 Sympathetic presynaptic outfow to the abdominopelvic organs is from the lower thoracic and upper lumbar segments of the spinal cord. These fibers travel to the prevertebral ganglia through abdominopelvic splanchnic nerves consisting of the greater, lesser, and least thoracic splanchnic and lumbar splanchnic nerves. Postsynaptic neurons have their cell bodies mostly in the prevertebral ganglia (see Fig. 12.24c). Only presynaptic fibers terminating on cells in the medulla of the suprarenal (adrenal) gland originate from paravertebral ganglia of the sympathetic trunk. The adrenal medullary cells function as a special type of postsynaptic neuron that release neurotrans mitter directly into the bloodstream instead of into the synaptic cleft.
 There is no parasympathetic outflow to the body wall and extremities. Anatomically, the autonomic innervation in the body wall is only sympathetic (see Fig. 12.24b). Each spinal nerve contains postsynaptic sympathetic fibers— that is, unmyelinated visceral efferents of neurons whose cell bodies are in paravertebral ganglia of the sympathetic trunk. For sweat glands, the neurotransmitter released by the “sympathetic” neurons is ACh instead of the usual NE.
The central nervous system consists of the brain located in the cranial cavity and the spinal cord located in the vertebral canal. The CNS is protected by the skull and vertebrae and is surrounded by three connective tissue membranes called meninges. The brain and spinal cord essentially float in the cerebrospinal fluid that occupies the space between the two inner meningeal layers. The brain is further subdivided into the cerebrum, cerebellum, and brain stem that connects with the spinal cord.
In the brain, the gray matter forms an outer covering or cortex; the white matter forms an inner core or medulla.
The cerebral cortex that forms the outermost layer of the brain contains nerve cell bodies, axons, dendrites, and central glial cells, and it is the site of synapses. In a freshly dissected brains, the cerebral cortex has a gray color, hence the name gray matter. In addition to the cortex, islands of gray matter called nuclei are found in the deep portions of the cerebrum and cerebellum.
The white matter contains only axons of nerve cells plus the associated glial cells and blood vessels (axons in fresh preparations appear white). These axons travel from one part of the nervous system to another. Whereas many of the axons going to, or coming from, a specific location are grouped into functionally related bundles called tracts, the tracts themselves do not stand out as delineated bundles. The demonstration of a tract in white matter of the CNS requires a special procedure, such as the  destruction of cell bodies that contribute fibers to the tract. The damaged fibers can be displayed by the use of appropriate staining or labeling methods and then traced. Even in the spinal cord, where the grouping of tracts is most pronounced, there are no sharp boundaries between adjacent tracts.
Cells of the Gray Matter
The types of cell bodies found in the gray matter vary according to which part of the brain or spinal cord is being  examined.
Each functional region of the gray matter has a characteristic variety of cell bodies associated with a meshwork of axonal, dendritic, and glial processes.
The meshwork of axonal, dendritic, and glial processes associated with the gray matter is called the neuropil. The organization of the neuropil is not demonstrable in H&E–stained sections. It is necessary to use methods other than H&E histology to decipher the cytoarchitecture of the gray matter (Plate 29, page 394).
Although general histology programs usually do not deal with the actual arrangements of the neurons in the CNS, the presentation of two examples will add to the appreciation of H&E sections that students usually examine. These examples present a region of the cerebral cortex (Fig. 12.27) and the cerebellar cortex (Fig. 12.28), respectively.