Definite segregation of cortical neurons projecting to the dorsal column nuclei in the rat

The dorsal column nuclei (DCN) receive a substantial contingent of projections that arise from a number of somatosensory and motor cortical areas. We investigated the spatial organization of these projections in the rat by placing small deposits of two retrograde fluorescent tracers in adjacent foci within the DCN. Single-labeled neurons were abundant in layer V of the primary somatosensory (SI) and motor areas, and in the posterior parietal cortex. More sparse labeling was found in the medial agranular cortex (or MII), and the second somatosensory area. A somatotopic-like arrangement of these neurons was more clearly noticed in the granular zones of SI. Double-labeled neurons were very uncommon, and appeared at border regions where single-labeled cells intermingled. The segregation of these projections supports some differential modulatory effects that the cortex exerts on the somatosensory processing that takes place in the DCN.