Response of C fibre nociceptors in the anaesthetized monkey to heat stimuli: estimates of receptor depth and threshold.

1. Responses to ramped or stepped temperature stimuli were obtained from fifty-three cutaneous C fibre mechano-heat nociceptors (CMHs) in the hairy skin of the pentobarbitone-morphine anaesthetized monkey. A three-layer heat transfer model was developed to describe the temperature distribution within the skin and to estimate receptor depth and heat threshold. 2. Surface heat threshold, defined as the surface temperature when the first action potential occurs, increased as: (a) the rate of temperature rise for the ramped stimuli increased from 0.095 to 5.8 degrees C s-1; (b) the duration of stepped heat stimuli decreased from 30 to 1 s; and (c) the base temperature of stepped heat stimuli decreased from 38 to 35 degrees C. These results suggest that the heat threshold for CMHs is determined by the temperature at the depth of the receptor. 3. Receptor depth estimates from responses to ramped stimuli ranged from 20 to 570 microns with a mean of 201 microns. The estimated mean receptor heat threshold was 40.4 +/- 2.2 degrees C (+/- S.D.). No correlation was observed between depth and thermal or mechanical threshold. The average receptor depth and threshold, estimated from the responses to stepped heat stimuli, were 150 microns and 40.2 degrees C, respectively. 4. We conclude that: (a) the receptor endings of CMHs occur in the epidermis and dermis; (b) temperature at the level of the receptor determines threshold; (c) temperature at the receptor ending is much lower than skin surface temperature at threshold; and (d) the tight distribution of receptor heat thresholds suggests a uniform transducer mechanism for heat in CMHs.