Enteric Plexuses

The myenteric (Auerbach) plexus is relatively coarse, with thicker meshes and larger ganglia. The main, or primary, meshes give off fascicles that form secondary networks in the interstices of the primary networks. These, in turn, split into minute bundles of fibers that ramify between the muscle tunics and supply them. The submucous (Meissner) plexus is more delicate and its meshes are more irregular. Its delicate offshoots mostly end in relation to cells forming the muscularis mucosae or form rarefied periglandular plexuses, while other offshoots end in almost invisible subepithelial plexuses.

The patterns and densities of these plexuses vary in different parts of the alimentary tract. They are less well defined in the upper part of the esophagus but are well developed from the stomach to the lower end of the rectum. The ganglia are not uniformly distributed. The density of ganglionic cells in the plexuses is lowest in the esophagus, rises steeply in the stomach until it reaches a peak at the pylorus, falls to an intermediate level throughout the small intestines, and gradually increases along the colon to reach another, lesser peak in the rectum.

The extrinsic nerves involved contain efferent and afferent sympathetic and parasympathetic fibers derived from thoracic, lumbar, and sacral branches of the sympathetic trunks and from the vagus and pelvic splanchnic nerves. Most of the sympathetic efferent fibers entering the enteric plexuses are postganglionic, while parasympathetic efferent fibers are still preganglionic. The vagal fibers form synapses with ganglion cells located in the enteric plexuses, from the esophagus to the distal third of the transverse colon; below this level, the preganglionic parasympathetic fibers are carried in branches of the pelvic splanchnic nerves. Thus in this as in other situations, the parasympathetic postganglionic fibers are very much shorter than their sympathetic counterparts.

Many interconnections exist between the myenteric and submucous plexuses. In general, the former are mainly concerned with the innervation of the muscle layers in the visceral walls, whereas the latter are chiefly involved with supplying the glands and muscularis mucosae and in forming delicate subepithelial plexuses. The enteric plexuses and their subdivisions are also responsible for supplying adjacent vessels and transmitting sensory impulses. The sympathetic innervation is primarily inhibitory to peristalsis and stimulatory to the sphincters, while the parasympathetic innervation is the opposite.

Afferent fibers from the alimentary tract are conveyed to the central nervous system (CNS) through the same sympathetic and parasympathetic nerves that carry the corresponding efferent fibers. There is also evidence that local reflex arcs exist.

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