Extrinsic (Autonomic) Nervous System
The enteric nervous system is situated within the walls of the GI tract, figure. It consists of two neural plexuses that extend from the esophagus to the anus. The myenteric plexus (Auerbach's
plexus) lies between the longitudinal and smooth muscle layers. The submucosal (Meissner's) plexus is found between the circular smooth muscle layer and the mucosa.
The plexuses consist of networks of ganglia connected by interneurons that facilitate information interchange among the parts of the system. The ganglia within each plexus are connected by interneurons, and the myenteric and submucosal plexuses are also linked by interconnecting nerve fibers.
Both afferent and efferent nerve fibers are present. Efferent fibers supply smooth muscle, epithelial, and endocrine cells. Sensory receptors in the gut wall include mechanoreceptors, chemoreceptors, and pain receptors. Afferent fibers from the sensory organs transmit information within and among the plexuses and also relay information to the CNS.
It has been proposed that, because of their anatomic location, the myenteric plexus controls the motility of the longitudinal and circular muscle and the submucosal plexus controls mucosal tissue activity. However, because of the extensive interconnections among the plexuses, it is likely that both
plexuses effect both motility and the secretory and absorptive functions of the mucosal tissue.
The enteric nervous system has been described as a visceral brain capable of maintaining motor and secretory functions of the GI tract independently of neural inputs from the CNS. It has been suggested that the enteric nervous system contains neural circuits that are programmed for unique patterns of activity. The role of the extrinsic innervation, provided by the parasympathetic and sympathetic systems, is to modulate these predetermined enteric system activity patterns.