Figure 1: The cortico-striatal-thalamic-cortical circuits, including the indirect and direct pathways; ENK = enkephalin; GPe = globus pallidus, external segment; GPi = globus pallidus, internal segment; SNc = substantia nigra, pars compacta; SNr = substantia nigra, pars reticulata; SP/DYN = substance P/dynorphin; STh = subthalamic nucleus; D1 or D2: medium-sized spiny neurons with D1 or D2 receptors.