Thalamic afferents to layer I of anterior sigmoid cortex originating from the VA-VL neurons with entopeduncular input.

Seventy seven thalamic neurons in the VA-VL nuclear complex of the cat which projected to the anterior sigmoid gyrus (ASG) were studied extracellularly, and their responses to stimulation of both the cerebellar nuclei (CN) and the entopeduncular nucleus (ENT) were examined. Forty two neurons were inhibited by ENT-stimulation but were not excited by CN-stimulation, while 27 neurons were excited by CN-stimulation but were not inhibited by ENT-stimulation. Eight neurons were inhibited by ENT-stimulation and also excited by CN-stimulation. Distribution of axons in each layer of ASG was examined by means of antidromic activation of the thalamic neurons by microstimulation at various depths of ASG. Almost all (13/14) of the ENT-inhibited neurons examined were antidromically activated by microstimulation in layer I with currents less than 35 microA. On the other hand, most (11/13) of CN-excited neurons were antidromically activated by weak microstimulation in layers deeper than the second, but were not activated by microstimulation in layer I with currents less than 40 microA. VA-VL neurons with inhibitory input from ENT were considered to project mainly to layer I of ASG.