The subthalamo-pallidal system constitutes the second layer of circuitry in the basal ganglia, lying downstream of the striatum. It consists of four nuclei. Two of them, the external segment of the globus pallidus (GPe) and subtha-lamic nucleus (STN), make their connections primarily within the basal gan-glia. The other two, the internal segment of the globus pallidus (GPi) and the substantia nigra pars reticulata (SNr), are the output nuclei of the basal gan- two categories of striatal principal cells, which are called the direct and indi-rect pathway neurons. Both of these are the axonal projections of striatal spiny principal cells. Striatal cells of the indirect pathway project to the GPe only, whereas the direct pathway cells project to both the GPe and the GPi/SNr. The dendritic trees of neurons of the GPe are roughly planar and oriented normal to the trajectory of descending striatal axons. The nucleus is divided into two parts on the basis of striatal axonal arborizations (Kawaguchi et al., 1990 ). One part is a thin layer subjacent to the boundary with the striatum, and the other is the remainder of GPe. Striatal axons entering the GPe each make an initial collateral arborization confi ned to this boundary layer, and then make one or more other arborizations deeper in the nucleus. Both sets of striatal arborizations form topographical representations. Thus, in the bound-ary layer there is a highly compressed projection of the striatal cells of origin, collapsed along the direction normal to the striato-pallidal boundary, whereas in the remainder of the GPe the striato-pallidal projection makes connections that approximate a three-dimensional map of the striatum. Direct pathway neurons make about 50 % fewer synapses in the GPe than indirect pathway neurons, so that the striatal innervation of the GPe is approximately two-thirds from the indirect and one-third from the direct pathway.