Comparison of heat and mechanical receptive fields of cutaneous C-fiber nociceptors in monkey

Receptive-field properties were investigated in cutaneous C-fiber nociceptive afferents (CMH) responsive to mechanical and heat stimuli. Teased-fiber techniques were used to record from 28 CMHs that innervated the hairy skin of upper or lower limb in anesthetized monkeys. The response to mechanical stimuli was studied with the use of calibrated von Frey probes. The response to heat stimuli was studied with the use of a laser thermal stimulator that provided stepped increases in skin temperature with rise times to the desired temperature near 100 ms. The size of the receptive field (RF) for mechanical stimuli was determined by use of a suprathreshold stimulus that consisted of a 0.5-mm-diam probe that exerted a 200-mN force (10 bar). The size of the heat RF was determined by use of a 49°C stimulus applied to a 7.5-mm-diam area for 1 s. Heat thresholds were determined with an ascending series of stimulus intensities and were found to be stable over many hours; they ranged from 37 to 46°C (mean, 41.1°C). Mechanical thresholds ranged from 1.3 to 7.3 bar (mean, 3.3 bar). There was no correlation between mechanical and heat thresholds. Both thresholds extended well below the corresponding psychophysical pain thresholds in the literature. This suggests that spatial and/or temporal summation of C-fiber input are important for pain induced by either stimulus modality. Mechanical RF diameters ranged from 3.3 to 9.6 mm (mean, 4.7 mm); heat RF diameters ranged from punctate (<1 mm) to 9.5 mm (mean, 4.3 mm). There was a significant linear correlation between mechanical and heat RF sizes with a slope of one. The distance between the center of the mechanical RF and the center of the heat RF along one axis ranged from 0 to 1.1 mm (mean, 0.4 mm). These data indicate that the heat RFs coincided with the mechanical RFs. Within the mechanical RF determined with the suprathreshold stimuli, all CMHs had one or more punctate areas of maximal mechanical sensitivity where mechanical threshold was lowest. Heat excitability extended >2 mm beyond these mechanically sensitive spots. Because lateral transmission of the heat stimulus is small, this indicates that heat transduction occurs outside the regions of maximal mechanical sensitivity. Both the threshold to heat and the response magnitude at suprathreshold intensities depended on the percentage of the RF area overlapped by the heat stimulus. This indicates that multiple transducer sites probably contribute to the total evoked response. In conclusion, the structure of the heat RF suggests that the terminal arborization pattern of most CMHs consists of more than just a few branches at the spots of maximal mechanical sensitivity, but in fact covers a fairly large skin area with a multitude of transduction sites. This skin area corresponds to the size of the mechanical RF, determined with suprathreshold stimuli.