Evidence for a monosynaptic mechanism in the tonic vibration reflex of the human masseter muscle.

Vibration of the masseter and temporalis muscles in normal human adult subjects elicits a tonic vibration reflex with unexpected features. The electromyographic response is not asynchronous as in the limb muscles, but involves well-defined spikes with a one-to-one temporal relation to the vibration cycles. The effect of various parameters such as muscle stretch, vibration frequency or amplitude, etc, has been investigated. The small latency fluctuation of the vibration-induced spikes is compatible with a monosynaptic reflex mechanism which is considered to be assisted by a polysynaptic facilitatory background of proprioceptive origin.