It is important to have a clear concept of the extent of the peripheral nervous system (PNS) and the mechanisms by which it is affected by disease. The PNS includes all neural structures lying outside the pial membrane of the spinal cord and brainstem with the exception of the optic nerves and olfactory bulbs, which are special extensions of the brain. The nerves within the spinal canal and attached to the ventral and dorsal surfaces of the cord are the spinal roots, which continue to form the numbered spinal nerves; those attached to the ventrolateral surface of the brainstem are the cranial nerve roots, or cranial nerves.
The dorsal, or posterior (afferent, or sensory), spinal roots consist of central axonal processes of the sensory and cranial ganglia. On reaching the spinal cord and brainstem, the roots extend for variable distances into the dorsal horns and posterior columns of the cord and into the spinal trigeminal and other tracts in the medulla and pons before synapsing with secondary sensory neurons, as described in Chaps. 7 and 8 that are devoted to the neurology of pain and sensation. The peripheral axons of the dorsal root ganglion cells are the sensory nerve fibers. They terminate as freely branching or specialized corpuscular endings—that is, the sensory receptors—in the skin, joints, and other tissues. The sensory nerve fibers vary greatly in size and in the thickness of their myelin covering; based on these dimensions, they are classified as type A, B, or C, as discussed in Chap. 7.
The ventral, or anterior (efferent, or motor), roots are composed of the emerging axons of anterior and lateral horn cells and motor nuclei of the brainstem. Large, heavily myelinated fibers terminate on muscle fibers and smaller unmyelinated ones terminate in sympathetic or parasympathetic ganglia. From these autonomic ganglia issue the axons that terminate in smooth muscle, heart muscle and conducting system, and glands. Traversing the subarachnoid space, where they lack well-formed epineurial sheaths, the cranial and spinal roots (both sensory and motor) are bathed in and are susceptible to substances in the cerebrospinal fluid (CSF), the lumbosacral roots having the longest exposure (Fig. 43-1).
The vast extent of the peripheral ramifications of cranial and spinal nerves is noteworthy, as are their thick protective and supporting sheaths of perineurium and epineurium that are endowed with a vascular supply through longitudinal arrays of richly anastomosing nutrient arterial branches. The perineurium comprises the connective tissue sheaths that surround and separate each bundle of nerve fibers (fascicles) of varying size, each fascicle containing several hundred axons. The sheath that binds and surrounds all the fascicles of the nerve is the epineurium. As the nerve root approaches the cord, the epineurium blends with the dura (see Fig. 43-1). The fine connective tissue covering of individual nerve fibers is the endoneurium. Longitudinally oriented and widely anastomotic endoneural vessels also nourish the nerve fibers and are susceptible to disease.
The nerves traverse narrow foramina (intervertebral and cranial) and a few pass through tight channels peripherally in the limbs (e.g., the median nerve between the carpal ligament and tendon sheaths of flexor forearm muscles that make up the carpal tunnel; the ulnar nerve in the cubital tunnel). These anatomic features explain the sites of susceptibility of certain nerves to compression and entrapment and also to ischemic damage.