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. 2022 May 24;12(4):1083–1113. doi: 10.3233/JPD-213082

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© 2022 – The authors. Published by IOS Press

This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial (CC BY-NC 4.0) License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Fig. 1 — Cortico-basal ganglia motor circuit. The depicted cortico-thalamic-basal ganglia (BG) motor circuit is inspired by the model of DeLong and colleagues [11, 12]. BG consist of several interconnected subcortical nuclei. Striatum is the input structure. It receives input from almost the entire cerebral cortex. In particular, somatosensory (SSC), motor (MC) and premotor (PMC) cortices and supplementary motor area (SMA) are of interest for motor functions. Two sets of cells in the striatum can be distinguished on the basis of the type of dopamine receptors - D1 and D2 –expressed. They give rise to the direct and indirect pathways, respectively. The direct pathway (DP) brings straightforwardly to the output structures of the BG, globus pallidus internal segment (GPi) or the substantia nigra pars reticulata (SNr). The indirect pathway (IP) involves the globus pallidus external segment (GPe) and the subthalamic nucleus (STN), before getting to the GPi/SNr. An hyperdirect pathway (HP) sees a direct cortical excitation of the STN, bypassing the striatum. The STN sends excitatory projections to GPi/SNr. The output from GPi/SNr is inhibitory and projects to the ventral anterior and ventrolateral (VA/VL) thalamus, which excites the motor areas in the cortex, closing the loop. This output most influences SMA function. Gpi and SNr project also to the brainstem premotor structures, in particular the pedunculopontine nucleus, and to the superior colliculus. Overall, increased activity in the DP facilitates movements, releasing the thalamus from tonic inhibition of the GPi/SNr, while increased activity in the IP inhibits movements, by disinhibiting the GPi/SNr. The HP seems to contribute to early stopping of competing motor programs [247]. The balance between the pathways is affected by the dopaminergic projection from the substantia nigra pars compacta (SNc) to the striatum: the release of dopamine in the DP increases the responsiveness of the D1 striatal cells to corticostriatal input, while the release of dopamine in the IP decreases the responsiveness of the D2 striatal cells. In hypokinetic movement disorder, such as Parkinson’s disease, the degeneration of the nigrostriatal dopamine projections let the IP overpower the DP, which results in stronger inhibition of the thalamus.