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 were strongly inhibited by conditioning mechanical, thermal (CO2 laser) and intracutaneous electrical stimulation of specific skin areas. By contrast, withdrawal reflexes in the biceps posterior-semitendinosus muscles 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 area, which would move towards the stimulation on contraction in the respective muscle. In addition, weak nociceptive inhibition was evoked from the corresponding skin areas on the contralateral hindlimb and, in some muscles, the tail. 4. The ipsilateral inhibitory and excitatory receptive fields of the withdrawal reflexes in single muscles overlapped somewhat. On stimulation of these transitional areas the reflex responses were preceded by a short-lasting inhibition. 5. Graded mechanical and thermal stimulation demonstrated prominent inhibitory effects from nociceptive receptors. Weak inhibitory effects were elicited by innocuous mechanical stimulation, suggesting a weak contribution from low threshold mechanoreceptors. Latency measurements indicated an inhibitory input from both myelinated and unmyelinated fibres. 6. In conclusion, the withdrawal reflex pathways receive a powerful nociceptive inhibitory input through spinal pathways. The movement-related organization of this input suggests that it serves to prevent inappropriate withdrawal reflexes.