Figure 1.

Direct and indirect basal ganglia pathways. The cerebral cortex sends input to the striatum. Dopaminergic projections from the SNc (violet connectors) target striatal neurons expressing D1 or D2 receptors. Direct pathway (orange connectors): D1 neurons send direct inhibitory projections to the SNr/GPi. Indirect pathway (blue connectors): D2 neurons connect indirectly to the GPi/SNr through the GPe and STN. The SNr inhibits the SC. In Parkinson’s disease, dopaminergic depletion leads to reduced inhibitory direct pathway output (thin lines) and increased excitatory indirect pathway output (thick lines) onto the GPi/SNr and, consequently, increased SNr inhibition onto the SC as net effect. SNc, substantia nigra pars compacta; GPe, external globus pallidus; STN, subthalamic nucleus; GPi, internal globus pallidus; SNr, substantia nigra pars reticulata; SC, superior colliculus.