Single C nociceptor responses and psychophysical parameters of evoked pain: effect of rate of rise of heat stimuli in humans.

1. Effects of rate of rise of temperature stimuli applied to skin on (i) unitary receptor threshold and frequency response often single C nociceptors, and (ii) on magnitude and reaction times of evoked pain were studied in fifteen healthy human volunteers. 2. Temperature ramps of 32 to 45 or 47 degrees C were applied at three consistent rates of rise to receptive fields of C nociceptors in dorsum of foot (n = 9) or hand (n = 1). For rates of rise of 0.3, 2.0 and 6.0 degrees C/s, mean receptor threshold for heat was remarkably uniform: 41.5 +/‐ 0.57, 41.5 +/‐ 0.61 and 41.9 +/‐ 0.71 degrees C respectively. 3. The mean discharge rate of the ten cutaneous C nociceptors increased with rate of rise of temperature stimuli: 1.22 +/‐ 0.13, 4.57 +/‐ 0.49 and 13.45 +/‐ 0.71 impulses/s, respectively, for stimulus temperature rates of 0.3, 2.0 and 6.0 degrees C/s. 4. Magnitude estimates of pain for thirteen subjects also increased with rate of rise of temperature stimuli. Mean normalized magnitude estimates of heat pain were: 11.8 +/‐ 1.55, 15.1 +/‐ 0.84 and 28.0 +/‐ 1.87 for stimulus rates of rise of 0.3, 2.0 and 6.0 degrees C/s, respectively. 5. Results of simultaneous recordings of reaction time for pain and of C nociceptor responses to heat ramps given at 2.0 degrees C/s, in three subjects, indicate that under those circumstances heat pain messages are exclusively mediated by C nociceptors.