Surface anodal stimulation of human peripheral nerves.

Bipolar surface stimulation of human peripheral nerves with short square pulses elicits action potentials in the A-alpha fibers at the beginning of the stimulus pulse both at the cathode (cathodal stimulation) and simultaneously in a hypopolarized region surrounding the anode (anodal stimulation). When recording with a bipolar surface electrode on the "anodal side" of the stimulating electrode a stimulus of medium strength gives a double peak response of submaximal amplitude. The peak with long latency is generated by the cathode and the peak with short latency by the anode. With a strong stimulus and recording on the "anodal side" only the short latency peak (with a maximal amplitude) is recorded. With a weak stimulus only the long latency peak (with a submaximal or maximal amplitude) is seen. These findings have importance in standard neurography investigations since mistakes concerning the polarity of the stimulating electrode affect the calculation of latencies, nerve conduction velocities, F-responses and SEPs. A nerve action potential with dual peaks may also be mistaken as a sign of anomalous innervation or activation of nerve axon subpopulations.