Electrophysiological studies of two types of thalamo-cortical neurones and their responses to stimulation of mesencephalic reticular formation.

1. Thalamo-cortical (T-C) neurones projecting their axons to the motor cortex were recorded intra- or extra-cellularly in the anterior ventral and lateral ventral nuclear complex of the thalamus in lightly nembutalized cats. The T-C neurones were identified as responding antidromically to stimulation of the motor cortex, and were examined in connection with effects of stimulation of the centrum medianumparafascicular complex (CM), pallidum (Pal), cerebellar nuclei (CN) and mesencephalic reticular formation (RF). 2. The T-C neurones were classified into two groups, A and B. The A-group neurones responded neither to stimulation of CM nor to that of Pal. By contrast, the B-group neurones were activated from either CM, Pal or both. The effect of RF stimulation was facilitatory in many of the A-group neurones whereas it was mostly inhibitory in the B-group neurones. CN stimulation activated most of the T-C neurones in both A and B groups. 3. The A-group neurones were categorized as the deep T-C neurones mediating the deep T-C response of the cortex (the early surface-positive-deep-negative component of the augmenting response) and the B-group neurones as the superficial T-C neurones mediating the superficial T-C response of the cortex (the recruiting response and the late surface-negative-deep-positive component of the augmenting response). Desynchronization of electrocorticograms in response to RF stimulation was suggested to result from inhibition of the superficial T-C neurones and facilitation of the deep T-C neurones.