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  • Title: Evidence for central summation of C and A delta nociceptive activity in man.
    Author: Andersen OK, Jensen LM, Brennum J, Arendt-Nielsen L.
    Journal: Pain; 1994 Nov; 59(2):273-280. PubMed ID: 7892025.
    Abstract:
    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.
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