Electrophysiologic properties and nature of ventrolateral thalamic nucleus neurons reactive to converging inputs of paleo- and neocerebellar origin.

Acute experiments were carried out in chloralose-anesthetized, curarized, and artificially ventilated cats to investigate the consistency and functional characteristics of converging cerebellar nuclear (fastigial, interpositus, dentate nuclei) inputs to single neurons of the ventrolateral thalamic nucleus, and the nature (thalamocortical relay or interneuronal) of its convergent neuronal population responsive to stimulation of the cerebellar nuclei. From unitary analysis of both extra- and juxtacellular recordings, we conclude that convergence of paleo- and neocerebellar impulses on thalamic ventrolateral neurons occurs widely, and that both excitatory (mono- and polysynaptic) and inhibitory (di- and polysynaptic) effects are induced on the same neurons by cerebellar nuclei stimulation; these effects concern about 40% of the thalamic ventrolateral neuronal population tested using the extracellular recording technique and about 75% of the ventrolateral nucleus neurons tested using the juxtacellular recording; activation of neocerebellar nuclear outputs generates mainly excitatory and excitatory-inhibitory response patterns, whereas activation of the paleocerebellar nuclear outputs evokes predominantly inhibitory and excitatory-inhibitory response patterns; the convergent paleo- neocerebellar neurons are found commonly in the more anterior ventrolateral nucleus stereotaxic planes; and from the results obtained by stimulation of the somatomotor cortical areas, a large proportion of the ventrolateral nucleus convergent units consists of thalamocortical relay cells. The results have implications for a possible integration between posture and voluntary movements at the level of the thalamic relay nucleus of the cerebellar-cortical pathway, and an interaction between central feedback and peripheral feedback in the programming, execution, and correction of voluntary movements.