Response of cutaneous A- and C-fiber nociceptors in the monkey to controlled-force stimuli

The goal of this study was to determine the capacity of primary afferent no ciceptive fibers (nociceptors) to encode information about noxious mechanic al stimuli in primates. Teased-fiber techniques were used to record from 14 A-fiber nociceptors and 18 C-fiber nociceptors that innervated the hairy s kin. Stimulus-response functions were examined with an ascending series of force-controlled stimuli. Stimulus-interaction effects were examined with u se of a series of paired stimuli in which the interval between the stimulus pairs was varied systematically. Both A-fiber and C-fiber nociceptors exhi bited a slowly adapting response to the stepped force stimuli. The response of the A fibers increased monotonically with increasing force, whereas the response of the C fibers reached a plateau at low force levels. The slope of the stimulus-response function for the A fibers was significantly steepe r than that for the C fibers, and the total response was greater. The A fib ers also provided more discriminative information regarding stimulus intens ity. The C fibers demonstrated a significant fatigue in response when the i nterstimulus interval between the paired stimuli was less than or equal to 150 s, whereas the A fibers did nut demonstrate a significant fatigue until the interstimulus interval was less than or equal to 30 s. This fatigue in response was not due to changes in tissue compliance. These results sugges t that A- and C-fiber nociceptors have different mechanical transduction me chanisms. A-fiber nociceptors exhibit steeper stimulus-response functions a nd less fatigue than C-fiber nociceptors.