Stimulus intensity and site of excitation in human median nerve sensory fibres.

Median nerve sensory fibres were stimulated with minimal and supramaximal stimuli at the base of the third digit in 30 normal subjects. Evoked nerve action potentials were simultaneously recorded from two points on the median nerve just above the wrist. As stimulus voltage was increased from minimal to supramaximal, amplitudes of nerve action potentials increased, latencies decreased, but conduction velocities remained unchanged. The shortening of latencies was interpreted as movement of the effective point of nerve excitation away from the stimulating cathode towards the recording electrodes. Therefore, the effective point of nerve excitation cannot be assumed to be underneath the cathode, but at some distance from it depending on stimulus intensity. Furthermore, the fastest conducting sensory fibres in the human median nerve do have a lower threshold than slower conducting fibres.