LOCALIZED PERIPHERAL NERVE (PN) injuries are very common and arise from a wide range of etiologies. They are seen in healthy individuals and also may arise from complications from other disorders or traumatic injuries. PN injuries may accompany or complicate rehabilitation management of other disorders, for example, nerve compressions due to use of assistive devices or bracing, or overuse injuries in less impaired extremities. An understanding of the anatomic course and innervation of peripheral nerves and common sites of entrapment is important in recognizing and diagnosing these injuries. As PN injuries can accompany other musculoskeletal disorders and trauma, their existence can sometimes be initially masked. Therefore, it is important to recognize high-risk scenarios for nerve injury as they may impede expected recovery from the traumatic injury.
PERIPHERAL NERVE ANATOMY AND TYPES OF INJURY
The peripheral nervous system contains 12 pairs of cranial and 31 pairs of spinal nerves that supply specific sensory territories and muscle groups, known as dermatomes and myotomes, respectively (Fig. 12–1). After leaving the spinal cord as nerve roots, the spinal nerves in the cervical and lumbosacral regions form plexuses. Different root levels then intermingle in the plexus to form individual peripheral nerves innervating the upper and lower extremities. When a spinal root, portion of the plexus, or peripheral nerve is injured, characteristic patterns of sensory and motor abnormalities will be seen.
Spinal nerve dermatomes and peripheral nerves. (Reproduced with permission from Flynn JA. Acute Back Pain. In: McKean SC, Ross JJ, Dressler DD, Scheurer DB, eds. Principles and Practice of Hospital Medicine, 2e New York, NY: McGraw-Hill; 2017.)
The neural structure is composed of an axon (the nerve fiber) which is enclosed in a Schwann cell. A single Schwann cell may encircle a multiple axons, in which case it is referred to as an unmyelinated axon. When a Schwann cell associates with a single axon, wrapping around it several times, the resultant fiber is referred to as myelinated. The myelinated fiber is invested by multiple Schwann cells in succession, which are separated by small uncovered regions called nodes of Ranvier (Fig. 12–2). This allows nerve depolarization to “jump” from node to node in a process known as saltatory conduction. This results in far more rapid conduction of the nerve impulse than in unmyelinated fibers (50 to 60 m/s vs. 1 to 2 m/s). The nerve fiber and its Schwann cells are enclosed in an endoneurium. Several of these are grouped into fascicles surrounded by a perineurium. Along its course individual axons may cross from one fascicle to another. The fascicles are then all enclosed within an epineurium which makes up the entire peripheral nerve (Fig. 12–3).
Anatomy of axon: Schwann cell and nodes of Ranvier. (Reproduced with permission from Excitable ...