Backward cortical projections to primary somatosensory cortex in rats extend long horizontal axons in layer I.

We have studied the origin and extent of axons within layer I of the primary somatosensory cortex (SI) of rats by using retrograde and anterograde tracers with an emphasis on reciprocal connections to layer I of SI from ipsilateral cortical areas that are the target of SI projections. Small crystals of 1,1',dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate (DiI) labeled horizontal axons projecting in all directions within layer I, which extended for up to 4 mm with numerous terminal branches. Applications of horseradish peroxidase, Diamidino yellow, or fast blue to the pial surface of SI labeled a characteristic pattern of neurons below the application site that excluded neurons in layer IV of the barrel fields, unless the dye penetrated deeper than layer II. This provided a control for the effective depth of the layer I dye applications. Retrograde transport from layer I of SI was traced to the primary motor area, the lateral parietal areas, including the secondary somatosensory (SII) and agranular insular cortex ipsilaterally, as well as the homotopic areas of SI contralaterally. Injections of the anterograde tracer dextran amine at the same site as the SI surface application labeled dense fiber terminations in middle layers of these same secondary areas in the primary motor cortex (MI) or SII in the midst of cells labeled by retrograde transport from layer I of SI. Injections of dextran amine into these secondary cortical areas labeled fibers that coursed through deep layers to SI, where they ascended to layer I. These reciprocal corticocortical inputs to SI were concentrated in layer I, where they branched and extended horizontally across several SI barrels.