The striatofugal fiber system in primates: a reevaluation of its organization based on single-axon tracing studies.

The current model of basal ganglia rests on the idea that the striatofugal system is composed of two separate (direct and indirect) pathways originating from distinct cell populations in the striatum. The striatum itself is divided into two major compartments, the striosomes and the matrix, which differ by their neurochemical makeup and input/output connections. Here, neurons located in either striosomes or the extrastriosomal matrix in squirrel monkeys were injected with biotin dextran amine, and their labeled axons were entirely reconstructed with a camera lucida. Twenty-four of 27 reconstructed axons arborized into the three main striatal targets (external pallidum, globus pallidus, and substantia nigra pars reticulata), a finding that is at odds with the concept of a dual striatofugal system. Axons of striosomal neurons formed several columnar terminal fields in the substantia nigra pars reticulata. These data indicate that the substantia nigra pars compacta is neither the only nor the main target of striosomal neurons, a finding that calls for a reevaluation of the organization of the striatonigral projection system.