Anatomy and Neurophysiology for Electrodiagnostic Studies

Anatomy

The strict definition of the peripheral nervous system includes that part of the nervous system in which the Schwann cell is the major supporting cell, as opposed to the central nervous system in which the glial cells are the major supporting cells. The peripheral nervous system includes the nerve roots, peripheral nerves, primary sensory neurons, neuromuscular junctions (NMJs), and muscles ( Fig. 2.1 ). Although not technically part of the peripheral nervous system, the primary motor neurons (i.e., anterior horn cells), which are located in the spinal cord, are usually included as part of the peripheral nervous system as well. In addition, cranial nerves III through XII are also considered to be part of the peripheral nervous system, being essentially the same as peripheral nerves, except that their primary motor neurons are located in the brainstem rather than the spinal cord.

The primary motor neurons, the anterior horn cells , are located in the ventral gray matter of the spinal cord. The axons of these cells ultimately become the motor fibers in peripheral nerves. Their projections first run through the white matter of the anterior spinal cord before exiting ventrally as the motor roots . In contrast to the anterior horn cell, the primary sensory neuron, also known as the dorsal root ganglion (DRG), is not found within the substance of the spinal cord itself but rather lies outside the spinal cord, near the intervertebral foramen. The DRG are bipolar cells with two separate axonal projections. Their central projections form the sensory nerve roots . The sensory roots enter the spinal cord on the dorsal side to ascend in either the posterior columns or synapse with sensory neurons in the dorsal horn. The peripheral projections of the DRGs ultimately become the sensory fibers in peripheral nerves. Because the DRGs lie outside the spinal cord, this results in a different pattern of sensory nerve conduction abnormalities, depending on whether the lesion is in the peripheral nerve or proximal to the DRG at the root level (see Chapter 3 ).

Motor and sensory roots at each spinal level unite distal to the DRG to become a mixed spinal nerve . There are 31 pairs of spinal nerves (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal; Fig. 2.2 ). Each spinal nerve divides into a dorsal and ventral ramus ( Fig. 2.3 ). Unlike the dorsal and ventral nerve roots, the dorsal and ventral rami both contain motor and sensory fibers. The dorsal ramus runs posteriorly to supply sensory innervation to the skin over the spine and muscular innervation to the paraspinal muscles at that segment. The ventral ramus differs, depending on the segment within the body. In the thoracic region, each ventral ramus continues as an intercostal nerve . In the lower cervical to upper thoracic (C5–T1) region, the ventral rami unite to form the brachial plexus ( Fig. 2.4 ). In the mid-lumbar to sacral regions, the ventral rami intermix to form the lumbosacral plexus ( Fig. 2.5 ).

Within each plexus, motor and sensory fibers from different nerve roots intermix to ultimately form individual peripheral nerves . Each peripheral nerve generally supplies muscular innervation to several muscles and cutaneous sensation to a specific area of skin, as well as sensory innervation to underlying deep structures. Because of this arrangement, motor fibers from the same nerve root supply muscles innervated by different peripheral nerves, and sensory fibers from the same nerve root supply cutaneous sensation in the distribution of different peripheral nerves. For instance, the C5 motor root supplies the biceps (musculocutaneous nerve), deltoid (axillary nerve), and brachioradialis (radial nerve), among other muscles ( Fig. 2.6 ). Similarly, C5 sensory fibers innervate the lateral arm (axillary nerve) and forearm (lateral antebrachial cutaneous sensory nerve), in addition to other nerves.

All muscles supplied by one spinal segment (i.e., one nerve root) are known as a myotome , whereas all cutaneous areas supplied by a single spinal segment are known as a dermatome ( Fig. 2.7 ). For both myotomes and dermatomes, there is considerable overlap between adjacent segments. Because of the high degree of overlap between spinal segments, a single root lesion seldom results in significant sensory loss and never in anesthesia. Likewise, on the motor side, even a severe single nerve root lesion usually results in only mild or moderate weakness and never in paralysis. For instance, a severe lesion of the C6 motor root causes weakness of the biceps; however, paralysis would not occur because C5 motor fibers also innervate the biceps. In contrast, a severe peripheral nerve lesion usually results in marked sensory and motor deficits because contributions from several myotomes and dermatomes are affected simultaneously.

At the microscopic level, nerve fibers are protected by three different layers of connective tissue: the epineurium, perineurium, and endoneurium ( Fig. 2.8 ). The thick epineurium surrounds the entire nerve and is in continuity with the dura mater at the spinal cord level. Within the epineurium, axons are grouped into fascicles, surrounded by perineurium . A final layer of connective tissue, the endoneurium , is present between individual axons. Effectively, a blood-nerve barrier is formed by the combination of vascular endothelium supplying the nerve and the connective tissue of the perineurium. Together, the three layers of connective tissue give peripheral nerve considerable tensile strength, usually in the range of 20–30 kg. However, the weakest point of a nerve occurs where the nerve roots meet the spinal cord, where the nerve can sustain only 2–3 kg of force. For this reason, nerve root avulsion may occur after a significant trauma and especially after a stretch injury.

Analogous to nerve fibers, muscle fibers have a very similar arrangement at the microscopic level with three layers of connective tissue: the epimysium, perimysium, and endomysium ( Fig. 2.9 ). The epimysium surrounds the entire muscle. Within the epimysium, muscle fibers (which are the actual muscle cells) are grouped into fascicles surrounded by the perimysium . The final layer of connective tissue, the endomysium , is present between individual muscle fibers. Muscle fibers are cylindrical and contain the actual muscle fibrils: the structural proteins that are responsible for muscle contraction. When muscle contraction occurs, the force is transmitted most often to a bone to move a joint (occasionally, muscle is connected to other connective tissue or the skin). This connection is most often made by way of a tendon, which is a thick rope-like piece of connective tissue that is in continuity with the epimysium of the muscle. In some muscles, the contraction is by way of an aponeurosis, which is a large, sheet-like piece of connective tissue. Most muscles have two tendons, one at their origin (typically proximal and more static) and one at their insertion (typically more distal and more mobile). In some muscles, the tendon originates from inside the muscle, known as an internal or central tendon.