Efferent fibers of the subthalamic nucleus in the monkey. A comparison of the efferent projections of the subthalamic nucleus,substantia nigra and globus pallidus

A study was made to determine the efferent projections of the subthalamic nucleus in the monkey. Because of the impossibility of producing lesions in this nucleus, not involving adjacent structures, lesions were produced by different stereotaxic approaches. Comparisons were made with degeneration resulting from localized lesions in substantia nigra and globus pallidus. Degeneration resulting from these lesions was studied in transverse and sagittal sections stained by the Nauta-Gygax method. Efferent fibers from the subthalamic nucleus pass through the internal capsule into the medial pallidal segment; a few fibers are distributed to the lateral pallidum. Some subthalamic efferent fibers pass to the contralateral globus pallidus via the dorsal supraoptic decussation, but none projection to the thalamus. Nigral efferent fibers project to parts of the ventral anterior (VAmc) and ventral lateral (VLm) thalamic nuclei. The medial pallidal segment gives fibers to: (1) ventral anterior (VA), ventral lateral (VLo) and centromedian (CM) thalamic nuclei, and (2) the pedunculopontine nucleus. The lateral pallidal segment projects exclusively to the subthalamic nucleus. Thalamic projections of the substania nigra and globus pallidus are distinctive. Subthalamic projections to the globus pallidus are more profuse than those of the substantia nigra. The following hypothesis is presented: Subthalamic dyskinesia, due to lesions in the subthalamic nucleus, is a consequence of removal of inhibitory influences acting upon the medial segment of the globus pallidus.