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

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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.

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