Organization of pallidothalamic projections in the rhesus monkey

Abstract Attempts were made to: (1) define the regions of origin of the ansa lenticularis (AL) and the lenticular fasciculus (LF), and (2) determine if particular regions of the medial pallidal segment (MPS) project in an organized fashion to thalamic nuclei. The MPS was divided into rostral and caudal (by a plane through the LF), dorsal and ventral (by an appropriate plane), and medial and lateral (by the accessory medullary lamina) parts. Attempts were made to categorize various lesions of the MPS and degeneration was studied in transverse, sagittal and horizontal sections stained by the Nauta and Wiitanen technics. Corresponding Nissl‐stained sections were used to delineate thalamic nuclei. Localized lesions in portions of the MPS (11 rostral and 10 caudal) were worthy of anatomical analysis in 21 monkeys. Data support the thesis that fibers of the AL arise predominantly from the outer part of the MPS and pass ventrally and medially through portions of the inner part of the MPS to their site of emergence. Fibers of the LF appear to arise from the inner part of the MPS and project dorsomedially through the internal capsule. Pallidothalamic projections to the rostral ventral tier thalamic nuclei the ventral anterior (VApc) and the ventral lateral (VLo) appear topographically organized in three cardinal dimensions. Rostral parts of the MPS project pre‐dominantly to parts of VApc, while caudal parts of the MPS project primarily to parts of VLo. There also is a dorsoventral and mediolateral correspondence in the pallidal projection to VApc and VLo which exhibits overlap. Pallidothalamic projections to the contromedian nucleus (CM) terminate preponderantly in rostral and medial regions of the nucleus, and there appears to be a definite correspondence between parts of the MPS and parts of CM, only in the dorsoventral dimension.