The pallidal complex includes the external (GPe) and internal segments (GPi) of the globus pallidus and the ventral pallidum (VP). The GPi and GPe receive inputs from the DS, both the caudate nucleus and putamen. The VP, generally located ventral to the anterior commissure, receives its input from the VS.18,25,26 The outputs from the GPi and SNr are to the thalamus, which then projects back to the cortex, completing the “direct” cortico-basal ganglia circuit. The GPe and parts of the VP project to the subthalamic nucleus (STN). The STN, in turn, provides inputs to the GPi. This pathway is considered the indirect pathway. Importantly, the STN also projects back to the GPe. In addition to connections with the basal ganglia, the STN also receives a direct projection from all parts of the frontal cortex.27 This is referred to as the hyperdirect pathway. The dopaminergic system includes the SNc, the ventral tegmental area (VTA), and the retrorubral group. Dopamine (DA) neurons from the SNc and the VTA project to the DS and VS, respectively.
