Current surgical therapies are based upon a model of basal ganglia function, which suggests that motor activity is regulated by the balanced activity of two separate circuits within the basal ganglia (8). This model was able to explain the effect of known ablative targets, such as the globus pallidus interna (GPi), STN, and thalamus. Although the model has been instructive, further study has demonstrated a far greater complexity within the basal ganglia than previously appreciated. Numerous anatomic studies have unveiled a far more complex connectivity in the basal ganglia than accounted for in the model. It is clear now that there are dopaminergic projections to the pallidum and STN, as well as to the striatum. The thalamic centromedian and parafascicular nuclei form important "stabilizing" circuits with the pallidum and STN (9,10). The pedunculopontine nucleus (PPN) is also considered to be a potentially important output nucleus, which receives projections from the STN (11). These findings point to a more complex, internally regulating system, of which the traditionally described direct and indirect pathways tell only part of the story (12-17). With this increased understanding of basal ganglia structure and complexity, new surgical targets have been suggested.
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