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 huma n subjects. A series of inhibitory, excitatory, inhibitory and excitat ory waves (the 'Q, R, S and T' waves of the post-stimulus electromyogr aphic complex (PSEC)) occurred in full-wave rectified and averaged ele ctromyograms. Conditioning by remote noxious stimulation (RNS; applica tion of 3 degrees C water to a hand) usually produced increases in act ivity 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 shorten ing 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 quan tified by integrating records of the difference between conditioned an d control PSECs. The RNS-induced effect on the entire PSEC was signifi cantly (P < 0.01) greater when the reflexes were evoked by applying ha rd (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 diffe r significantly between these two intensities of tap stimuli. RNS-indu ced sensations of pain and increases in systemic arterial blood pressu re were not correlated with the RNS-induced effects on the different s egments of the PSEC. The results suggest that RNS may affect particula rly those elements of the PSEC evoked by higher threshold afferents an d that the effects are mediated by mechanisms acting directly at the b rainstem level and are not secondary to pain or autonomic responses.