Anodal excitation of intact peripheral nerves in humans

The median nerves of six normal subjects were electrically stimulated at different locations on the wrist in a bipolar fashion. The evoked compound sensory nerve action potentials (CSNAPs) were recorded at the index finger. Electrical stimuli consisted of constant-current monophasic rectangular pulses. Initially, the nerve was stimulated with a cathodal current and a just supramaximal CSNAP was obtained. Then, at the same stimulus location, the nerve was excited with an anodal current. A just supramaximal response in the anodal-CSNAP was obtained, or until a maximum current of 100 mA was delivered. CSNAPs obtained from anodal excitation required several times more electrical stimulus intensity when compared to those derived from cathodal stimulation. Morphologically, they were similar, except the onset latency of the anodal-CSNAPs was approximately 0.2 to 0.3 msec early compared to that derived from cathodal stimulation. This implies that during anodal excitation, the site of nerve depolarization was not located underneath the anode, but was situated at some distance from it. Also, it appears that a minimum nerve length (10 mm or more) should be exposed to the anodal current in order to induce depolarization. A very localized application of the anodal stimulus often failed to elicit nerve depolarization.