Many functions of the SNS are similar to those of the adrenal medulla, an endocrine gland. This functional similarity is partly explained by the developmental relationships between the cells of the adrenal medulla and postsynaptic sympathetic neurons. Both are derived from the neural crest, are innervated by presynaptic sympathetic neurons, and produce closely related physiologically active agents, EPI and NE. A major difference is that the sympathetic neurons deliver the agent directly to the effector, whereas the cells of the adrenal medulla deliver the agent indirectly through the bloodstream. The innervation of the adrenal medulla may constitute an exception to the rule that autonomic innervation consists of a two-neuron chain from the CNS to an effector unless the adrenal medullary cell is considered the functional equivalent of the second neuron (in effect, a neurosecretory neuron).
Enteric Division of the Autonomic Nervous System
The enteric division of the ANS consists of the ganglia and their processes that innervate the alimentary canal.
The enteric division of the ANS represents a collection of neurons and their processes within the walls of the alimentary canal. It controls motility (contractions of the gut wall), exocrine and endocrine secretions, and blood flow through the gastrointestinal tract; it also regulates immunologic and inflammatory processes.
The enteric nervous system can function independently from the CNS and is regarded as the “brain of the gut.” However, digestion requires communication between enteric neurons and the CNS, which is provided by parasympathetic and sympathetic nerve fibers. Enteroceptors located in the alimentary tract provide sensory information to the CNS regarding the state of digestive functions. The CNS then coordinates sympathetic stimulation that inhibits gastrointestinal secretion, motor activity, and contraction of gastrointestinal sphincters and blood vessels and parasympathetic stimuli that produce opposite actions. Interneurons integrate information from sensory neurons and relay this information to enteric motor neurons in the form of reflexes. For instance, the gastrocolic reflex is elicited when distention of the stomach stimulates contraction of musculature of the colon, triggering defecation.
Ganglia and postsynaptic neurons of the enteric division are located in the lamina propria, muscularis mucosae, submucosa, muscularis externa, and subserosa of the alimentary canal from the esophagus to the anus (Fig. 12.26). Because the enteric division does not require presynaptic input from the vagus nerve and sacral outflow, sympathetic trunk with paravertebral ganglion splanchnic nerve containing axons of presynaptic neurons
FIGURE 12.24 • Schematic diagram comparing somatic efferent and visceral efferent neurons. a. In the somatic efferent (motor) system, one neuron conducts the impulses from the CNS to the effector (skeletal muscle). b. In the visceral efferent system (repre sented in this diagram by the sympathetic division of the ANS), a chain of two neurons conducts the impulses: A presy naptic neuron located within the CNS and a postsynaptic neuron located in the paravertebral or prevertebral ganglia. Moreover, each presynaptic neuron makes synaptic contact with more than one postsynaptic neuron. Postsy naptic sympathetic fibers supply smooth muscles (as in blood vessels) or glandular epithelium (as in sweat glands). c. Neurons of the ANS that supply organs of the abdomen reach these organs by way of the splanchnic nerves. In this example, the splanchnic nerve joins with the celiac ganglion, where most of the synapses of the two-neuron chain occur. Note that one presynaptic neuron makes contact with several postsynaptic neurons. (Redrawn with permission from Reith EJ, Breidenbach B, Lorenc