Cortical projections of the lateral geniculate nucleus in the cat

Abstract A new application of the retrograde transport method designed to demonstrate neurons that project to two cortical areas has been developed. This method depends on the retrograde axonal transport of two markers, each of which is uniquely detectable by histological methods. In this study, horseradish peroxidase (detectable by the enzymatic reation product only) and tritiated proteins (either enzymatically inactive tritiated horseradish peroxidase or tritiated bovine serium albumin, both of which are detectable by the tritium label only) were used. One of these markers was injected into cortical area 17 and the other was injected into area 18. In layers A and A 1 of the lateral geniculate nucleus, 10% of the cells project to both area 17 and area 18 by axons that branch, 70% of the neurons project to area 17 only, less than 1% of the neurons project to area 18 only, and approximately 20% of the cells are probably interneurons. In the C laminae 50% of the cells project to both areas 17 and 18 by axons that branch, approximately 20% of the neurons project to area 17 only, 10% of the cells project to area 18 only, and about 20% of the neurons are unlabeled. The cells in the medial interlaminar nucleus project to area 17 only, to area 18 only, or to both of these areas by axons that branch. In addition the retrograde markers were injected into single cortical areas, either area 17, area 18, or area 19. The injections of area 17 and those of area 18 confirmed the results of the double‐label experiments, with 80% of the cells in the A laminae projecting to area 17 and approximately 10% projecting to area 18. Following injections of area 19, labeled neurons were seen in the medial interlaminar nucleus and the C laminae. Therefore, the medial interlaminar nucleus contains cells that project to areas 17, 18, and 19, with some cells projecting to both area 17 and area 18 by axons that branch. In the C laminae the analysis of the projection pattern could be carried further, for it was possible to determine the percentage of labeled neurons in this region. Since 80% of the cells project to areas 17 and 18, and since 60% of the cells of the C laminae were labeled following injections of area 19, many of the cells which project to area 19 must also project to either area 17 or area 18, and some cells must project to all three (areas 17, 18, and 19) by branching axons.