Intrinsic Innervation of Alimentary Tract
From the esophagus to the rectum, the intrinsic innervation of the alimentary tract is effected through the enteric nervous system. This network is composed of numerous ganglion cells interconnected by their axons and dendrites. They are found primarily in two locations, between the longitudinal and circular layers of the muscularis externa, as the myenteric (Auerbach) plexus, and between the muscularis externa and the submucosa, as the submucosal (Meissner) plexus. The former is relatively coarse, and its meshes consist of thick, medium, and thin bundles of fibers, which are described as its primary, secondary, and tertiary parts. The submucosal plexus is more delicate. Other subsidiary plexuses have been described, such as a rarefied subserous plexus in those parts of the alimentary canal covered by peritoneum, but minute details of these need not be given.
The enteric plexuses vary in pattern and density in different parts of the alimentary tract and in different species of animals. They are well developed in the regions from the stomach to the lower end of the rectum and are less well formed in the esophagus, particularly in its upper half, which is primarily skeletal muscle. The ganglion cells are also not distributed uniformly; thus, the density of cell distribution in the myenteric plexus is lowest in the esophagus, rises steeply in the stomach until it reaches its peak at the pylorus, falls to an intermediate level throughout the small intestine, and then increases again along the colon and especially in the rectum. The density of cell population in the submucosal plexus seems to run roughly parallel to that in the myenteric plexus.
The enteric plexuses contain postsynaptic sympathetic axons, as well as presynaptic and postsynaptic parasympathetic axons. These exist alongside afferent axons from the alimentary tract, and the intrinsic ganglia of each plexus. Vagal presynaptic fibers form synapses with the ganglion cells whose axons are the postsynaptic parasympathetic fibers. The sympathetic presynaptic fibers have already relayed in paravertebral or prevertebral ganglia, and so the sympathetic fibers in the plexuses pass through to their terminations without synaptic interruptions. The afferent fibers from the esophagus, stomach, and duodenum are carried to the brainstem and cord through the vagal and sympathetic nerves supplying these parts, but they form no synaptic connections with the ganglion cells in the enteric plexuses.
Two chief forms of nerve cells, types I and II, occur in the enteric plexus. Type I cells are multipolar and confined to the myenteric plexus, and their dendrites branch close to the parent cells. Their axons run for varying distances through the plexuses to establish synapses with cells of type II, which are more numerous and are found in both myenteric and submucosal plexuses. Most type II cells are multipolar, and their longer dendrites proceed in bundles for variable distances before ramifying in other cell clusters. Many of their axons pass outward to end in the muscle coats, and others proceed inward to supply the muscularis mucosae and to ramify around vessels and between epithelial secretory cells; their distribution suggests that they are motor or secretomotor in nature.
Another group of cells in the area are the interstitial cells of Cajal, which are associated with the terminal networks of all autonomic nerves and act as pacemaker cells of the smooth muscle layers of the gastrointestinal tract. The frequency of this pacemaker activity varies between different organs. Under experimental conditions peristaltic movements occur in isolated portions of the gut, indicating the importance of the intrinsic neuromuscular mechanism, but the parasympathetic and sympathetic nerves regulate the activity of the gut tube. Local reflex arcs may exist in the enteric plexuses; this possibility is supported by the observation that in addition to type I and II multipolar cells, much smaller numbers of pseudounipolar and bipolar cells can be detected in the submucosa, potentially acting as the afferent links in local reflex arcs.
In congenital megacolon (Hirschsprung disease) the enteric plexuses are apparently undeveloped or degenerated over a segment of the alimentary tract, although the extrinsic nerves are intact. The affected segment is tonically contracted and peristaltic movements are defective or absent. This results in distenti n of the proximal region of the gastrointestinal tract.