Basal Ganglia Motor Circuit and Physiology of Parkinsonism

The basal ganglia are a group of nuclei located in the deep portion of the brain. Along with the cerebellum, the basal ganglia have a major role in controlling human voluntary movements, and their dysfunction is apparently responsible for various involuntary movements. Although the exact mechanism of how the basal ganglia control movements has yet to be clarified, the model of focused selection (through the direct pathway) and tonic inhibition (via the indirect path- way) is proposed to be a principal functional model of the basal ganglia. Parkinson' s disease (PD) is classically charac- terized by bradykinesia, rigidity and tremor-at-rest. All features seem to be associated with dopamine depletion result- ing from the degeneration of the nigrostriatal pathway, which produces reduced activity of the direct pathway and a concurrent enhancement of excitatory output from STN. This change may result in increased tonic background inhibi- tion and reduced focused selection via the direct pathway, causing difficulties in performing voluntary movements selectively. However, it has not been possible to define a single underlying pathophysiologic mechanism that explains all parkinsonian symptoms. Here the data that give separate understanding to each of the three classic features are dis- cussed.