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.