The muscularis mucosae, the deepest portion of the mucosa, consists of smooth muscle cells arranged in an inner circular and outer longitudinal layer. Contraction of this muscle produces movement of the mucosa, forming ridges and valleys that facilitate absorption and secretion. This localized movement of the mucosa is independent of the peristaltic movement of the entire wall of the digestive tract.
The submucosa consists of a dense, irregular connective tissue layer containing blood and lymphatic vessels, a nerve plexus, and occasional glands.
The submucosa contains the larger blood vessels that send branches to the mucosa, muscularis externa, and serosa. The submucosa also contains lymphatic vessels and a nerve plexus. The extensive nerve network in the submucosa contains visceral sensory fibers mainly of sympathetic origin, parasympathetic (terminal) ganglia, and preganglionic and postganglionic parasympathetic nerve fibers. The nerve cell bodies of parasympathetic ganglia and their postganglionic nerve fibers represent the enteric nervous system, the third division of the autonomic nervous system. This system is primarily responsible for innervating the smooth muscle layers of the alimentary canal and can function totally independently of the central nervous system. In the submucosa, the network of unmyelinated nerve fibers and ganglion cells constitute the submucosal plexus (also called Meissner’s plexus).
As noted, glands occur occasionally in the submucosa in certain locations. For example, they are present in the esophagus and the initial portion of the duodenum. In histologic sections, the presence of these glands often aids in identifying the specific segment or region of the tract.
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 (also called 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 into 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. During 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 the following:  Pharyngoesophageal sphincter. Actually, the lowest part of the cricopharyngeus muscle is physiologically referred to as the superior (upper) esophageal sphincter. It prevents the entry of air into the esophagus.