The gut receives extrinsic neural innervation through the parasympathetic and sympathetic divisions of the autonomic nervous system (ANS). Parasympathetic innervation is provided by the vagus and pelvic nerves. The vagus nerve, also known as the 10th cranial nerve, innervates various parts of the gastrointestinal (GI) tract, including the esophagus, stomach, gallbladder, pancreas, first part of the intestine, cecum, and proximal part of the colon. On the other hand, the pelvic nerves innervate the distal part of the colon, anorectal region, and other pelvic organs not part of the GI tract.

The parasympathetic nervous system follows its typical organization, with preganglionic nerve cell bodies located in the brainstem (vagus) or sacral spinal cord (pelvic). Axons from these neurons travel through the vagus and pelvic nerves, respectively, and synapse with postganglionic neurons present in the organ wall. In this case, the postganglionic neurons are enteric neurons within the gut wall. Efferent nerves do not directly innervate effector cells in the gut wall; instead, the transmission always involves a neuron in the enteric nervous system (ENS).

The synapse between preganglionic and postganglionic neurons in the parasympathetic nervous system is an obligatory nicotinic synapse. Acetylcholine is released from the preganglionic nerve terminal and acts on nicotinic receptors present on the postganglionic neuron, which, in this context, is an intrinsic neuron.

Sympathetic innervation is provided by cell bodies in the spinal cord and fibers that terminate in the prevertebral ganglia, including the celiac, superior, and inferior mesenteric ganglia. These preganglionic neurons synapse with postganglionic neurons in the ganglia. The fibers then leave the ganglia and reach the end organ through major blood vessels and their branches. In some cases, a synapse may occur in the paravertebral (chain) ganglia, as observed in sympathetic innervation of other organ systems. Sympathetic fibers directly innervate blood vessels of the GI tract for vasoconstriction, while other sympathetic fibers innervate glandular structures in the gut wall.

Both the parasympathetic and sympathetic divisions of the ANS also carry afferent (sensory) neurons towards the central nervous system (CNS). Vagal afferents have their cell bodies in the nodose ganglion. These neurons project centrally and terminate in the nucleus of the tractus solitarius in the brainstem, as well as in the gut wall. Spinal afferent neurons that travel alongside the sympathetic pathway are segmentally organized and found in the dorsal root ganglia. The peripheral terminals of both spinal and vagal afferents are distributed across all layers of the gut wall. These afferent neurons detect information about the state of the gut, including luminal contents (e.g., acidity, nutrient content, osmolality), as well as the degree of stretch or contraction in smooth muscle. Afferent innervation is also responsible for transmitting painful stimuli to the CNS.
Extrinsic neural innervation to the gut is via the two major subdivisions of the ANS, namely, parasympathetic and sympathetic innervation (
Fig. 27.7 ). Parasympathetic innervation to the gut is via the vagus and pelvic nerves. The vagus nerve, the 10th cranial nerve, innervates the esophagus, stomach, gallbladder, pancreas, first part of the intestine, cecum, and the proximal part of the colon. The pelvic nerves innervate the distal part of the colon and the anorectal region, in addition to the other pelvic organs that are not part of the GI tract.
Consistent with the typical organization of the parasympathetic nervous system, the preganglionic nerve cell bodies lie in the brainstem (vagus) or the sacral spinal cord (pelvic). Axons from these neurons run in the nerves to the gut (vagus and pelvic nerves, respectively), where they synapse with postganglionic neurons in the wall of the organ, which in this case are enteric neurons in the gut wall. There is no direct innervation of these efferent nerves to effector cells within the wall of the gut; the transmission pathway is always via a neuron in the ENS.
Consistent with transmission in the ANS, the synapse between preganglionic and postganglionic neurons is an obligatory nicotinic synapse. That is, the synapse between preganglionic and postganglionic neurons is mediated via acetylcholine released from the nerve terminal and acting at nicotinic receptors localized on the postganglionic neuron, which in this case is an intrinsic neuron.
Sympathetic innervation is supplied by cell bodies in the spinal cord and fibers that terminate in the prevertebral ganglia (celiac, superior, and inferior mesenteric ganglia); these are the preganglionic neurons. These nerve fibers synapse with postganglionic neurons in the ganglia, and the fibers leave the ganglia and reach the end organ along the major blood vessels and their branches. Rarely there is a synapse in the paravertebral (chain) ganglia, as seen with sympathetic innervation of other organ systems. Some vasoconstrictor sympathetic fibers directly innervate blood vessels of the GI tract, and other sympathetic fibers innervate glandular structures in the wall of the gut.
The ANS, both parasympathetic and sympathetic, also carries the fibers of afferent (toward the central nervous system [CNS]) neurons; these are sensory in nature. The cell bodies for the vagal afferents are in the nodose
SuperiorcervicalganglionPrevertebral ganglia1. Celiac2. Superior mesenteric3. Inferior mesenteric 123 •Fig. 27.7 ExtrinsicinnervationoftheGItract,consistingoftheparasympathetic(A)andsympathetic(B)subdivisionsoftheautonomicnervoussystem.
ganglion. These neurons have a central projection terminating in the nucleus of the tractus solitarius in the brainstem and the other terminal in the gut wall. The cell bodies of the spinal afferent neurons that run with the sympathetic pathway are segmentally organized and are found in the dorsal root ganglia. Peripheral terminals of the spinal and vagal afferents are located in all layers of the gut wall, where they detect information about the state of the gut. Afferent neurons send this information to the CNS. Information sent to the CNS relays the nature of the luminal contents (e.g., acidity, nutrient content, osmolality of the luminal contents), as well as the degree of stretch or contraction in smooth muscle. Afferent innervation is also responsible for transmitting painful stimuli to the CNS.