Previous studies from our laboratory have indicated that the long-lasting changes in withdrawal reflexes induced by noxious stimuli are dependent on the location at which the noxious stimulus is applied (Clarke, 1999). We have explored this idea further by studying the effects of noxious stimuli applied at fourteen different locations on three withdrawal reflexes in the hindlimbs of decerebrated rabbits.

Experiments were performed in 34 rabbits decerebrated under nitrous oxide/halothane (2-2.5 %) anaesthesia. Reflexes were evoked in the semitendinosus (ST, knee flexor) and tibialis anterior (TA, ankle flexor) muscles by electrical stimulation of the plantar skin near the two central toes, while responses were elicited in the medial gastrocnemius (MG, ankle extensor) muscle by electrical stimulation of the skin just distal to the heel. Reflexes were recorded as compound EMG signals and integrated. Up to seven conditioning stimuli, separated by intervals of at least 62 min, were applied to each rabbit depending upon conditions. The stimulus used was 100 µl 20 % mustard oil in paraffin oil, and care was taken to keep this stimulus away from the stimulating electrodes. Four stimulation sites were on the plantar surface of the ipsilateral foot, one on the dorsal surface, two on the plantar surface of the contralateral hindpaw, one on the dorsal surface of the ipsilateral forepaw and one on the snout. The other locations stimulated were the TA and MG muscles of the ipsilateral hindlimb, the contralateral MG muscle and the ankle joint capsules on either side. Friedman’s ANOVA on ranks was used to determine if a mustard oil stimulus had significantly altered a reflex response. Experiments were terminated by I.V. injection of saturated KCl solution.

Application of mustard oil to the ipsilateral toe tips, plantar metatarsophalangeal (MT) joint or mid-point of the sole augmented the toes-TA and toes-ST reflexes (P < 0.05) for periods of ▓ge│ 19 min, while the heel-MG reflex was inhibited or unaffected from these sites. Noxious stimulation of the heel potentiated the toes-ST and heel-MG reflexes (P < 0.02) but had no effect on toes-TA. Stimulation of deep tissues had no consistent effects in all but one case. Stimulation of the dorsal surface of the ankle inhibited heel-MG and toes-TA (P < 0.02) but did not affect toes-ST. Stimuli applied off-limb generally produced the same pattern of changes, although applying mustard oil to the snout inhibited all three reflexes (P < 0.01) and stimulation of the forelimb had no consistent effects.

Thus the location at which a noxious stimulus is applied is a key determinant of the adaptive changes in reflexes that flow from the stimulus. The present data are consistent with the view that reflexes which protect the injured site are enhanced while other reflexes are inhibited or unaffected.This work was supported by BBSRC.

Clarke, R.W. (1999). J. Physiol. 521.P, 25S.