Figure 3.

Direct and indirect basal ganglia loops. The entire cortex projects to the basal ganglia. The diagram shows the direct and indirect pathways through the basal ganglia. Excitatory pathways are shown in green and inhibitory in red. Pathways that are common to the direct and indirect pathways are shown by solid green arrows. The cortex projects to the striatum (arrows a, b). Cortical axons terminate on inhibitory neurons of either the caudate or putamen, depending on their origin in the cortex. The direct pathway (heavy dotted line, solid arrowheads) projects from the striatum to GPi/SNr, which projects to the thalamus. Neurons in the GPi/SNr are inhibited by the direct pathway and therefore they can’t inhibit the thalamus (X, inhibited pathway). The thalamus remains free to project to the frontal cortex and back for movement. The indirect pathway (fine dotted line, silhouette arrowheads) takes a different route: from the striatum it projects and inhibits GPe. The GPe projects to the excitatory subthalamic nucleus (STN). The inhibited GPe leaves the STN free to activate GPi/SNr, which inhibits the thalamus, preventing activation and movement. Dopaminergic pathways from the substantia nigra pars compacta (SNc) have opposite effects on the two parallel pathways because they interact with distinct types of dopamine receptors found on neurons in the striatum. In the direct pathway, dopaminergic axons interact with D1 receptors, which are excitatory. In the indirect pathway, dopaminergic axons interact with D2 receptors, which are inhibitory. Abbreviations: GPe, external segment of the globus pallidus; GPi, internal segment of the globus pallidus. SNc, substantia nigra pars compacta; STN, subthalamic nucleus; MD, mediodorsal thalamic nucleus; VA, ventral anterior thalamic nucleus; VL, ventral lateral thalamic nucleus.