Normal functioning of cortico-BG motor loop as proposed by Alexander, Delong and Strick (1986), Albin, Young, and Penney (1989) and Nambu (2002). Cx, cerebral cortex; GPe, external globus pallidum; GPi, internal globus pallidum; SNc, subtantia nigra pars compacta; SNr, substantia nigra pars reticulata; STN, subthalamic nucleus; Str, striatum; Th, thalamus. Illustration adapted with permission from Nambu (2011).