The innervation of the gastrointestinal tract is complex. The myenteric and submucosal plexuses contain many interneurons. These possess a number of neurotransmit-ters and neuromodulators, including several peptides, such as enkephalins, substance P, and vasoactive intestinal peptide. Reflex activity within the plexuses regulates peristalsis and secretion locally. The effects of sympathetic and parasympathetic nerve stimulation are superimposed on this local neural regulation.
The myenteric and submucosal plexuses contain ganglion cells giving rise to excitatory cholinergic fibers that directly innervate the smooth muscle and gland cells of the gut. The sympathetic fibers that enter the gastrointestinal tract are postganglionic noradrenergic fibers, stimulation of which inhibits gut motility and gland secretion and contracts sphincters. Most of the noradrenergic fibers terminate either in blood vessels or on the cholinergic ganglionic cells of the intramural plexuses. These fibers alter gut motility by inhibiting acetylcholine release from the intramural nerves. Direct noradrenergic innervation of smooth muscle of the non-sphincter portion of the gut is sparse.
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