An inhibitory interaction of human cortical responses to stimuli preferentially exciting Aδ or C fibers

Finely myelinated (type Aδ) and unmyelinated (type C) fibers are the major afferent inputs to spinothalamic tract neurons mediating sensory and reflex responses to noxious and thermal stimuli. These two fiber types differ in their sensory and biophysical properties, raising questions about the interaction of their supraspinal responses. Therefore, we investigated the interaction of cortical responses to stimuli that preferentially excite these fibers in human subjects using evoked potential recordings in a paired conditioning stimulation (CS) and test stimulation (TS) paradigm. There were two experiments, one with Aδ as CS and C as TS (Aδ-C) and another with these stimuli reversed (C-Aδ). We used intra-epidermal electrical pulses applied to the dorsal left hand at 2× and 1× pinprick threshold (pp) for the preferential stimulation of Aδ fibers and 37-50 °C contact heat pulses applied to the left or right thenar and left hypothenar eminences for the preferential stimulation of C fibers. We found that the cortical response to preferential Aδ or C fiber stimulation was attenuated whenever either cortical response preceded the other. Standardized values of peak and integrated amplitudes were <1 in all pairing conditions and in all subjects in both experiments. The suppressive effect varied in magnitude with the intensity of the conditioning stimulus in both Aδ-C and C-Aδ experiments. Furthermore, intra-segmental interaction was differentially effective for Aδ conditioning (peak amplitude, P<0.008; analysis of variance). Our experiments provide the first neurophysiological evidence for a somatotopically distributed, mutually suppressive interaction between cortical responses to preferentially activated Aδ and C afferents in humans. This suppressive interaction of cortical responses suggests contrasting and possibly mutually exclusive sensorimotor functions mediated through the Aδ and C fiber afferent channels.