Burst activities of cremasteric motor units.

Electromyographic activities of the ipsilateral cremaster muscle evoked by electrical stimulation of either the thigh or the penis were analyzed for latencies and burst durations. Response latencies were 30 ms on thigh stimulation and 30-35 ms on penile stimulation. With the increase in stimulus intensity the latencies decreased to 30 ms in both cases. The changes were stepwise, indicating the switching between different reflex arcs. The burst duration on thigh stimulation was about 50 ms, and that for penile stimulation was 45 ms. Impulses producing the burst consisted of larger and smaller spikes, and this indicates the grouping of motor units for wing tonic and phasic units. The results suggest the existence of parallel neuronal circuitries for the spinal cremasteric reflex, and it is expected to provide a clue for evaluating the clinical significance of the cremasteric reflex.