ON THE CUTANEOUS RECEPTORS CONTRIBUTING TO WITHDRAWAL REFLEX PATHWAYSIN THE DECEREBRATE SPINAL RAT

Previous studies indicate that the withdrawal reflex system in the rat has a ''modular'' organization, each reflex pathway performing a spec ific sensorimotor transformation. Here, we wished to clarify which cut aneous receptors contribute to this system and to determine whether th ere are differences in this respect between reflex pathways of differe nt muscles. Withdrawal reflexes of the peroneus longus, extensor digit orum longus, and semitendinosus muscles were recorded with EMG techniq ues during high reflex excitability in decerebrate spinal rats (n=26). While maintained innocuous pressure on glabrous skin could elicit a s ustained reflex activity in all muscles studied, vibration of glabrous skin (10-300 Hz) always failed to evoke a reflex response, suggesting that slowly adapting, but not rapidly adapting, low-threshold mechano receptive fibers from this type of skin contribute to withdrawal refle x pathways. Thermal stimulation in the innocuous range, i.e., cooling from 32 to 17 degrees C, or warming the skin from 32 to 41 degrees C, always failed to produce reflex responses, indicating that neither col d nor warm receptors contribute to withdrawal reflex pathways. When ei ther cooling or warming the skin to the noxious temperatures of 1 degr ees C or above 45 degrees C, respectively, a reflex discharge was ofte n evoked in the muscles studied. Intradermal administration of histami ne, a potent pruritogenic substance, produced very weak, or no, reflex response. In contrast, mustard oil produced vigorous reflex responses in all muscles studied. These findings suggest that some chemonocicep tors contribute only weakly, or not at all, to withdrawal reflex pathw ays. The present data suggest that a selective set of cutaneous recept ors contribute to withdrawal reflex pathways and that different withdr awal reflex pathways receive input from essentially the same cutaneous receptor types.