Figure 1.

Functional anatomy of the BG. Excitatory glutamatergic cortical input drives cells in the two input nuclei of the BG, the striatum and the STN. The GABAergic striatal projection neurons can be subdivided into two populations on the basis of the dominant dopamine receptor type (D₁ or D₂ type) that modulates their firing (see Materials and Methods). The D₁-dominant cells predominantly send inhibitory projections to the output nuclei, the SNr (shown here) and the EP nucleus (the model presented includes only the SNr, because the two nuclei are similar with respect to their intra-BG connections). These GABAergic cells in turn send tonic inhibitory output to their targets in the thalamus and brainstem. It is the removal of this tonic inhibition that is thought to signal the selection of an action; hence, the combination of striatal D₁ cells and the output nuclei are termed the selection pathway. The inset box shows the proposed off-center, on-surround network formed by this pathway, with hypothetical activity of the channel populations. The D₂-dominant cells of the striatum send inhibitory projections to the GABAergic cells of the GP; the GP cells in turn send inhibitory projections to the STN and SNr/EP and are a source of control signals for the basal ganglia (hence, this circuit is termed the control pathway). Glutamatergic cells of the STN send excitatory projections to the GP and the SNr/EP, contributing to both the tonic activity of the SNr/EP cells and the activity within the control pathway.