Effects of heterotopic- and segmental counter-stimulation on the nociceptive withdrawal reflex in humans

A nociceptive withdrawal reflex in 12 human volunteers was elicited by pain ful electrical stimulation applied to the cutaneous innervation area of the sural nerve. The evoked electromyographic reflex activities were recorded with surface electrodes placed on the short head of the biceps femoris musc le ipsi-lateral to sural nerve stimulation, before, during and after condit ioning stimuli. The nociceptive withdrawal reflex was conditioned by nocice ptive and non-nociceptive, heterotopic and segmental counter-stimulation. H eterotopic nociceptive counter-stimulation and segmental nociceptive counte r-stimulation were induced by ice water applied to the contra-lateral hand and foot, respectively. Heterotopic nociceptive counter-stimulation produce d a significant inhibition of the nociceptive withdrawal reflex and the ass ociated pains rating to sural nerve stimulation. Segmental nociceptive coun ter-stimulation inhibited the pain rating and tended to inhibit the nocicep tive withdrawal reflex. There was no obvious correlation between visual ana logue scale (VAS) and the nociceptive withdrawal reflex elicited by sural n erve stimulation neither in the group nor in the individual subject. This s uggests that the nociceptive withdrawal reflex cannot always be used as a q uantitative measure of pain.