Synaptic responses of lumbar alpha-motoneurones to selective stimulation of cutaneous nociceptors and low threshold mechanoreceptors in the spinal cat.

The reflex projection from afferents of cutaneous nociceptors and low threshold mechanoreceptors onto intracellularly recorded lumbar alpha-motoneurones was tested in high spinal unanaesthetized cats. Low threshold mechanoreceptors were activated by light stroking of the hairy skin of the foot, nociceptors by radiant heat and in a few cases, for comparison, by pinching of the skin. In each experiment only one cutaneous nerve (Sur, SPC, Saph or Tib) was left intact. Flexor motoneurones (PBSt) generally showed a depolarization together with an increase of synaptic noise to both types of stimulation, the amplitude during nociceptive stimulation being larger in most cases. In extensor motoneurones (GS, ABSm) the responses to noxious and mechanical skin stimulation were more variable. Only a slight depolarization or no change of the level of the membrane potential, together with an increase of synaptic noise, was observed in most cases. Besides that, minor hyperpolarizations or transitions from light depolarization to hyperpolarization were induced during stimulation. The effects of noxious and mechanoreceptive skin stimulation were not strictly related to the effects of electrical stimulation of flexor reflex afferents. It is assumed that particularly for extensor motoneurones, the excitatory and the inhibitory segmental reflex pathways were activated in parallel during skin stimulation. The simultaneous action of the inhibitory pathway in addition to the excitatory one may serve as a mechanism to neutralize unwanted surplus excitation.