The thalamic input to area 17 in the cat can be divided into at least 3 parallel pathways, the W, X and Y. While the latter two are some of the best studied synaptic connections in the brain, the former remains poorly understood both in structure and in function. By combining light and electron microscopy we have reconstructed in 3-D single W axons and described quantitatively the synapses that they form. We have also made a structural comparison of reconstructed synapses from the three visual pathways.
Thalamic axons were labeled in vivo by injections of Biotinylated Dextran-Amine into the dLGN. W axons originating from C laminae injections arborized in layers 1, 2/3 and 5. Axons that traversed layer 1 supplied a few descending collaterals to layer 2/3, but the most extensive innervation in layer 2/3 was provided by axons ascending from the white matter. Most W boutons formed a single synapse, dendritic spines being the most common target, with dendritic shafts forming the remaining targets. In layer 1 the area of the postsynaptic density of spine synapses (0.16µm2) was significantly larger then those of layer 2/3 (0.11µm2) and 5 (0.09µm2). Synapses from X and Y axons in layer 4 were similar in size to synapses formed by W boutons in layer 1. In layer 1 the main targets of the W axons are likely the apical dendrites of pyramidal cells, so that both proximal and distal regions of pyramidal cell dendritic trees can be excited by the W pathway.