1. Ramped heat stimuli were used to compare the effects of rate of temperature change on the responses of monkey nociceptors and on heat pain threshold in human subjects. Recordings were made from twenty-five cutaneous C fibre mechano-heat nociceptors (CMHs) innervating the hairy skin in the anaesthetized monkey. Heat pain thresholds were determined on the volar forearm of eight human subjects using a converging staircase technique. 2. The heat pain threshold decreased as stimulus ramp rate increased. In contrast, the CMH heat threshold, defined as the surface temperature at which the first action potential occurred, increased as stimulus ramp rate increased. Thus, the properties of the heat stimulus that dictate heat pain threshold are different from the properties of the heat stimulus that govern the initiation of a response in nociceptors. 3. Peak discharge frequency of CMHs during the heat ramp increased with stimulus ramp rate.Heat pain threshold was correlated with achievement of a minimum discharge rate in nociceptors (0 5 Hz), rather than with the threshold for action potential initiation.

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°C s'; (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 'C. These results suggest that the heat threshold for CMHs is determined by the temperature at the depth of the receptor. 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.

1. The responses to mechanical stimulation of myelinated fibers that originate from an acutely cut nerve or a neuroma were studied in the anesthetized monkey. The superficial radial or sural nerve was tightly ligated and cut. Either immediately (acute experiment) or 2-6 wk later (chronic experiment), single-unit recording techniques were used to record the evoked neural activity after vibratory mechanical stimulation (5-100 Hz; 50-800 microns) near the injury site. 2. The 30 myelinated afferents studied in the chronic experiments displayed an entrained response (1 action potential for each stimulus cycle) to vibratory stimuli applied at or near the nerve injury site. For 19 fibers, the minimum amplitude for entrainment was determined as a function of frequency (tuning curve). For 11 others, complete tuning curves were not obtained, although the frequency range over which they were most sensitive could be estimated. The afferents could be classified into three groups on the basis of the frequency range over which they were most sensitive: 1) a low-frequency group that was most sensitive to frequencies less than or equal to 5 Hz (n = 7), 2) a mid-frequency group that was most sensitive to a broad range of frequencies (i.e., 20-75 Hz, n = 13), and 3) a high-frequency group that was most sensitive to frequencies greater than or equal to 100 Hz (n = 10). These three response classes are similar to the three classes of response associated with the different low-threshold mechanoreceptors (i.e., slowly and rapidly adapting and Pacinian-like mechanoreceptors).(ABSTRACT TRUNCATED AT 250 WORDS)