The fast-acting neurotransmitters released by visceral afferent fibers are not well documented, although many of these neurons release an excitatory amino acid transmitter such as glutamate. However, visceral afferent fibers also contain many neuropeptides or combinations of neuropeptides, including angiotensin II, arginine vasopressin, bombesin, calcitonin gene–related peptide, cholecystokinin, galanin, substance P, enkephalin, oxytocin, somatostatin, and vasoactive intestinal polypeptide.
Visceral afferent fibers that can mediate conscious sensation include nociceptors that travel in sympathetic nerves, such as the splanchnic nerves. Visceral pain is caused by excessive distention of hollow viscera, contraction against an obstruction, or ischemia. The origin of visceral pain is often difficult to identify because of the diffuse nature of the pain and its tendency to be referred to somatic structures (see ). Visceral nociceptors in sympathetic nerves reach the spinal cord via the sympathetic chain, white rami, and dorsal roots. The terminals of nociceptive afferent fibers project to the dorsal horn and to the region surrounding the central canal. They activate not only local interneurons, which participate in reflex arcs, but also projection cells, which include spinothalamic tract cells that signal pain to the brain.
A major visceral nociceptive pathway from the pelvis involves a relay in the gray matter of the lumbosacral spinal cord. These neurons send axons into the fasciculus gracilis that terminate in the nucleus gracilis. Thus the dorsal columns not only contain primary afferents for somatic sensation (their main component) but also second-order neurons of the visceral pain pathway (recall that second-order axons for somatic pain travel in the lateral funiculus as part of the spinothalamic tract). Visceral nociceptive signals are then transmitted to the ventral posterior lateral nucleus of the thalamus and presumably from there to the cerebral cortex. Interruption of this pathway accounts for the beneficial effects of surgically induced lesions of the dorsal column at lower thoracic levels to relieve pain produced by cancer of the pelvic organs.
Other visceral afferent fibers travel in parasympathetic nerves. These fibers are generally involved in reflexes rather than sensation (except for taste afferent fibers; see ). For example, the baroreceptor afferent fibers that innervate the carotid sinus are in the glossopharyngeal nerve. They enter the brainstem, pass through the solitary tract, and terminate in the nucleus of the solitary tract (see
Fig. 4.6E ). These neurons connect with interneurons in the brainstem reticular formation. The interneurons, in turn, project to the autonomic preganglionic neurons that control heart rate and blood pressure (see
The nucleus of the solitary tract receives information from all visceral organs, except those in the pelvis. This nucleus is subdivided into several areas that receive information from specific visceral organs.
The Enteric Nervous System
The enteric nervous system, which is located in the wall of the gastrointestinal tract, contains about 100 million neurons. The enteric nervous system is subdivided into the myenteric plexus, which lies between the longitudinal and circular muscle layers of the gut, and the submucosal plexus, which lies in the submucosa of the gut. The neurons of the myenteric plexus primarily control gastrointestinal motility (see ), whereas those in the submucosal plexus primarily regulate body fluid homeostasis (see