Classical model of the basal ganglia (Albin et al., 1989; DeLong, 1990). Death of dopaminergic neurons in the SNc reduces inhibition of the indirect pathway via dopamine D₂ receptors and reduces activation of the direct pathway via dopamine D₁ receptors. Accordingly, the net effect of dopamine loss, as in Parkinson's disease, is increased inhibition of thalamocortical activity and impaired movement. This model predicts that basal activity of the STN should be increased in Parkinson's disease. GPe, External segment of the globus pallidus; GPi, internal segment of the globus pallidus.