Differential patterns of arborization of striatal and subthalamic fibers in the two pallidal segments in primates.

Double-anterograde tract-tracing experiments in the squirrel monkey (Saimiri sciureus) reveal that fibers from the striatum and the subthalamic nucleus converge onto the same neurons in both the external (GPe) and internal (GPi) segments of the globus pallidus. However, these two pallidal afferents arborize according to a different pattern in GPe and GPi. Whereas the striatal fibers closely entwined the distal dendrites of pallidal neurons in a similar fashion in both pallidal segments, the subthalamic fibers display a tight pericellular arrangement that is much more obvious in GPi than in GPe. This perisomatic arborization is similar to the pericellular contacts made by the GPe fibers terminating on GPi neurons. Such a resemblance suggests that these two types of afferents exert an opposite effect upon the cell body and proximal dendrites of GPi neurons. It also raises the possibility for striatal neurons to influence the same GPi neuron by acting directly on its distal dendrites and indirectly on its cell body via a relay in GPe.