Crossed reflex actions from group II muscle afferents in the lumbar spinal cord of the anaesthetized cat.

1. The effects of electrical activation of muscle nerves at strengths sufficient to activate group II afferents in contralateral hindlimb motoneurones have been investigated by intracellular recording from motoneurones in alpha-chloralose-anaesthetized cats in which the spinal cord was intact or transected at a low thoracic level. 2. A large proportion of the motoneurones encountered could be influenced by activation of contralateral group II afferents. With the spinal cord intact, the predominant effect of contralateral group II afferents was inhibitory in almost all motoneurones, including extensors. After spinal transection the same stimuli elicited EPSPs in extensor motoneurones and IPSPs in flexors. 3. Both with the spinal cord intact and following spinal transection, group II afferents from particular nerves had powerful effects, whereas others were without effect in most neurones and had weak effects in others. Stimulation of gastrocnemius-soleus or of the hamstring nerves was largely ineffective, evoking weak effects in a small minority of motoneurones. Stimulation of the quadriceps, tibialis anterior-extensor digitorum longus and flexor digitorum longus nerves had powerful effects and in a high proportion of motoneurones. 4. The minimal latencies of the IPSPs evoked prior to spinalization were directly comparable to those of IPSPs evoked from ipsilateral group II afferents. The possibility that the mid-lumbar region, where many neurones which mediate ipsilateral group II reflexes are located, might mediate these crossed effects is discussed.