Information processing in cutaneous nociceptors in relation to sensations of pain.

The phenomena of fatigue and sensitization have been observed in the responses to heat of A-fiber and C-fiber mechanoheat nociceptors (AMHs and CMHs, respectively). After heat stimulation that is sufficiently intense to sensitize AMHs, CMHs within the stimulated area become fatigued, which suggests a dominant role of AMHs in contributing to cutaneous hyperalgesia under these conditions. Mild heat injuries, on the other hand, not sufficiently intense to sensitize AMHs, result in a characteristic time course in development of sensitization of CMHs that is matched by similar changes in pain threshold and in magnitude ratings of pain. Although available evidence does not suggest a simple relation between the shapes of psychophysical magnitude scaling functions and the average responses of nociceptors, it does support the conclusions that CMHs encode the intensity of painful heating of skin and that the alterations in responses of CMHs after mild heat injury contribute to the observed changes in scaling functions during the development of hyperalgesia.