On the question of the ventral intermediate nucleus in primate thalamus. A study of the lesser bushbaby, Galago senegalensis.

Cytoarchitectural and hodological evidence is given which supports the contention that the large-celled, intermediate zone between ventral posterior lateral (VPL) and ventral lateral (VL) nuclei is an independent thalamic nucleus, designated as the ventral intermediate nucleus (Vim), in the thalamic ventral tier of Galago senegalensis. Observation of Nissl-stained sections indicates that Vim contains cells which are larger than those in VL and more scattered in appearance than those in VPL. Fink and Heimer stained brains show that after cord hemisection the spinothalamic degeneration in Vim is sparse in amount and scattered in organization as opposed to the 'bursts' of dense degeneration in VPL. Following cortical injection of horseradish peroxidase, thalamocortical relay cells in Vim are distinct form those in VPL in both size and organization. Consequently, this study corroborates previous results on another primate and suggests that a distinct VPL-VL transitional nucleus, Vim, is present in the ventral tier of the prosimian primate galago senegalensis. The functional significance of Vim and its possible role in the evolution of primate locomotor behavior are discussed.