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 extracellular 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
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