Cortico-basal ganglia and cortico-cerebellar circuits in Parkinson's disease: pathophysiology or compensation?

The basal ganglia and the cerebellum are anatomically and functionally linked to the cerebral cortex through a series of well-established circuits. The disruption of dopaminergic projections in Parkinson's disease (PD) leads to an imbalance within these circuits, leading to motor and cognitive symptoms. The cortico-cerebellar (CC) network has often been viewed as a compensatory network, helping the dysfunction of the cortico-basal ganglia (CBG) circuits in PD. However, evidence for this compensatory role is scarce; most changes in cerebellar activity could equally be attributed to pathophysiological changes underlying PD. This paper will review the anatomy, interaction and function of the CBG and CC circuits, the pathophysiological, metabolic, and functional changes observed in PD, as well as the effect of levodopa and deep brain stimulation on these changes. We will use this framework to discuss the pathophysiological and compensatory mechanisms behind CBG and CC circuit activity in PD.