The topographic arrangement and cytoarchitecture of cells in the ventral posterolateral nucleus (VPL) of a prosimian primate (Galago senegalensis) were studied using horseradish peroxidase (HRP) and Golgi impregnation techniques. Following cortical implants of HRP, reactive neurons in VPL are organized into medially concave lamellae which extend through the dorsoventral and rostrocaudal dimensions of the nucleus. After implant in forelimb and hindlimb areas of motor-sensory cortex, labeled cells are confined to the medial (VPLm) and lateral (VPLl) portions of VPL, respectively. HRP-positive cells in the ventral part of each lamella are organized into clusters which correspond to the clusters of cells and “parcellated-bursts” of preterminal debris previously described in bushbaby VPL (Pearson and Haines, this volume). HRP-reactive cells in the ventral intermediate nucleus (Vim) are evenly distributed as contrasted to the tightly clustered groups of somata in the adjacent VPL. This evidence argues in favor of the presence of Vim in dorsal thalamus of this prosimian.
Golgi impregnations reveal two main types of relay cells in Galago VPL. Type I cells have multiangular somata, straight distal dendrites, and primary dendritic branch points which are free of appendages. Type II cells have rounded somata, sinuous distal dendrites, and clusters of appendages located at primary branch points. Intermediate cells (i.e., cells with morphological features in between types I and II) are also present in VPL. Comparison of tufted Golgi impregnated cells with neurons labeled with HRP shows definite similarities in somata size and shape, and in the orientation of proximal dendrites. This evidence corroborates the relay nature of the tufted neurons in VPL. Relay cells in Galago VPL have morphological features which are similar to those of relay cells in lateral and medial geniculate nuclei of cat and primate.
Type III cells have small round somata, radiate dendrites with elaborate appendages, and axons which appear to be intrinsic to VPL. Consequently, these cells are considered to be interneurons in the VPL of Galago. Glial-like neurons (type IV cells) were also observed. These have beaded dendritic processes similar to those which presumably represent presynaptic boutons in other species. Consequently, these cells are also assumed to function intrinsically within VPL.