as a circularly oriented layer; those in the outer layer form a loose spiral, described as a longitudinally oriented layer. Located between the two muscle layers is a thin connective tissue layer. Within this connective tissue lies the myenteric plexus (Auerbach's plexus) containing nerve cell bodies (ganglion cells) of postganglionic parasympathetic neurons and neurons of the enteric nervous system, as well as blood vessels and lymphatic vessels.
Contractions of the muscularis externa mix and propel the contents of the digestive tract
Contraction of the inner circular layer of the muscularis externa compresses and mixes the contents by constricting the lumen; contraction of the outer, longitudinal layer propels the contents by shortening the tube. The slow, rhythmic contraction of these muscle layers under the control of the enteric nervous system produces peristalsis, i.e., waves of contraction. Peristalsis is marked by constriction and shortening of the tube, which moves the contents through the intestinal tract.
A few sites along the digestive tube exhibit variations in the muscularis externa. For example, in the wall of the proximal portion of the esophagus (pharyngoesophageal sphincter) and around the anal canal (external anal sphincter), striated muscle forms part of the muscularis externa. In the stomach, a third, obliquely oriented layer of smooth muscle is present deep to the circular layer. Finally, in the large intestine, part of the longitudinal smooth muscle layer is thickened to form three distinct, equally spaced longitudinal bands called teniae coli. Dur ing contraction, the teniae facilitate shortening of the tube to move its contents.
The circular smooth muscle layer forms sphincters at specific locations along the digestive tract
At several points along the digestive tract the circular muscle layer is thickened to form sphincters or valves. From the oropharynx distally, these structures include
• Pharyngoesophageal sphincter. Actually, the lowest part of the cricopharyngeus muscle is physiologically referred to as the superior esophageal sphincter. It prevents the entry of air into the esophagus. The inferior esophageal sphincter creates a pressure difference between the esophagus and stomach that prevents reflux of gastric contents into the esophagus.
• Pyloric sphincter. Located at the junction of the pylorus of the stomach and duodenum (gastroduode-nal sphincter), it controls the release of chyme, the partially digested contents of the stomach, into the duodenum.
• Ileocecal valve. Located at the junction of the small and large intestines, it prevents reflux of the contents of the colon with its high bacterial count into the distal ileum, which normally has a low bacterial count.
• Internal anal sphincter. This, the most distally located sphincter, surrounds the anal canal and prevents passage of the feces into the anal canal from the undistended rectum.
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