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 elicited by electrical stimulation of the cutaneous sural nerve. The stimulus intensity was adjusted to produce a moderate pricking-pain sensation. 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 stimulator and consisted of a 100-ms pulse of heat with a beam diameter of 20 mm. Its power was 22.7 +/- 4.2 W (7.2 mJ/mm(2)), and it produced a sensation of warmth. The maximum surface temperature reached at the end of the period of stimulation was calculated to be 7 degrees C above the actual reference temperature of the skin (32 degrees C). The interval 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 nociceptive flexion reflex was facilitated by the thermal stimulus; this modulation occurred with particular conditioning-test intervals, which peaked at 500 and 1,100 ms with an additional late, longlasting phase between 1,600 and 2,300 ms. It was calculated that the conduction velocities of the cutaneous afferent fibers responsible for facilitating the R-III reflex, fell into three ranges: one corresponding to A delta 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 nociceptors converges. In this respect, the present data show, for the first time, that in man, conditioning nonnociceptive warm thermoreceptive AS and C fibers results in an interaction at the spinal level with a nociceptive reflex. This interaction may constitute a useful means whereby signals add together to trigger flexion reflexes in defensive reactions and other basic motor behaviors. It also may contribute to hyperalgesia in inflammatory processes. The methodology used in this study appears to be a useful noninvasive tool for exploring the thermoalgesic mechanisms in both experimental and clinical situations.