Organization of thalamostriatal terminals from the ventral motor nuclei in the macaque

This study examines the organization of thalamostriatal projections from ventral tier nuclei that relay basal ganglia output to the frontal cortex. Although previous thalamostriatal studies emphasize projections from the intralaminar nuclei, studies in primates show a substantial projection from the ventral anterior (VA) and ventral lateral (VL) nuclei. These nuclei make up the main efferent projection from the basal ganglia to frontal cortical areas, including primary motor, supplementary, premotor, and cingulate motor areas. Functionally related motor areas of the frontal cortex and VA/VL have convergent projections to specific regions of the dorsal striatum. The distribution of VA/VL terminals within the striatum is crucial to understanding their relationship to motor cortical afferents. We placed anterograde tracer injections into discrete VA/VL thalamic areas. VA/VL thalamostriatal projections terminate in broad, rostrocaudal regions of the dorsal striatum, corresponding to regions innervated by functionally related cortical motor areas. The pars oralis division of VL projects primarily to the dorsolateral, postcommissural putamen, whereas the parvicellular VA targets more medial and rostral putamen regions, and the magnocellular division of VA targets the dorsal head of the caudate nucleus. Whereas these results demonstrate a general functional topography, specific VA/VL projections overlap extensively, suggesting that functionally distinct VA/VL projections may also converge in dorsal striatal areas. Within striatal territories, VA/VL projections terminate in a patchy, nonhomogeneous manner, indicating another level of complexity. Moreover, terminal fields contain both terminal clusters and scattered, long, unbranched fibers with many varicosities. These fiber morphologies resemble those from the cortex and raise the possibility that VA/VL thalamostriatal projections neurons have divergent connectional features.