Crossed actions of group I muscle afferents in the cat.

Reflex actions evoked by electrical stimulation of contralateral quadriceps, hamstring and gastrocnemius-soleus muscle afferents were investigated using intracellular recording from motoneurones, in chloralose-anaesthetized, acute, low spinal cats. Contralateral group I afferents were found to evoke excitatory post-synaptic potentials only occasionally. Contralateral group II afferents evoked post-synaptic potentials (excitatory to extensor motoneurones and inhibitory to flexor motoneurones) from the quadriceps nerve, whereas contralateral group III afferents evoked post-synaptic potentials (with the same pattern as quadriceps group II) from all muscles tested. Contralateral group I afferents were found to facilitate Ia reciprocal inhibition, and both the excitatory and inhibitory reflex actions from Ib afferents. This is taken to indicate an action of these afferents on interneurones interposed in ipsilateral reflex pathways.