Central suppression of cold-induced C fibre pain by myelinated fibre input.

Changes in thermal sensibility for warmth, cold, heat pain and cold pain during nerve compression block of impulse conduction in myelinated fibres were studied in 20 healthy subjects. When mainly unmyelinated fibres were conducting, after 30-36 min of nerve compression, the pain threshold, induced by cold stimulation, was shifted towards higher temperatures (from 19.1 degrees C to 22.8 degrees C, mean values). Furthermore, the sensation of cold pain became more unpleasant and had a hot burning rather than a cold quality. These results indicate that a change in central decoding of the afferent input has occurred, possibly due to lack of inhibition normally exerted by concomitant activation of myelinated fibres. Whereas dramatic changes in the sensation of cold pain were observed during the course of nerve compression, no alteration in heat pain threshold was seen. This implies that heat pain threshold in hairy skin is due to activation of C nociceptor fibres without any significant contribution from myelinated nociceptor fibres. Furthermore, no gating from heat-sensitive myelinated fibre input was evident on heat pain threshold.