RESPONSE OF C-FIBER NOCICEPTORS IN THE ANESTHETIZED MONKEY TO HEAT STIMULI - CORRELATION WITH PAIN THRESHOLD IN HUMANS

1. Ramped heat stimuli were used to compare the effects of rate of tem perature change on the responses of monkey nociceptors and on heat pai n threshold in human subjects. Recordings were made from twenty-five c utaneous C fibre mechano-heat nociceptors (CMHs) innervating the hairy skin in the anaesthetized monkey. Heat pain thresholds were determine d on the volar forearm of eight human subjects using a converging stai rcase technique. 2. The heat pain threshold decreased as stimulus ramp rate increased. In contrast, the CMH heat threshold, defined as the s urface temperature at which the first action potential occurred, incre ased as stimulus ramp rate increased. Thus, the properties of the heat stimulus that dictate heat pain threshold are different from the prop erties of the heat stimulus that govern the initiation of a response i n nociceptors. 3. Peak discharge frequency of CMHs during the heat ram p increased with stimulus ramp rate. Heat pain threshold was correlate d with achievement of a minimum discharge rate in nociceptors (0.5 Hz) , rather than with the threshold for action potential initiation.