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Coupling between feline cerebellum (fastigial neurons) and motoneurons innervating hindlimb muscles.

The aims of the study were twofold: (1) to verify the hypothesis that neurons in the fastigial nucleus excite and inhibit hindlimb alpha-motoneurons and (2) to determine both the supraspinal and spinal relays of these actions. Axons of fastigial neurons were stimulated at the level of their decussation in the cerebellum, within the hook bundle of Russell, in deeply anesthetized cats with only the right side of the spinal cord intact. The resulting excitatory postsynaptic potentials and inhibitory postsynaptic potentials were analyzed in motoneurons on the left side of the lumbar enlargement. Postsynaptic potentials evoked by the first effective stimulus were induced at latencies <2 ms from descending volleys and <1 ms from interneuronally relayed volleys, indicating a trisynaptic coupling between the fastigial neurons and alpha-motoneurons, via commissural interneurons on the right side. Cerebellar stimulation facilitated the synaptic actions of both vestibulospinal and reticulospinal tract fibers. However, the study leads to the conclusion that trisynaptic fastigial actions are mediated via vestibulospinal rather than reticulospinal tract fibers [stimulated within the lateral vestibular nucleus (LVN) and the medial longitudinal fascicle (MLF), respectively]. This is indicated firstly by collision between descending volleys induced by cerebellar stimulation and volleys evoked by LVN stimuli but not by MLF stimuli. Second, similar cerebellar actions were evoked before and after a transection of MLF. Mutual facilitation between the fastigial and reticulospinal, as well as between the fastigial and vestibulospinal actions, could be due to the previously reported integration of descending vestibulospinal and reticulospinal commands by spinal commissural interneurons.