Functional anatomy of the basal ganglia. I. The cortico-basal ganglia-thalamo-cortical loop.

This paper reviews some of the recent findings on different aspects of the anatomical organization of the basal ganglia. Attempts have been made to delineate the anatomical substrate of information processing along the cortico-basal ganglia-thalamo-cortical loop. Emphasis has been placed on data obtained with highly sensitive anterograde tract-tracing methods applied to the study of the main axis of the loop, which is composed of the striatum, the pallidum, and the substantia nigra. These findings have highlighted the complexities of the organization of the intrinsic basal ganglia circuitry, which comprises multiple modular units that are distributed according to highly ordered and repetitive patterns. Such an arrangement is well suited to convey cortical information in a highly specific manner throughout the basal ganglia. The basal ganglia circuitry is also designed so as to modulate in a precise manner the neuronal activity of several brain functional systems, which are involved in the direct control of different aspects of psychomotor behavior. Of utmost importance is the action of the basal ganglia on thalamocortical premotor neurons. It is through these neurons, which can be considered as a sort of final common pathway, that the basal ganglia ultimately influence the complex neuronal computation that goes on at cortical level.