Basal ganglia circuits

Abstract The basal ganglia constitute a forebrain system associated with affecting motor and other behaviors that involve the cerebral cortex. The principal input nucleus is the striatum, which receives excitatory projections from the cerebral cortex and the intralaminar thalamus as well as neuromodulatory input from midbrain dopamine neurons. Output of the basal ganglia arises from GABAergic inhibitory neurons in the substantia nigra and internal segment of the globus pallidus that project to the thalamus and midbrain motor circuits. Two systems connect the striatum to the output nuclei: a direct pathway from D1 dopamine receptor expressing striatal neurons that project directly to the output nuclei, and an indirect pathway from D2 expressing striatal neurons that project to the external segment of the globus pallidus and connect indirectly to the output nuclei through the subthalamic nucleus. The direct and indirect striatal pathways, respectively, promote and constrain behaviors affected by the basal ganglia.