The effect of stimulus intensity and gape on electrically-evoked jaw reflexes in man.

The changes in masseter EMG and biting force evoked by electrical stimulation of the lip were measured in normal people. Stimulation at noxious intensities elicited a characteristic, biphasic inhibitory response in the masseter EMG and a transient reduction in the biting force. The pattern of masseter inhibition and force change varied both with stimulus intensity and with inter-incisal separation. In general, the decrease in biting force was greater at intermediate jaw positions than in the open and closed positions. The duration of the EMG inhibitory period increased as the stimulus intensity increased but was independent of the jaw separation. Reflex thresholds could not be correlated with pain thresholds because subjects found it difficult to decide at what level an electrical stimulus became painful. Nevertheless this stimulation of high-threshold afferents may be useful in estimating the rate of relaxation of tension in the jaw muscles.