Evidence for central summation of C and Aδ nociceptive activity in man

Abstract

Using two different stimulators, we have induced activity in Aδ and C afferents in order to investigate a possible summation of nociceptive activity from these two fiber types. We used nociceptive electrical stimulation to evoke activity in Aδ fibers. High-intensity light from a xenon lamp, focused into a liquid light guide which was positioned on a spot painted black under the sole of the foot, resulted in a characteristic delayed burning sensation, indicating selective C-fiber activation. By varying the delay between radiant heat and electrical stimuli (0–3000 msec), sensations evoked by these stimuli were brought to coincide. When we elicited the electrical stimulation during on-going burning pain, corresponding to a delay of approximately 1 sec between the stimulations, we found a significantly higher nociceptive withdrawal reflex in tibialis anterior ( P < 0.01) and a higher overall pain rating ( P < 0.05). The existence of a summation mechanism at the spinal cord is the most likely explanation for our findings. Furthermore, the results demonstrate that the nociceptive reflex may be modulated by on-going C-fiber activity.