Neurophysiologic studies can play a significant role in the assessment of peripheral nervous system disorders and suspected neuromuscular dysfunction. Neurophysiologic evaluation provides objective information about the diagnosis, localization, nature, and severity of the pathologic processes involved. Information obtained about the peripheral neuromuscular system can be used to plan and prescribe a therapeutic program. It also provides information on prognosis and the effectiveness of a treatment program. This chapter reviews basic principles of electrodiagnostic studies and the technical aspects of obtaining biologic signals from study subjects.
NEUROMUSCULAR ANATOMY & PHYSIOLOGY
The peripheral nervous system consists of the nerve root, the motor efferent from the anterior horn cell, and the afferent sensory root with dorsal root ganglion, usually located in the neural foramina—both of which combine to form a mixed spinal nerve. The mixed spinal nerve splits into the ventral and dorsal rami. The ventral rami from these nerves coalesce to form trunks, divisions, and cords, which ultimately become terminal nerve branches of the brachial plexus (to innervate the upper limbs) and the lumbosacral plexus (to innervate the lower limbs). The dorsal rami of the spinal nerves innervate the paraspinal muscles (Figure 16–1).
Motor neuron and its components.
Nerve axons have electrical properties common to all excitable cells. The axons not only conduct the propagating electrical potential but also transport nutritional and trophic substances, the latter maintaining the metabolic integrity of the peripheral nerve. There is extracellular preponderance of sodium (Na) ions and intracellular preponderance of potassium (K) ions. At rest an excess positive charge outside the cell and negative charge inside the cell is maintained as a resting transmembrane potential by the cell wall. Voltage-gated sodium (Na+) and potassium (K+) channels are present in the cell wall. The density of the Na channels is high at the nodes of Ranvier in the myelinated axons. The influx of Na with efflux of K that occurs upon opening of these channels produces the action potential that is propagated along the axon. When a weak current is applied to a nerve, negative charges from the negative pole (cathode) make the inside of the cell relatively more positive, causing depolarization. After 10–30 mV of depolarization, the membrane potential reaches a critical threshold for generation of an action potential, which is bidirectional.
Motor action potentials that are generated at the cortical level travel through the brainstem and spinal cord to the muscle. At the level of the muscle the action potentials traverse the neuromuscular junction. The neuromuscular junction is a simple relay between the nerve terminals of a motor neuron and the skeletal muscle fibers innervated by that nerve. The electrical potential that arrives at the nerve terminal is converted at the neuromuscular junction to a chemical signal. The neuromuscular junction contains ...