SINGLE UNIT STUDIES ON VENTROLATERAL NUCLEUS OF THE THALAMUS IN CAT: ITS RELATION TO THE CEREBELLUM, MOTOR CORTEX AND BASAL GANGLIA

Abstract

Extracellular recordings were made from single neurons in cat''s ventrolateral nucleus (VL) of the thalamus to study the mode of their activity as relay neurons mediating cerebellar impulses to the cerebral cortex. The following results were obtained. 144 VL units were recorded which fired with a short latency to brachium conjunctivum stimulation. Latencies ranged from 1.2 to 4.6 msec and the greatest number was between 1.5 and 2.0 msec. These units were presumably excited monosynaptically. 68 out of 144 VL neurons could be driven antidromically by a stimulus to the pericruciate cortex. Lowest threshold points for the antidromic activation were predominantly distributed on the precruciate gyrus. Cortical stimulation also produced a prolonged inhibition of VL neurons, which may be attributed to recurr?nt inhibition. The inhibition was demonstrated after chronic ablation of the cortical gray matter. Twenty-one out of 144 VL neurons were activated synaptically by pericruciate cortex stimulation, suggesting the existence of corticofugal facilitatory projection to VL. Globus pallidus and caudate nucleus stimulation usually had a facilitatory effect on VL neurons with occasional evoked discharges. Stimulation of nonspecific thalamic nuclei produced discharges of VL neurons followed by a prolonged inhibition. Functional relationships between VL relay neurons and the motor cortex were discussed and the role of VL neurons as the specific relay neuron was analyzed.