The cell bodies of peripheral nerves may be located within the CNS or outside the CNS in peripheral ganglia. Ganglia contain clusters of neuronal cell bodies and the nerve fibers leading to and from them (see Fig. 12.16). The cell bodies in dorsal root ganglia as well as ganglia of cranial nerves belong to sensory neurons (somatic afferents and visceral afferents that belong to the autonomic nervous system [discussed below]), whose distribution is restricted to specific locations (Table 12.1 and Fig. 12.3). The cell bodies in the paravertebral, prevertebral, and terminal ganglia belong to postsynaptic “motor” neurons (visceral efferents) of the autonomic nervous system (see Table 12.1 and Fig. 12.16).
To understand the PNS, it is also necessary to describe some parts of the CNS.
Motor neuron cell bodies of the PNS lie in the CNS.
The cell bodies of motor neurons that innervate skeletal muscle (somatic efferents) are located in the brain, brain stem, and spinal cord. The axons leave the CNS and travel in peripheral nerves to the skeletal muscles that they innervate. A single neuron conveys impulses from the CNS to the effector organ.
Sensory neuron cell bodies are located in ganglia outside of, but close to, the CNS.
In the sensory system (both the somatic afferent and the visceral afferent components), a single neuron connects the receptor, through a sensory ganglion, to the spinal cord or brain stem. Sensory ganglia are located in the dorsal roots of the spinal nerves and in association with sensory components of cranial nerves V, VII, VIII, IX, and X (see Table 12.1).
Connective Tissue Components of a Peripheral Nerve
The bulk of a peripheral nerve consists of nerve fibers and their supporting Schwann cells. The individual nerve fibers and their associated Schwann cells are held together by connective tissue organized into three distinctive components, each with specific morphologic and functional characteristics (Fig. 12.23; also, see Fig. 12.3).
 The endoneurium includes loose connective tissue sur rounding each individual nerve fiber.  The perineurium includes specialized connective tissue surrounding each nerve fascicle.  The epineurium includes dense irregular connective tis sue that surrounds a peripheral nerve and fills the spaces between nerve fascicles.
Endoneurium constitutes the loose connective tissue associated with individual nerve fibers.
The endoneurium is not conspicuous in routine light microscope preparations, but special connective tissue stains permit its demonstration. At the electron microscope level, collagen fibrils that constitute the endoneurium are readily apparent (see Figs. 12.11 and 12.12). The fibrils run both parallel to, and around, the nerve fibers, binding them together into a fascicle, or bundle. Because fbroblasts are relatively sparse in the interstices of the nerve fibers, it is likely that most of the collagen fibrils are secreted by the Schwann cells. This conclusion is supported by tissue culture studies in which collagen fibrils are formed in pure cultures of Schwann cells and dorsal root neurons.
Other than occasional fibroblasts, the only other connective tissue cells normally found within the endoneurium are mast cells and macrophages. Macrophages mediate immunologic surveillance and also participate in nerve  tissue re-