CUTANEOUS INHIBITORY RECEPTIVE-FIELDS OF WITHDRAWAL REFLEXES IN THE DECEREBRATE SPINAL RAT

1. The inhibitory cutaneous input to the withdrawal reflex pathways to single hindlimb muscles was investigated in decerebrate spinal rats ( n = 53) using electromyography. 2. Withdrawal reflexes in the peronei, extensor digitorum longus and tibialis anterior muscles of the leg we re strongly inhibited by conditioning mechanical, thermal (CO2 laser) and intracutaneous electrical stimulation of specific skin areas. By c ontrast, withdrawal reflexes in the biceps posterior-semitendinosus mu scles of the thigh could only be weakly inhibited by conditioning skin stimulation. 3. Powerful inhibition of withdrawal reflexes in single lower leg muscles was elicited from the ipsilateral hindpaw plantar ar ea, which would move towards the stimulation on contraction in the res pective muscle. In addition, weak nociceptive inhibition was evoked fr om the corresponding skin areas on the contralateral hindlimb and, in some muscles, the tail. 4. The ipsilateral inhibitory and excitatory r eceptive fields of the withdrawal reflexes in single muscles overlappe d somewhat. On stimulation of these transitional areas the reflex resp onses were preceded by a short-lasting inhibition. 5. Graded mechanica l and thermal stimulation demonstrated prominent inhibitory effects fr om nociceptive receptors. Weak inhibitory effects were elicited by inn ocuous mechanical stimulation, suggesting a weak contribution from low threshold mechanoreceptors. Latency measurements indicated an inhibit ory input from both myelinated and unmyelinated fibres. 6. In conclusi on, the withdrawal reflex pathways receive a powerful nociceptive inhi bitory input through spinal pathways. The movement-related organizatio n of this input suggests that it serves to prevent inappropriate withd rawal reflexes.