Response of C fibre nociceptors in the anaesthetized monkey to heat stimuli: correlation with pain threshold in humans.

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.