Electrophysiological and morphological studies on thalamic neurons receiving entopedunculo- and cerebello-thalamic projections in the cat.

Electrophysiological studies on the entopedunculo- and cerebello-thalamic projections were performed by intracellular recordings in the thalamic VA, VL and VM nuclei of cats under sodium pentobarbital anesthesia. Identification of the thalamic neurons were performed electrophysiologically by antidromic activation on stimulation of the precruciate cortex (areas 4 and 6) and the caudate nucleus, and morphologically by intracellular staining with HRP through recording microelectrodes. One hundred and sixty-three neurons were collected in the VA, VL and VM nuclei. In 79 neurons penetrated in the medial and ventral parts of the VA and VL nuclei, stimulation of the entopeduncular nucleus induced monosynaptic IPSPs (latency of 1.1-3.5 ms, mean 2.07 ms). Sixteen neurons were identified as thalamo-cortical relay neurons and 3 were activated only orthodromically by precruciate stimulation. Seventy-eight neurons located dorsolaterally to the entopeduncular-influenced neurons received only cerebellar EPSPs. Only 6 neurons showed convergence of entopeduncular and cerebellar inputs. They were scattered around the border between the entopeduncular and cerebellar projection areas. Sixteen neurons could be stained intracellularly by HRP injection. From the pattern of dendritic arborization, two types of neurons can be distinguished: neurons whose dendrites spread radially in all directions and neurons whose dendrites extend mainly along the long axis of the soma for a long distance in the frontal plane, respectively. The former are relay cells to the cerebral cortex or the caudate nucleus (i.e. projection neurons) and the latter appear to be interneurons in the thalamus.