Fig. 1.

Motor circuit components of importance to Parkinson's disease (PD). Black arrows indicate inhibitory [gamma-aminobutyric acid (GABA)-ergic] connections; gray arrows indicate excitatory (glutamatergic) connections. Cortex includes the cortical motor areas: primary motor, supplementary motor area (SMA), premotor areas, and cingulate motor areas. GPe, external segment of the globus pallidus; GPi, internal segment of the globus pallidus; SNpc, substantia nigra pars compacta; SNr, substantia nigra pars reticulata; STN, subthalamic nucleus. [Figure adapted from Delong and Wichmann (2007) with permission.]