Motor Pathways, Basal Ganglia Physiology, and Pathophysiology

The main axis of the basal ganglia (BG) connects the thalamocortical networks to the BG input stages (striatum and subthalamic nucleus) and continues through the BG output stages (internal segment of the globus pallidus and the substantia nigra reticulata) to the cortical and subcortical motor centers. This chapter reviews the unique anatomical and physiological features of the BG network and argues that they support the main computational goal of the BG, namely, a behavioral policy that maximizes future cumulative gains and minimizes costs. The BG critics differentially modulate both the excitability of striatal neurons and the efficacy of the corticostriatal synapses, enabling instantaneous optimal tradeoffs between exploratory and exploitative behavior and long-term learning of optimal behavioral policy (state-to-action associations). The multiobjective optimization model provides insights into the role of the nondopaminergic critics in the BG physiology and pathophysiology (e.g. dopamine-acetylcholine motor balance and serotonin-related depression in Parkinson's disease).