Effects of a prepulse stimulus on the masseteric inhibitory reflex in humans

We investigated the effects of a weak cutaneous stimulus applied to the digital nerves (prepulse) on the masseter inhibitory reflex (MIR) to electrical stimulation of the mental nerve in nine healthy volunteers. The prepulse stimulus, which had no direct effect on the masseter muscle EMG activity, induced a significant decrement in the area of the second phase of the MIR at interstimulus intervals between 50 and 100 ms. The percentage reduction induced on the inhibitory phase of the MIR was not different from that induced by the same prepulse on the late excitatory phase (R2) of the orbicularis oculi muscle to supraorbital nerve electrical stimulation during voluntary contraction. The effects observed during contraction were significantly less marked than those observed in the blink reflex at rest. We conclude that prepulse inhibition occurs in various brainstem reflexes, is similarly active on excitatory and inhibitory responses, and is reduced during voluntary activation.