Evidence for central summation of C and A delta nociceptive activity in man.
Using two different stimulators, we have induced activity in A delta 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 A delta 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.
Andersen
Ole Kæseler
OK
Department of Medical Informatics, Laboratory for Experimental Pain Research, Aalborg University, DK-9220 AalborgDenmark Department of Neurology, Laboratory for Sensory Physiology, Gentofte University Hospital, DK-2900 HellerupDenmark.
Jensen
Lars Mohr
LM
Brennum
Jannick
J
Arendt-Nielsen
Lars
L
D000328:Adult; D004558:Electric Stimulation; D004576:Electromyography; D005260:Female; D006801:Humans; D008297:Male; D009046:Motor Neurons / Q000502:physiology; D009412:Nerve Fibers / Q000502:physiology*; D009413:Nerve Fibers, Myelinated / Q000502:physiology*; D009619:Nociceptors / Q000502:physiology*; D010147:Pain Measurement; D010775:Photic Stimulation; D011930:Reaction Time / Q000502:physiology; D013497:Sural Nerve / Q000502:physiology