Laminar origin of cortical neurons which project upon the caudate nucleus: a horseradish peroxidase investigation in the cat.

The horseradish peroxidase method was used to study the cortical projections upon the caudate nucleus in the cat. The cells of origin of the corticostriatal projection arise from both the supragranular (II and III) and infragranular (V and VI) cortical layers, in contrast to all other known corticofugal pathways to subcortical structures, which appear to arise only from the infragranular layers. This finding has been verified in the present study by several novel surgical procedures, which include direct injections into the caudate nucleus, after the overlying tissues have been removed. The present study also shows that there is a truly widespread distribution of the cells contributing to the corticostriatal projection. Some portion of every neocortical gyrus contains labeled cells, though their relative numbers vary greatly. Also, simple topographical relationships are maintained, but there is much overlap. In addition, present findings show that the corticostriatal projection is strikingly bilateral and originates from regions which are homotopic to the ipsilateral projection.