LOCALIZATION OF FACILITORY AND INHIBITORY SITES IN AND AROUND THE CEREBELLAR NUCLEI AFFECTING LIMB POSTURE, ALPHA AND GAMMA MOTONEURONS

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Abstract
In decerebrate cats, the effects of repetitive, near-threshold stimulation of cerebellar fastigial and interpositus nuclei and their vicinity were observed on: (a) postural tone of the forelimbs, (b) tonic stretch reflexes of hindlimb extensor alpha motoneurons, (c) tonic gamma (fusimotor) neuron activity of same hindlimb extensors. The stimulation sites were histologically controlled. Ipsilateral fastigial stimulation, with respect to the hind-limb under study, usually inhibited the alpha extensor stretch reflex, contralateral fastigial stimulation facilitated the same alpha response. The concomitant postural changes in the forelimbs were, as a rule, synergistic on the ipsilateral side and reciprocal on the contralateral side. Ipsilateral hindlimb and/or forelimb extensor facilitation resulted from stimulation sites in the rostro-dorsal white substance adjacent to the fastigial nucleus. Mixed effects were obtained from interpositus stimulation; extensor inhibition dominated with more medially located sites, extensor facilitation with more laterally located sites in and near the ipsilateral interpositus nucleus. The great majority of ipsi- as well as contralateral cerebellar stimulation sites enhanced the hindlimb extensor gamma activity. The few inhibitory or diphasic (inhibitory/facilitory) gamma effects which were observed were obtained from regions which exerted similar effects upon the extensor alpha units. Occasional stimulation of the lateral cerebellar nuclei and their vicinity revealed inconsistent effects upon forelimb posture and hindlimb extensor motoneurons.