Red nucleus inputs to retractor bulbi motoneurones in the cat

Rubral projections to retractor bulbi motoneurones were investigated by intracellular recording in the accessory abducens nucleus in the cat. Their post-synaptic effects were compared with those of trigeminal afferent pathways. Post-synaptic potentials evoked by stimulation of the contralateral red nucleus were predominantly excitatory, with latencies in the monosynaptic range. Post-synaptic potentials evoked by stimulation of the ipsilateral red nucleus, at threshold, were predominantly inhibitory with latencies indicating a di- or trisynaptic pathway. At above threshold intensity, an earlier excitatory post-synaptic potential (e.p.s.p.) preceded the inhibitory post-synaptic potential (i.p.s.p.) in about half the recorded neurones. Convergence of rubral and trigeminal inputs was also observed in other neurones, not identified as retractor bulbi motoneurones, but situated within the accessory abducens nucleus.