The vestibulospinal tract: crossed and uncrossed effects on hindlimb motoneurones in the cat.

Effects produced by stimulation of Deiters' nucleus on contralateral hindlimb motoneurones were investigated in the cat with intracellular and monosynaptic reflex recording. Polysynaptic PSPs were evoked in all species of motoneurone examined. The minimal synaptic linkage was found to be disynaptic in the excitatory path and prisynaptic in the inhibitory path. Experiments with various lesions to the spinal funiculus and those with mapping of the stimulated sites in the brain stem showed that the effects were evoked almost exclusively via the lateral vestibulospinal tract of the stimulated side. The crossed effects were excitatory in extensor motoneurones of all species studied except for hip extensors (ABSm) in which EPSPs and/or IPSPs were evoked. The effects on flexors, by contrast, were not simple and consisted of EPSPs, IPSPs or a mixture of both. The difference in effects depended, though not entirely, on the species of motor nucleus. Between the excitatory and inhibitory effects on knee flexor (PBSt) motoneurones the former predominated under chloralose and the latter under pentobarbital anaesthesia. By recording PSPs evoked from the two vestibulospinal tracts in the same motoneurone, the convergence pattern and the magnitude of effects from the two tracts were studied. The interneuronal organization of the crossed vestibulo-motoneuronel pathway and its functional significance are discussed.