Neurons of the enteric division are not supported by and neurofibrillary tangles associated with Alzheimer’s Schwann or satellite cells; instead, they are supported by disease have been found in the walls of the large intesenteric neuroglial cells that resemble astrocytes (see tine. This finding may lead to development of routine page 367). Cells of the enteric division are also affected rectal biopsies for early  diagnosis of these conditions
FIGURE 12.26 • Enteric nervous system. This diagram shows the organization of the enteric system in the wall of the small intestine. Note the location of two nerve plexuses containing ganglion cells. The more superficial plexus, the myenteric plexus (Auerbach’s plexus), lies between two muscle layers. Deeper in the region of submucosa is a network of unmyelinated nerve fibers and ganglion cells, forming the submucosal plexus (Meissner’s plexus). Parasympathetic fibers originating from the vagus nerve enter the mesentery of the small intestine and synapse with the ganglion cells of both plexuses. Postsynaptic sympathetic nerve fibers also contribute to the enteric nervous system.
rather than the more complex and risk-associated biopsy of the brain.
A Summarized View of Autonomic Distribution
Figures 12.24 and 12.25 summarize the origins and distribution of the ANS. Refer to these figures as you read the descriptive sections. Note that the diagrams indicate both the paired innervation (parasympathetic and sympathetic) common to the ANS as well as the important exceptions to this general characteristic.
Head  Parasympathetic presynaptic outfow to the head leaves the brain with the cranial nerves, as indicated in Figure 12.25, but the routes are quite complex. Cell bodies may also be found in structures other than head ganglia listed in Table 12.1 and Figure 12.25 (e.g., in the tongue). These are “terminal ganglia” that contain nerve cell bodies of the parasympathetic system. Sympathetic presynaptic outfow to the head comes from the thoracic region of the spinal cord. The postsynaptic neurons have their cell bodies in the superior cervical ganglion; the axons leave the ganglion in a nerve network that hugs the wall of the internal and external carotid arteries to form the periarterial plexus of nerves. The inter nal carotid plexus and external carotid plexus follow the branches of the carotid arteries to reach their destination.
 Parasympathetic presynaptic outfow to the thoracic viscera is via the vagus nerve (X). The postsynaptic neurons have their cell bodies in the walls or in the parenchyma of the organs of the thorax.
 Sympathetic presynaptic outfow to the thoracic organs is from the upper thoracic segments of the spinal cord. Sympathetic postsynaptic neurons for the heart are mostly in the cervical ganglia; their axons make up the cardiac nerves. Postsynaptic neurons for the other thoracic viscera are in ganglia of the thoracic part of the sympathetic trunk. The axons travel via small splanchnic nerves from the sympathetic trunk to organs within the thorax and form the pulmonary and esophageal plexuses.
 Parasympathetic presynaptic outfow to the abdominal viscera is via the vagus (X) and pelvic splanchnic nerves. Postsynaptic neurons of the parasympathetic system to abdominopelvic organs are in terminal ganglia that generally are in the walls of the organs, such as the ganglia of the submucosal (Meissner’s) plexus and the myenteric (Auerbach’s) plexus in the alimentary canal. These ganglia are part of the enteric division of the ANS.