Nigrostriatal and nigrothalamic fibers in the rhesus monkey

Abstract Lesions in caudal parts of the substantia nigra (SN) produced in 11 monkeys were localized to lateral, medial and central regions. Resulting degeneration was studied by the Wiitanen technic in transverse and sagittal sections. Ascending nigral efferents project into Forel's field H where they divide into medial and lateral bundles. Fibers of the medial bundle terminate in portions of the ventral lateral (VLm) and ventral anterior (VAmc) thalamic nuclei. Quantitatively larger numbers of fibers project laterally dorsal to, and within, the subthalamic nucleus. These nigral efferents traverse the internal capsule and globus pallidus to enter the putamen (P) and parts of the caudate nucleus (CN). Nigral efferent fibers in the globus pallidus appeared entirely en passage. The location of degeneration in the putamen appeared related to the part of the SN destroyed. An apparent correspondence exists between: (1) lateral parts of SN and dorsal regions of P, and (2) medial parts of SN and ventral regions of P. The pars reticularis of SN gives rise to nigrothalamic fibers while nigrostriatal fibers arise from the pars compacta. Data suggest that reciprocal topographic relationships may exist between nigrostriatal and strionigral projections and that these fibers appear to form a closed feedback loop concerned with dopamine transport. Lesions in SN concomitantly interrupt large numbers of corticotegmental and corticothalamic fibers.