Figure 1.

Subcortical connectivity of the striatum. Widespread cortical projections reach the striatal neurons which are engaged in direct (blue) and indirect (red) pathways before reaching the thalamus which in turn projects back to the cortex. The former engages neurons which project to GPi and SNr, the latter involves neurons and detours through two synaptic stations, namely the GPe and the STN, prior to reach the GPi/SNr complex. Neurons involved in the direct pathway co-express dopamine receptor D1, substance P and dynorphin while those engaged in the indirect pathway co-express dopamine receptor D2 and enkephalin. These subsets of neurons demonstrated characteristic patterns of gene expression.