The striatopallidal projection displays a high degree of anatomical specificity in the primate

The striatopallidal projection in the squirrel monkey (Saimiri sciureus) was studied with two highly sensitive anterograde tracers, the lectin Phaseolus vulgaris leucoagglutinin (PHA-L) and biocytin. After small PHA-L injections into various sectors of the striatum, the striatopallidal projection was found to display a very precise topographical organization. Fibers from the head of the caudate nucleus emerge as several distinct fascicles that penetrate the dorsal portion of the pallidum at various points along its rostrocaudal extent. Each fascicle arborizes into the dorsal third of the pallidum as dense plexuses composed of numerous fibers that closely entwined the dendrites of pallidal neurons, hence forming typical 'woolly' fiber arrangements. In contrast, fibers from the postcommissural putamen emerge as a few compact bundles that reach the pallidum through its lateral surface. In the pallidum, thin fibers detach themselves from these compact bundles, sweep caudally, and arborize in the form of narrow and elongated bands aligned parallel to the medullary laminae. Each band appears composed of numerous, thin and weakly varicose fibers that make only en passant type of contact with pallidal cell bodies rostrally, but form a dense field of woolly fibers caudally. In cases in which two PHA-L injections were made at two different rostrocaudal levels in the putamen, two rostrocaudally distant fields of woolly fibers, separated one another by thin varicose fibers, occur in each band. Furthermore, each PHA-L injection site in the striatum gives rise to at least two bands in each pallidal segment, indicating that the primate striatum has a dual representation at pallidal level. Finally, injections of PHA-L and biocytin into two small and mediolaterally adjacent areas of the postcommissural putamen lead to the formation of two clearly distinguishable sets of bands in each pallidal segment. Even though they lie very close to one another these two types of bands never really overlap. This experiment shows that, in contrast to previous beliefs, axons of striatal neurons from two small adjacent populations do not converge upon the same pallidal neurons but instead project to several distinct subsets of pallidal neurons. The findings of the present study reveal that the striatopallidal projection system in primates is highly ordered and displays a high degree of specificity with respect to its target sites in the pallidum. Different anatomical strategies are used to maximally exploit the relatively small pallidal space and ensure that the finely tuned corticostriatal information is not blurred as it flows through the funnel-shaped pallidum.