FACILITATION OF A NOCICEPTIVE FLEXION REFLEX IN MAN BY NONNOXIOUS RADIANT-HEAT PRODUCED BY A LASER

Electromyographic recordings were made in healthy volunteers from the knee-flexor biceps femoris muscle of the nociceptive R-III reflex elic ited by electrical stimulation of the cutaneous sural nerve. The stimu lus intensity was adjusted to produce a moderate pricking-pain sensati on. The test responses were conditioned by a nonnoxious thermal (less than or equal to 40 degrees C) stimulus applied to the receptive field of the sural nerve. This stimulus was delivered by a CO2 laser stimul ator and consisted of a 100-ms pulse of heat with a beam diameter of 2 0 mm. Its power was 22.7 +/- 4.2 W (7.2 mJ/mm(2)), and it produced a s ensation of warmth. The maximum surface temperature reached at the end of the period of stimulation was calculated to be 7 degrees C above t he actual reference temperature of the skin (32 degrees C). The interv al between the laser (conditioning) and electrical (test) stimuli was varied from 50 to 3,000 ms in steps of 50 ms. It was found that the no ciceptive flexion reflex was facilitated by the thermal stimulus; this modulation occurred with particular conditioning-test intervals, whic h peaked at 500 and 1,100 ms with an additional late, longlasting phas e between 1,600 and 2,300 ms. It was calculated that the conduction ve locities of the cutaneous afferent fibers responsible for facilitating the R-III reflex, fell into three ranges: one corresponding to A delt a fibers (3.2 m/s) and two in the C fiber range (1.3 and 0.7 m/s). It is concluded that information emanating from warm receptors and nocice ptors converges. In this respect, the present data show, for the first time, that in man, conditioning nonnociceptive warm thermoreceptive A S and C fibers results in an interaction at the spinal level with a no ciceptive reflex. This interaction may constitute a useful means where by signals add together to trigger flexion reflexes in defensive react ions and other basic motor behaviors. It also may contribute to hypera lgesia in inflammatory processes. The methodology used in this study a ppears to be a useful noninvasive tool for exploring the thermoalgesic mechanisms in both experimental and clinical situations.