Inhibition of motoneuron discharge by peripheral nerve stimulation: an F response analysis.

F waves were recorded from the abductor pollicis brevis stimulating the median nerve at the wrist. These data were compared to responses obtained after preceding supramaximal stimulation of digital fibers of the second (II) finger or the median nerve at the wrist. The time between conditioning and test stimuli were 50 msec. Following conditioning stimuli, F wave latencies were significantly increased while F amplitudes, durations, and persistences were all decreased. Chronodispersion was not significantly affected. These changes were associated with increased repetition of individual F responses. The most prominent changes were found after stimulation of fibers of digit II but only at levels of stimulation supramaximal for the sensory nerve action potential. Some, but relatively limited, changes were present after stimulation of digital fibers of the fifth finger. The results are consistent with afferent fiber, probably A delta, inhibition of antidromic motoneuron activation with associated decrease in central motor neuron pool excitability. The study also demonstrates that, except for chronodispersion, changes in F waves found with peripheral nerve injury may also occur due to physiological changes in the central nervous system.