Neurophysiology of Nerve Conduction Studies
James B. Caress, Gregory J. Esper, and Seward B. Rutkove
Summary
The methodology for performing standard nerve conduction studies has been established by iden- tifying the most helpful and consistent physiological data obtainable while being constrained by a variety of technical and practical limitations. Nerve stimulation occurs underneath the negatively charged anode of the applied stimulator and simultaneous hyperpolarization of the nerve occurs beneath the positively charged cathode. Referential or bipolar recording techniques are used for all types of measurements. Sensory conduction studies can be performed either antidromically or ortho- dromically, although, for technical reasons, the former are usually preferred; the recorded sensory nerve action potential is made up of the simultaneous depolarization of all of the cutaneous sensory axons. In motor studies, the compound motor action potential is recorded from the motor point of the muscle of interest and represents the depolarization of the underlying muscle fibers rather than the nerve itself and is, thus, of considerably greater amplitude and duration. F-waves and H-reflexes rep- resent the two most commonly evaluated forms of late responses and assist with assessing the entire length of the neurons, from spinal cord to distal muscle.
Key Words: Compound motor action potential; depolarization; late responses; nerve conduction study;
sensory nerve action potential; stimulation.
1. PHYSIOLOGY OF STIMULATION
Stimulators used in routine nerve conduction studies (NCS) have a cathode and an anode and are, therefore, bipolar. The cathode is negatively charged, whereas the anode is positively charged. The depolarization of axons occurs under the cathode because the negativity in the region of the cathode leads to a reduction in the potential difference between the inside and the outside of the cell (the inside of the cell is relatively negative at baseline). On the other hand, the extracellular environment under the anode is positively charged, leading to hyper- polarization of the underlying axons.
Much of the current supplied by the stimulator travels in the very low resistance extracel- lular space because current follows the path of least resistance. The cross-sectional resistance of an axon will determine whether some of that current will enter and depolarize the nerve.
Cross-sectional resistance is reduced as diameter of the axon increases, resulting in a greater area in which current can flow. The result relevant to NCS is that large axons will depolarize with relatively less stimulus current than smaller axons. Hence, with low stimulus intensities, large axons will be preferentially stimulated.
From: The Clinical Neurophysiology Primer
Edited by: A. S. Blum and S. B. Rutkove © Humana Press Inc., Totowa, NJ