Differential labeling of neural pathways converging on the ventrobasal complex of cat thalamus (original) (raw)

Several experiments on the cat have demonstrated that cells in the dorsal column nuclei (DCN) project to the ventrobasal nuclear complex of the thalamus (VB) 1,10,14. These projections are contralateral, involve large-diameter axons, and terminate in a patchy pattern within VB. Electron microscopic evidence shows that the synaptic endings of these fibers are large (3-5 #m diameter) and contain round vesicles 14. Each terminal makes multiple synaptic contacts usually with the larger, more proximal portions of the dendritic tree8,1~,14,16. Other experiments on the cat have demonstrated that cells in the second somatosensory region of the cerebral cortex ($2), like those in DCN, also project to VB 4,9,10, ~,~a. These corticothalamic projections are ipsilateral, involve small-diameter axons, and terminate diffusely within VB. Electron microscope evidence shows that the synaptic endings of these fibers are small (1-2/zm diameter) and contain round vesicles 9A5. Each terminal forms synaptic contacts (usually only one) with a peripheral branch of the dendritic tree8, 9A5,16. The reports mentioned above are based on the use of a single method of 'labeling' pathways in the nervous system. This method makes use of the fact that degenerating neural tissue 'looks' different from normal tissue-both under the electron and the light microscope. Taken together, these experiments suggest that, although the distribution and synaptic morphology of DCN and 2terminalsaredifferent,bothDCNand2 terminals are different, both DCN and 2terminalsaredifferent,bothDCNand2 cells send axons which converge on the same regions within VB, and possibly on the same cells. This suggestion of input convergence, however, has not yet been demonstrated directly. In order to provide such direct evidence it would be necessary to 'label' differentially in the same preparation the pathway from DCN to VB and the pathway from $2 to VB. Since the degeneration method of labeling has, until recently, been the only one available to neuroanatomists, it has been impossible to demonstrate directly the existence of convergence anywhere within the nervous system. A new method for 'labeling' pathways in the nervous system has been reported recently by Lasek et al. 1~ and by Cowan et al. 3. The method makes use of the normal transport system of the nerve cell in which proteins synthesized in the soma are