Peripheral Coding of Tonic Mechanical Cutaneous Pain: Comparison of Nociceptor Activity in Rat and Human Psychophysics

Abstract

These experiments investigated temporal summation mechanisms of tonic cutaneous mechanical pain. Human volunteers provided psychophysical estimates of pain intensity, which were compared with discharge patterns of rat cutaneous nociceptors tested with identical stimulus protocols. Human subjects made either intermittent or continuous ratings of pain intensity during stimulation of the skin between the thumb and first finger. Stimulus intensities of 25, 50, and 100 g were applied with a probe of contact area of 0.1 mm2 for 2 min. Pain perception significantly increased during stimulation (temporal summation) for the 50- and 100-g stimulus intensities. Sequential conduction block of the myelinated fibers supplying the stimulated skin was used to investigate the role of A-fiber mechanoreceptors and nociceptors in this temporal summation. Conduction block of the Aβ fibers resulted in an increase in mechanically evoked pain estimates and an increase in temporal summation, consistent with loss of Aβ-mediated inhibition. When only conduction in the unmyelinated fibers remained, pain estimates were reduced to the preblock levels, but temporal summation was still present. Electrophysiological recordings were made from filaments of the sciatic nerve supplying receptors in the plantar skin of barbiturate-anesthetized rats. Forty units fulfilled the identification criteria for nociceptors: 20 A-fiber and 20 C-fiber nociceptors. Each unit was characterized by recording its responses to graded mechanical and heat stimuli. Nociceptors were also tested with stimuli identical to those applied to the human subjects. The responses of all units to sustained mechanical stimuli were adaptive—that is, they exhibited a gradual decline in response with time. However, the time course of adaptation varied among units. All the C-fiber nociceptors and one-half of the A-fiber nociceptors had rapidly adapting responses. The remainder of the A-fibers displayed slowly adapting responses. One-third of all units also showed short-duration increases in firing rate during stimulation. The latency after stimulus onset of this rate acceleration was inversely related to stimulus intensity. Despite the apparent disparity between perceptual temporal summation and nociceptor adaptation, central and peripheral mechanisms are proposed that can reconcile the relationship between nociceptor activity and pain perception.