EFFECTS OF STIMULATION OF CEREBELLAR AND THALAMIC NUCLEI UPON SPINAL ALPHA MOTONEURONS OF THE CAT

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Abstract
Effects of stimulation of cerebellar and thalamic nuclei on spinal function were tested by recording monosynaptic reflex discharges from ventral roots and recording intracellular potentials from alpha motoneurons in the lumbar segments of the cat. The effect elicited by single pulse stimulation of nuclei was generally the same between animals under very light anesthesia. However, it varied with a slight change in an animal''s condition. Single pulse stimulation of the fastigial nucleus (FN) facilitated the monosynaptic reflex set up by tibial nerve stimulation (extensor), first at a latency of about 10 msec with a relatively short duration of effect, and than at a latency of 20-30 msec with a long-lasting time course of 50-100 msec. The facilitations appeared bilaterally but were more marked on the ipsilateral side than the contralateral. Little effect was observed with FN stimulation upon the relfex set up by peroneal nerve stimulation (flexor). Single pulse stimulation of the interpositus nucleus (IN) produced facilitation of the ipsilateral flexor reflex at a latency of 10-15 msec, while inhibiting the ipsilateral extensor reflex at a latency of about 15 msec. Stimulation of the dentate nucleus (DN) produced long-lasting facilitation of bilateral peroneal and tibial reflexes. The facilitation usually appeared after a latency of 50-100 msec and continued for about 100 msec or more. It was often preceded by a brief facilitatory period with a latency of 10-15 msec. Single pulse stimulation of the ventral-lateral nucleus of the thalamus (VL) elicited facilitation of the bilateral peroneal and tibial reflexes at a latency of about 15 msec which was more pronounced upon the contralateral side. The time course of the facilitation showed an initial steep increases and a subsequent slow decay. The lapse of the decay was always longer in the extensor reflex than the flexor one. Effects of repetitive stimulation of a nucleus were, usually, deducible from the effect of single pulse stimulation of that nucleus as its temporal summation. However, they were rather complicated when compared with the single stimulation. The facilitation and inhibition of the monosynaptic reflex by stimulating cerebellar and thalamic nuclei seemed, in almost all cases, to be elicited by PSP mechanism in motoneurons which was produced directly or indirectly with descending impulses through spinal tracts. DN stimulation set up the inhibitory effect upon some motoneurons without any remarkable membrane potential change. Analyses of the effect showed, however, that the inhibition may be due to the shunting effect of the motoneuronal membrane by both EPSP''s and IPSPs which converged upon the moto-neuron and were neutralized nutually at the resting potential level. Most of PSP''s in motoneurons, elicited by the cerebellar and thalamic stimulation, especially the late group PSP''s, consisted of both EPSP''s and IPSP''s. Facilitation or inhibition of the motoneurons by stimulation was the net result of both PSP''s.