Sensations evoked by intraneural microstimulation of C nociceptor fibres in human skin nerves.

1. Seventy-one C polymodal nociceptors supplying glabrous and hairy skin in limbs of awake human volunteers were identified on the basis of cutaneous stimulus-response characteristics recorded intraneurally by microneurography (MNG). The large majority of such units were primarily detected during intraneural microstimulation (INMS) on the basis of subjective quality and cutaneous localization of evoked painful sensation. Electrophysiological studies were supplemented with rigorous psychophysical testing during microstimulation delivered at intraneural C recording sites. 2. The conduction velocity of single C nociceptor units could be shown to become transiently slowed following repetitive INMS at threshold intensity for conscious sensation. Such 'marking' witnessed that particular C units, identified by recording, had been effectively activated during INMS and psychophysical testing. 3. Cognitive attributes of sensations evoked from C recording sites by INMS at threshold intensity for perception were estimated psychophysically for subjective quality, temporal attributes and localized projection. There was remarkable matching of physiological unit type (C polymodal nociceptor) with subjective quality of evoked sensation (dull or burning pain). Further, there was remarkable spatial matching of receptive field of given C nociceptors with projected field of the pain sensation evoked from the C recording site by INMS delivered at threshold intensity for conscious sensation. 4. Dissociated A nerve fibre blocks caused by compression-ischaemia did not abolish the sensation of burning pain projected to hairy skin, evoked by INMS delivered at C recording sites. 5. While the double matching of (a) subjective quality and spatial localization with (b) objective physiological unitary type and receptor location, coupled with the results of A blocks, provide evidence that C nociceptor fibres can be fairly selectively activated during INMS, the results also attest that C polymodal nociceptors from human skin evoke delayed dull or burning pain, accurately projected to a defined locus in skin, even after spatial summation is reduced to a minimum.