Nociceptor discharges and sensations due to prolonged noxious mechanical stimulation--a paradox.

Forceps were used to induce controlled squeeze stimuli to small skin folds of the dorsal surface of the hand of human volunteers. Psychophysical and microneurographic experiments were performed to characterize sensations and afferent input induced by stimuli exerting a constant force during a period of 120 s. The psychophysical data showed that forces greater than 4 N exerted on forceps faces of 30 mm2 elicited pain, which had two main characteristics: (a) stronger stimuli evoked a stronger pain response, (b) pain had a tendency to increase throughout the 120-s stimulus duration. In contrast, discharges of polymodal C-fibre nociceptors showed an initial high frequency dynamic discharge followed by adaptation. It was found that the ratio of C-fibre activity to activity in myelinated SA-fibres provided a better correlate of the time course of the stimulus-induced pain sensations than C-fibre activity alone. Possible reasons for the mismatch between nociceptor input and pain sensations are discussed.