Anatomy of Peripheral Nerve

A single nerve, such as the median or ulnar nerve, is composed of thousands of axons that are grouped into several distinct nerve fascicles (see Plate 6-1, top left). Each of the fascicles is held together by the epineurium, the connective tissue sheath that maintains the structure of the entire nerve. Longitudinal vessels, arterioles and venules, course along the surface of the epineurium; these provide the necessary vascular supply to the nerve. Compromise of this vascular supply—by compression, ischemia, or inflammation—can result in infarction of an entire nerve or individual fascicles, producing acute or subacute nerve dysfunction. Clinical examples of nerve injury resulting from vascular compromise include those that occur with peripheral nerve vasculitis. The group of axons within a single nerve fascicle is surrounded by additional connective tissue called the perineurium. Within the perineurium are also small vessels providing vascular supply to the individual fascicles and axons. Each fascicle is composed of a number of individual nerve fibers, the main functional component of a nerve.

The epineurium and perineurium have important roles in maintaining the structure of the nerve, but also in providing a safety mechanism protecting the nerve from physical stresses that may injure the axons. Not only do they provide the framework for the nerves to remain adjacent and in close proximity to each other, but they protect the nerves from physical stress or injury (see Plate 6-1, top right). Physical compromise of a nerve can occur by direct external compression, such as from repetitive physical compression of superficial nerves (e.g., habitual leg crossing compressing the fibular nerve at the fibular head, habitual leaning on the elbow compressing the ulnar nerve as it courses superficially behind the medial epicondyle, or long-distance bicycle riding compressing the ulnar nerve at the wrist). Although the connective tissue structures provide some degree of protection, with severe or repetitive compression, damage to the myelin sheaths and eventually the axons can occur.

In addition, the supportive structures and the spiral configuration of the nerve fiber bundles within each fascicle help to protect the nerve from traction injuries, in which the nerve may be suddenly extended longitudinally. This type of injury often occurs with sudden, direct blunt trauma to the limb or neck region in the case of the nerves in the brachial plexus, such as with trauma after high-speed collisions. Although the spiral configuration protects the nerves from relatively minor traction injuries, it does not prevent injury from more severe injuries (e.g., nerve root avulsion from the spinal cord).

There are two main anatomic types of individual nerves: those that are myelinated and those that are unmyelinated (see Plate 6-1, bottom). Myelinated fibers consist of an axon surrounded by multiple Schwann cells that are present longitudinally along the course of the axon. The cytoplasm of the Schwann cells wrap around a 0.5- to 1.0-mm longitudinal segment of the axon in a spiral formation, producing lamellae. The plasma membrane of the Schwann cells that encircle the axon is composed of lipids (including cholesterol, cerebrosides, sulfatides, proteolipids, sphingomyelin, glycolipids, and glycoproteins) and proteins, and the concentric layers of the membrane that wrap around the axons form the “myelin.” The function of the myelin is to form “insulation” around segments of the nerve. Between the segments of myelin are small unmyelinated regions called nodes of Ranvier, the sites at which there is a high concentration of sodium and potassium channels. As a result, the action potentials that are generated along the nerve are rapidly transmitted from node to node, producing a very rapid “saltatory” conduction.

In contrast to myelinated fibers, unmyelinated fibers consist of an axon that is embedded within several Schwann cells, and a single Schwann cell surrounds a number of unmyelinated axons. In these fibers, rather than each axon having multiple Schwann cells wrap their cytoplasm around the axon, many axons are embedded in fewer Schwann cells. Although there is a small amount of protective myelin, there are no nodes of Ranvier, and sodium channels are equally dispersed along the entire course of the axon, resulting in slower action potential propagation along the nerve.

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