Red and black arrows are used to show projections from distinct neuronal
populations. The thickness of arrows in these diagrams indicates relative
differences in the extent of target innervation by the afferent inputs. For
simplicity, some corticostriatal neuron subtypes are not shown. M1, primary
motor cortex; RRA, retrorubral area; SNc, substantia nigra pars compacta; STN,
subthalamic nucleus; VTA, ventral tegmental area.