Nociceptive input to spinal interneurones in reflex pathways from group II muscle afferents in cats

Effects of noxious stimulation of the skin by radiant heat were tested on responses of first order interneurones in reflex pathways from group II muscle afferents in mid-lumbar, lower-lumbar and sacral segments of the spinal cord. In mid- and lower-lumbar segments both background discharges and monosynaptically evoked responses of intermediate zone interneurones were facilitated. Those of mid-lumbar dorsal horn interneurones were also facilitated suggesting that both these interneuronal populations contribute to the facilitation of flexion reflexes by nociceptors. In contrast, the dominating effects of noxious heat on sacral dorsal horn group II interneurones were inhibitory. The effects evoked by selective activation of C fibres, after A-delta fibres had been blocked by TTX, were similar to those obtained before TTX application.