Functional anatomy of the basal ganglia. II. The place of subthalamic nucleus and external pallidum in basal ganglia circuitry.

The subthalamic nucleus and the external pallidum (GPe) are classically viewed as part of the so-called indirect pathway, which acts in concert with the direct pathway. The direct and indirect pathways form the conceptual framework of the anatomical and functional organization of the basal ganglia. A review of recent data regarding the connections of the subthalamic nucleus and the GPe has revealed a lack of firm anatomical support for the existence of the indirect pathway. However, newly recognized projections of the subthalamic nucleus and the GPe place these structures on various novel routes that change the conceptual architecture of the basal ganglia circuitry. These new findings force us to modify our view of the functional identity of the subthalamic nucleus and the GPe. In this new perspective, the GPe stands as an additional integrative station, together with the striatum and the internal pallidum and substantia nigra pars reticulata (GPi/SNr), along the main steam of information processing within the basal ganglia circuitry. Because of its crucial position between the input and output stations of the basal ganglia, the GPe can markedly influence the neuronal computation that occurs at GPi/SNr levels. The subthalamic nucleus can still be regarded as a 'control structure' lying alongside the main stream of information processing. However, because of its widespread efferent projections, the subthalamic nucleus exerts its driving effect on most components of the basal ganglia. Its action is mediated not only by the indirect pathway, but by a multitude of mono- and polysynaptic projections that ultimately reach the basal ganglia output cells.