Corticothalamic projections from layer 5 of the vibrissal barrel cortex in the rat.

This study bears on the projections of layer 5 cells of the vibrissal sensory cortex to the somatosensory thalamus in rats. Small groups of cells were labeled with biotinylated dextran amine (BDA), and their axonal arborizations were individually reconstructed from horizontal sections counterstained for cytochrome oxidase. Results show that the vast majority ( approximately 95%) of layer 5 axons that innervate the somatosensory thalamus are collaterals of corticofugal fibers that project to the brainstem. The anterior pretectal nucleus, the deep layers of the superior colliculus, and the pontine nuclei are among the structures most often coinnervated. In the thalamus, layer 5 axons terminate exclusively in the dorsal part of the posterior group (Po), where they form clusters of large terminations. Because dorsal Po projects to multiple cortical areas, we sought to determine whether all recipient areas return a layer 5 projection to this part of the thalamus. Additional experiments using fluoro-gold and BDA injections provided evidence that the primary somatosensory area is the sole source of layer 5 projections to dorsal Po but that this thalamic region receives convergent layer 6 projections from the primary and second somatosensory areas and from the motor and insular cortices. These results show that layer 5 projections do not overlap in associative thalamic nuclei, thus defining area-related subdivisions. Furthermore, the coinnervation of brainstem nuclei by layer 5 CT axons suggests that this pathway conveys to the thalamus a copy of the cortical output aimed at brainstem structures.