[Anatomical connections of the basal ganglia]

The basal ganglia consist of the striatum, which includes the caudate-putamen and the core of the nucleus accumbens, the external segment of the globus pallidus (GPe), the subthalamic nucleus, the internal segment of the globus pallidus (GPi), and the substantia nigra (SN). The major input to the basal ganglia is glutamatergic striatopetal projections from nearly all areas of the neocortex and the intralaminar and midline nuclei. The striatofugal projection neurons are divided into 2 groups; one includes neurons projecting to the SN and GPi, and the other includes those projecting to the GPe. The former neurons are called 'direct pathway' neurons in the basal ganglia circuit, since they directly extend their axons to the output nuclei of the basal ganglia, i.e. pars reticulata of the SN (SNr) and GPi. On the other hand, the latter striato-GPe neurons are called 'indirect pathway' neurons and contain striato-GPe, GPe-subthalamic, and subthalamo-GPi/SNr projections. The output of the basal ganglia is the projection of GABAergic neurons in the GPe and SNr. In addition to the 2 segregated striatofugal groups, the neostriatum possesses a mosaic organization composed of patch and matrix compartments. The patch compartment occupies 10-15% of the neostriatal volume and is characterized by its projection to the pars compacta of the SN (SNc). The basal ganglia output targets the thalamic nuclei, which are part of cortico-basal ganglia circuits, the intermediate layers of the superior colliculus, and the pedunculopontine nucleus. The basal ganglia possess not only the topographic organization that provides parallel and functionally defined loops but also the divergence and convergence connections, which may reflect the organizational features of the basal ganglia.