Thalamic terminal fields of individual axons from the ventral part of the dentate nucleus of the cerebellum in Macaca mulatta

This study examined organization of the projection from the dentate nucleus of the cerebellum to the ventral lateral nucleus (VL) of the thalamus in Macaca mulatta. Small injections of biotinylated dextran amine were placed in the ventral parts of dentate nuclei. The distribution of all contralateral terminal fields in the thalamus was charted, and representative individual axons that terminated in the VL were traced in serial sections under the light microscope. These axons were reconstructed with all their branches and terminal fields in the thalamus. The geometry and size of the terminal fields as well as the number and distribution of boutons and neurons in them were analyzed. The terminal fields of all labeled axons were distributed widely over the VL either singly or in clusters. Two types of axons were found: simple axons formed only one terminal field and complex axons formed multiple terminal fields at a distance. Individual terminal fields were focal, had the form of flattened discs, and generated up to 200 boutons distributed between 10 and 29 nerve cell bodies. These findings suggest that a simple axon activates a small group of neurons at one site. The complex axons, in turn, influence similar size cell groups at different VL locations. The total number of boutons generated by a single complex axon was up to 300. Future studies should determine whether simple axons could be branches of complex axons that took off below the thalamus. The results reveal a complex organization of the input from the ventral dentate to the VL that only partially fits into the traditional concept of somatotopic organization of the nucleus.