Figure 1.

BG architecture. The STN is the only excitatory nucleus within the BG. STN activity excites the GPi and substantia nigra pars reticulata (SNr), the two BG output structures, via direct projections, but also has an indirect inhibitory impact on the GPi via the GPe (Smith et al., 1994; Shink and Smith, 1995; Nambu et al., 2000). The projections between the STN and the GPe, as well as the GPe and the striatum form two recurrent loops potentially promoting oscillations. Excitatory projections are shown in red, inhibitory projections in blue.