Nociceptive masseter inhibitory reflexes evoked by laser radiant heat and electrical stimuli

Electrical stimulation of the mental nerve evokes two suppression periods SP1 and SP2 in masseter muscle activity bilaterally. In order to investigate a possible nociceptive origin of the suppression periods, we compared the reflex responses evoked by electrical stimulation and by selective activation of nociceptors in hairy skin using painful infrared laser stimuli. The SP was elicited during more than 90% maximal voluntary contraction. Thresholds for detection, pain, and SP in the mental nerve area were determined by the method of limits. A suppression period was evoked by laser stimuli in nine of ten subjects bilaterally. The mean onset latency was 46.9 ms, the mean duration 58.9 ms. The electrical threshold of SP1 (9 mA) was 7.7 x I(0), about 20% smaller than I(P), and significantly higher than I(SP2) (4.7 mA). The onset latencies and durations were 11.7 ms and 21 ms for SP1, and 45 ms and 42.7 ms for SP2 (stimulus intensity 2 x I(P)). The mean difference in onset latencies between laser SP and electrically evoked SP1 was 35.1 +/- 6.2 ms, which closely matches the nociceptor response latency to a laser heat pulse. Based on the threshold and the onset latency we conclude that at least SP1 and laser SP are nociceptive in origin and mediated by group III fibers.