Disynaptic tectal and pyramidal excitation of hindlimb motoneurons mediated by pontine reticulospinal neurons in the cat.

1. The pathway mediating disynaptic tectal and pyramidal excitation of hindlimb motoneurons was analyzed in cats anesthetized with alpha-chloralose or pentobarbital. Stimulation of the contralateral tectofugal fibers induced EPSPs in flexor and extensor hindlimb motoneurons (118/171). EPSP latencies, measured from the monosynaptically evoked descending volley, were 0.8 ms or less in 34 of the 118 motoneurons, suggesting disynaptic linkage from the tectum. The latencies tended to be shorter in motoneurons innervating proximal muscles than in those innervating distal muscles. 2. Stimulation of the cerebral peduncle induced EPSPs only in a small proportion of motoneurons (7/32). But the peduncular stimulation exhibited a marked facilitatory effect on the tectal EPSPs in most of the tested motoneurons (23/27), showing convergence of tectal and peduncular inputs onto relay cells. 3. In animals whose pyramid was transected, the tectal EPSPs were still facilitated by peduncular stimulation in 45 of 48 tested motoneurons. The time course of facilitation indicated convergence of tectofugal and corticofugal fibers onto brainstem relay neurons. 4. Projection of single neurons in the nucleus reticularis pontis caudalis (NRPC) to the gray matter of the hindlimb segments was examined by mapping thresholds of antidromic activation. Twelve of 13 tested neurons were excited by contralateral tectal stimulation at short latencies, probably monosynaptically. Four of them were found to project to lamina IX. Two of the 3 tested neurons projecting to lamina IX were found to receive excitatory input from the cerebral peduncle. 5. Stimulation of NRPC induced monosynaptic EPSPs in hindlimb motoneurons. In 19 motoneurons, the NRPC-induced monosynaptic EPSPs were facilitated by a conditioning tectal shock. This indicated that the tectal stimulus lowered thresholds of direct activation of cell bodies of premotor NRPC neurons. The time course of the facilitation indicated that the NRPC neurons received monosynaptic tectal excitation. The results provide strong evidence that NRPC neurons are involved in mediating disynaptic tectal excitation of hindlimb motoneurons.