Electrodiagnostic (edx) testing provides information about the peripheral nervous system and is an extension of the physical examination. Although most commonly comprised of nerve conduction studies (NCS) and electromyography (EMG), it can also include special testing, such as late responses, repetitive stimulation, and single-fiber EMG. Each test sheds light on a different aspect of the peripheral nervous system—from the alpha motor neuron, through the neuromuscular junction, and down to individual muscle fibers. This chapter provides an introduction to EDX testing and reviews nerve electrophysiology and common instrumentation used in EDX studies. Basic descriptions of the different types of EDX studies will also be examined.
In neurons, depolarization and repolarization cycles create action potentials. Action potentials are brief changes in the electrical potential on the surface of neurons or muscle cells. Action potentials create a current I (measured in milliamperes) and potential difference or voltage V (measured in millivolts) along the membrane. The resistance to current of an alternating-current (AC) circuit (such as in human tissue) is termed impedance Z (measured in kilo-/megaohms). In electrophysiology, V = I × Z.
Every axon and motor cell has a resting membrane potential of around −70 µV. When a stimulus is strong enough to reach threshold potential (−30 µV in most cells), the membrane depolarizes. Sodium channels open, allowing sodium ions to flood into the cell. Sodium channels are slowly deactivated as potassium ions exit the cell, causing an initial hyperpolarization, before the Na+/K+ pump restores the resting membrane potential. This period of hyperpolarization is also known as the “relative refractory period” because the hyperpolarized membrane requires an even greater stimulus than normal to bring the cell membrane to threshold and cause a subsequent depolarization. The period just prior to the relative refractory period is called the “absolute refractory period” because the sodium channels have not yet reset and a second action potential cannot be elicited until they have done so. The refractory period promotes unidirectional action potential propagation (Fig. 70–1).
(A) Nerve action potential. The upstroke of the action potential results from increased Na+ conductance. Repolarization results from a declining Na+ conductance combined with an increasing K+ conductance; afterhyperpolarization is due to sustained high K+ conductance. (B) Action potential propagation. Local current flow causes the threshold potential to be exceeded in adjacent areas of the neuron membrane. Because the upstream region is refractory, an action potential is only propagated downstream. In myelinated axons, action potentials propagate faster by “jumping” from one node of Ranvier to the next node by saltatory conduction. ARP = absolute refractory period; RRP = relative refractory period. (Reproduced with permission from General Physiology. In: Kibble JD, Halsey CR, eds. Medical Physiology: The Big Picture, New York, NY: McGraw-Hill; 2014.)