Relations between the ventrolateral thalamic nucleus and motor cortex and their possible role in the central organization of motor control

Abstract

1. (1) In cats anesthetized with chloralose, the topographical relations between ventorlateral nucleus and motor cortex were investigated with extracellular microelectrodes and with macroelectrodes. 2. (2) A rough mediolateral topography of the projections from VL cells to motor cortex was observed. The medial VL neurons project mainly to area 6 and the medial part of area 4 (area for axial and proximal muscles) whereas the lateral neurons project to area 4 predominantly in its lateral part (distal forelimb area). The cells in the middle of the nucleus send their axons to the entire motor cortex. 3. (3) Eight neurons were antidromically invaded from more than one site in the motor cortex. These neurons with branching axons are thought to play a special role in central organization of postures and movements. 4. (4) Postsynaptic responses from the motor cortex were almost always (with only 2 exceptions) elicited by the same electrode used to antidromically activate a VL cell. Other motor cortical electrodes were generally ineffective. These results suggest that the synaptic responses observed in our experiments are induced by axonal recurrent collaterals or by some autoexcitatory process between the soma and the dendrites. 5. (5) The inhibitory effect induced by stimulation of motor cortex can be graded by varying the intensity of stimulation. Inhibitory effects from various cortical sites on the same neuron were frequently observed. 6. (6) The topography of the inhibitory effect is comparable to the topography of the projections from VL to cortex. The lateral part of the VL is inhibited from the lateral and medial part of area 4 (distal and proximal forelimb area), the medial