Afferents to different layers of the dorsolateral isocortex in rats

Fluorescent somatopetal tracers were used to infiltrate, by diffusion rather than injections, the dorsolateral cortex of one hemisphere in rats. In different animals the tracers penetrated into the cortex to different depths. We found several interesting features of the commissural system: first, there were no areas without commissural neurons. At least a few labelled cell bodies were present in a single-cell layer also in "acallosal" cortical areas. Secondly, there is a considerable variety of laminar distribution patterns of labelled perikarya in different areas. Thirdly, some cortical fields, which cytoarchitecturally appear uniform, can be subdivided according to different distributions of cell bodies with commissural projections. Fourthly, when only supragranular layers were infiltrated, labelled cell bodies were present mainly in the supragranular layers of the contralateral cortex. Infiltration of the first layer alone did not label any neurons in the contralateral cortex but did label neurons in layer VIb ipsilaterally. In the subcortex, the labelled perikarya were found in the structures already known to project directly to the cortex. In rats with the tracer restricted mainly to the supragranular layers, a conspicuously reduced labelling was found in the basal forebrain and the thalamus. In the thalami of those animals, labelled neurons were found only in paralamellar nuclei. The high sensitivity of the tracer used, together with infiltration of the entire dorsolateral cortex, allows us to conclude that probably all sources of innervation of the isocortex in rats have been seen.