Modulation of exteroceptive suppression periods in human jaw-closing muscles induced by summation of nociceptive and non-nociceptive inputs

Convergence of various afferent inputs onto brain-stem neurons may play an important role in the regulation of trigeminal motor function. The present study examined the efficacy of nociceptive and non-nociceptive perioral stimulation for the modulation of the two exteroceptive suppression periods (ES1 and ES2) in human jaw-closing muscles. The inhibitory jaw-reflexes, ES1 (10-15 ms onset) and ES2 (40-50 ms onset), were recorded as the surface electromyogram of masseter and temporalis muscles in 14 healthy subjects. Non-painful electrical stimuli were applied to the right mental nerve while the subject was biting at 50% of the maximal voluntary force. Five conditions were compared: baseline, repetitive tactile stimulation (1 Hz) on the right cheek, topical application of capsaicin (5%) on the right cheek, repetitive tactile stimulation plus capsaicin, and post-baseline. The perceived intensity of the electrical stimuli was evaluated by the subjects on a 0-10 rating scale. Capsaicin alone failed to induce significant changes of ES1 and ES2; tactile stimulation alone induced a significant delay in the onset of ES2 (P<0.001). During repetitive tactile stimulation plus capsaicin, the duration of ES1 and ES2 was shortened (ES1 and ES2: P<0.001) and the degree of suppression was reduced (ES1: P<0.05; ES2: P<0.005). Perceived intensity of the electrical Stimuli was unchanged during the different experimental conditions. The present results suggest summation of simultaneous nociceptive and non-nociceptive inputs plays an important role in the modulation of the neural pathways involved in the short-latency ES1 and long-latency ES2.