Somatosensory input to the ventrolateral thalamic region in the macaque monkey: A potential substrate for parkinsonian tremor

In the present study, we determined the anatomic relationships between somatosensory and motor pathways within ventrolateral (VL) thalamic nuclei of the motor thalamus of macaque monkeys. In labeling experiments, four macaque monkeys (Macaca mulatta) received injections of biotinylated dextran amine and wheat germ agglutinin conjugated to horseradish peroxidase into the cerebellar nuclei or internal segment of the globus pallidus and cervical segments of the spinal cord, respectively. Each tracer was visualized in brain sections by sequentially using a different chromogen. Labeled terminals were plotted and superimposed on adjacent brain sections processed for Nissl substance, acetylcholinesterase, and the antigens for calbindin and Cat-301 to reveal thalamic nuclei. The labeled cerebellar terminals were distributed throughout the posterior VL (VLp), whereas the labeled pallidothalamic terminals were concentrated in the anterior VL and the ventral anterior nucleus. The spinothalamic input was directed mostly to the ventral posterior complex and cells just caudal to it. In addition, the patches of spinothalamic terminations intermingled and partly overlapped with the cerebellothalamic, but not with the pallidothalamic terminations within VLp. The regions of overlap of somatosensory and cerebellar inputs within the VLp of the present study appear to correspond to the reported locations of the tremor-related cells in parkinsonian patients. Thus, the overlapping spinothalamic and cerebellar inputs may provide a substrate for the altered activity of motor thalamic neurons in such patients.