The Role of the Internal Segment of the Globus Pallidus in Mediating Dyskinesias

Changes in neuronal activity within the internal segment of the globus pallidus (GPi), which subsequently influence thalamo-cortical pathways, are believed to reflect movement-related information processed within the basal ganglia. Previous work from this laboratory would support the view that in the hypokinetic movement disorder of Parkinson’s disease, the GPi is overactive, being driven by a disinhibited subthalamic nucleus (STN) mediating excitatory transmission. In contrast it is well documented that lesion or damage within the STN in man gives rise to choreic or ballistic dyskinesias (hyperkinesias), which, if STN output is reduced, would imply the presence of an underactivity of GPi neurones in the production of these movements. Therefore the current view is that an overall increase or decrease in GPi neuronal activity explains the two extremes of the movement spectrum. In this chapter we examine the sufficiency of this view by discussing more specifically the role of the GPi in mediating dyskinesias (chorea, ballism and dystonia), by drawing upon results from our own and other related studies.