Effects of remote noxious stimulation on exteroceptive reflexes in human jaw-closing muscles.

Reflexes evoked by applying non-painful taps to an incisor tooth were recorded from the jaw closing masseter and temporal muscles of 21 human subjects. A series of inhibitory, excitatory, inhibitory and excitatory waves (the 'Q, R, S and T' waves of the post-stimulus electromyographic complex (PSEC)) occurred in full-wave rectified and averaged electromyograms. Conditioning by remote noxious stimulation (RNS; application of 3 degrees C water to a hand) usually produced increases in activity at the Q-R and S-T transitions of the PSEC (at mean latencies of 24 and 54 ms respectively), which resulted principally from a shortening of the inhibitory Q and S waves. Changes in the amplitudes of the excitatory R and T waves were also found. The effects of RNS were quantified by integrating records of the difference between conditioned and control PSECs. The RNS-induced effect on the entire PSEC was significantly (P < 0.01) greater when the reflexes were evoked by applying hard (7.4 mN.s) as opposed to soft (3.4 mN.s) taps to the tooth. However the ratio between the effects on the ST and QR segments did not differ significantly between these two intensities of tap stimuli. RNS-induced sensations of pain and increases in systemic arterial blood pressure were not correlated with the RNS-induced effects on the different segments of the PSEC. The results suggest that RNS may affect particularly those elements of the PSEC evoked by higher threshold afferents and that the effects are mediated by mechanisms acting directly at the brainstem level and are not secondary to pain or autonomic responses.