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FIGURE 2 The basal ganglia motor loop. Major connections linking motor cortex, basal ganglia (blue box), and thalamus are shown in this simplified scheme. Cortical areas representing all three levels of the motor hierarchy project to the neostriatum (caudate and putamen). Output from the basal ganglia is via the globus pallidus and substantia nigra to three nuclei of the thalamus. The thalamus completes the loop pathway with projections back to the motor cortex, particularly the supplemental motor area.

FIGURE 3 Connections of the direct and indirect pathways in the basal ganglia motor loop. The overall role of the basal ganglia (blue box) is to maintain inhibitory control over the thalamus. Inhibitory influence (—) is shown with blue arrows and excitatory influence ( + ) is shown with black arrows. Pathways through the basal ganglia are tonically active and coordinate converging excitatory input from the cortex through two separate routes, both of which feed back to the cortex through the thalamus. The direct path includes an inhibitory connection between the neostriatum and the globus pallidus/substantia nigra. Activity in this pathway decreases the level of inhibitory control over the thalamus, which leads to an increase in excitatory feedback to the SMA. The opposite occurs with activity in the indirect pathway. Note that the indirect pathway stimulates the inhibitory fibers to the thalamus, thus decreasing the excitatory feedback to the cortex. The appropriate balance between direct and indirect pathways maintains a suitable excitatory tone within cortical motor pathways.

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