RESPONSE OF C-FIBER NOCICEPTORS IN THE ANESTHETIZED MONKEY TO HEAT STIMULI - ESTIMATES OF RECEPTOR DEPTH AND THRESHOLD

1. Responses to ramped or stepped temperature stimuli were obtained fr om fifty-three cutaneous C fibre mechano-heat nociceptors (CMHs) in th e hairy skin of the pentobarbitone-morphine anaesthetized monkey. A th ree-layer heat transfer model was developed to describe the temperatur e distribution within the skin and to estimate receptor depth and heat threshold. 2. Surface heat threshold, defined as the surface temperat ure 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 d ecreased from 38 to 35 degrees C. These results suggest that the heat threshold for CMHs is determined by the temperature at the depth of th e receptor. 3. Receptor depth estimates from responses to ramped stimu li ranged from 20 to 570 mu m with a mean of 201 mu m. The estimated m ean receptor heat threshold was 40.4 +/- 2.2 degrees C (+/- S.D). No c orrelation was observed between depth and thermal or mechanical thresh old. The average receptor depth and threshold, estimated from the resp onses to stepped heat stimuli, were 150 mu m and 40.2 degrees C, respe ctively. 4. We conclude that: (a) the receptor endings of CMHs occur i n the epidermis and dermis; (b) temperature at the level of the recept or determines threshold; (c) temperature at the receptor ending is muc h 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.