Mechanisms underlying the effects of remote noxious stimulation and mentalactivities on exteroceptive jaw reflexes in man

Successive inhibitory, excitatory, inhibitory and excitatory reflexes (the Q, R, S and T waves of the post-stimulus electromyographic complex (PSEC)), evoked by applying non-painful taps to an incisor tooth, were recorded fro m the jaw-closing muscles of 15 subjects. The effects on these reflexes of the subjects undertaking mental exercises (MEx) in the form of arithmetic c alculations were compared with those of remote noxious stimulation (RNS; ap plication of 3 degrees C to a hand). This was done to investigate whether t he previously established effects of RNS were likely to be related to a cha nge in the subject's mental state and/or to direct nociceptive mechanisms. Both MEx and RNS caused increases in EMG activity around the Q-R and S-T tr ansitions of the PSEC, which resulted principally from shortenings of the i nhibitory Q and S waves. Reducing the intensity of the tap stimuli, which m imicked condition-induced disinhibition, caused shortenings of the inhibito ry waves at-latencies similar to the shortenings induced by MEx or RNS. The magnitude of the RNS-induced effect on the ST segment of the PSEC was grea ter (P < 0.01) than that on the QR segment. By contrast, MEx induced simila r effects on both segments. Regression analyses were performed for the rela tionship between condition-induced changes in amplitude of the excitatory w aves and their control amplitudes. These analyses were performed to reveal any condition-induced inhibition or facilitation of the tap-induced influen ces on the motoneurons. Overall, the evidence suggested that: (1) mental ex ercise induced a similar degree of inhibition of the two tap-induced inhibi tory jaw reflexes and a facilitation of the excitatory ones, and (2) remote noxious stimulation induced an inhibition of the second tap-induced inhibi tory reflex which was greater than that of the first one, and an inhibition of the first excitatory reflex. Thus, although factors related to altered mental activity could play a role in the modulation of jaw reflexes by RNS, the differences between the effects of MEx and RNS suggest that alternativ e or complementary mechanisms are also likely to be involved. (C) 2000 Inte rnational Association for the Study of Pain. Published by Elsevier Science B.V.