Comparison of responses of warm and nociceptive C-fiber afferents in monkey with human judgments of thermal pain

Radiant-heat stimuli of different intensities were delivered every 28 s to the thenar eminence of the hand of human subjects and to the receptive fields (RFs) of 58 'mechanothermal nociceptive' and 16 'warm' C-fibers, most of which innervated the glabrous skin of the monkey hand. A CO₂ infrared laser under control via a radiometer provided a step increase in skin temperature to a level maintained within ±0.1°C over a 7.5-min-diameter spot. Human subjects categorized the magnitude of warmth and pain sensations evoked by stimuli that ranged in temperature from 40 to 50°C. The scale of subjective thermal intensity constructed from these category estimates showed a monotonically increasing relation between stimulus temperature and the magnitude of warmth and pain sensations. The mechanothermal fibers had a mean RF size of 18.9 ± 3.2 mm² (SE), a mean conduction velocity of 0.8 ± 0.1 m/s, mean thresholds of 43.6 ± 0.6°C for radiant heat and 5.95 ± 0.59 bars for mechanical stimulation, and no spontaneous activity. In contrast, warm fibers had punctate RFs, a mean conduction velocity of 1.1 ± 0.1 m/s, heat thresholds of less than 1°C above skin temperature, no response to mechanical stimulation, and a resting level of activity in warm skin that was suppressed by cooling. The cumulative number of impulses evoked during each stimulation in the nociceptive afferents increased monotonically as a function of stimulus temperature over the range described by humans as increasingly painful (45-50°C). Nociceptive fibers showed little or no response to stimulus temperatures less than 45°C that elicited in humans sensations primarily of warmth but not pain. In contrast, the cumulative impulse count during stimulation of each warm fiber increased monotonically with stimulus temperature over the range of 39-43°C. However, for stimuli of 41-49°C the cumulative impulse count in warm fibers was nonmonotonic with stimulus temperature. Warm-fiber response to stimuli of 45°C or greater usually consisted of a short burst of impulses followed by cessation of activity. The subjective magnitude of warmth and pain sensations in humans and the cumulative impulse count evoked by each stimulus in warm and nociceptive afferents varied inversely with the number, delivery rate, and intensity of preceding stimulations. The results of these experiments suggest the following: a) that activity in the mechanothermal nociceptive C-fibers signals the occurrence of pain evoked by radiant heat, and that the frequency of discharge in these fibers may encode the intensity of painful stimulation; b) that activity in warm fibers may encode the intensity of warmth at lower stimulus temperatures, but is unlikely to provide a peripheral mechanism for encoding the intensity of painful stimulation at higher stimulus temperatures.