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.
