This study investigated the organization of a vibrissal pathway that arises from the interpolar division of the spinal trigeminal complex (SP5i), transits through the ventral posterior medial nucleus (VPM), and innervates the somatosensory cortical areas in the rat. Using Fluoro-Gold and biotinylated dextran amine, respectively, as retrograde and anterograde tracers, the following organization plan was disclosed. The SP5i projection arises from a population of small-sized neurons that selectively innervate the ventral lateral part of VPM. In cytochrome oxidase-stained material, this region does not display any barreloid arrangement, but Fluoro-Gold injections in single barrel columns labeled rods of cells that extend caudally into the ventral lateral division of VPM. Thus, on the basis of retrograde labeling, barreloids were divided into core and tail compartments, which correspond to the rod segments running across the dorsal and ventral lateral parts of VPM, respectively. Double-labeling experiments revealed that SP5i afferents innervate the tail of barreloids. The anterograde labeling of thalamocortical axons show that most “core cells” project to a single barrel column, whereas some “tail cells” give rise to branching axons that innervate the second somatosensory area and the dysgranular zone of the barrel field. Injections that straddled the transition zone between the core and tail regions disclosed cells projecting to a single barrel column and to the surrounding dysgranular zone. These results suggest that the projection of “barreloids cells” to the granular and/or dysgranular zones relates to the class of prethalamic input(s) they receive.
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