The ventral posteromedial thalamic nucleus (VPM) of the rat contains at least two
major vibrissa-representing compartments: the dorsomedial (VPMdm), which belongs to
the lemniscal afferent pathway, and the ventrolateral (VPMvl), which belongs to the
extralemniscal afferent pathway. Although input-output projections and functional
characteristics that distinguish these two compartments were recently clarified, a
comprehensive structural analysis of these compartments and the border between them
was lacking. This paper addresses structural and functional relationships between
the VPMdm and VPMvl. We found that the size of the VPM is almost constant across
individual rats. Next, we computed a canonical map of the VPM in the oblique plane,
where structural borders are best visualized. Using the canonical map, and sequential
slices cut in oblique and coronal planes, we determined the border between the VPMdm
and VPMvl in the standard coronal plane, and verified it with in vivo extracellular
recordings. The position of the border between these two vibrissal sub-nuclei changes
along the rostrocaudal extent within the VPM due to the relative sizes of these
sub-nuclei at any point. The border between the VPMdm and VPMvl, which was revealed
by this technique, can now be included in atlases of the rat brain and should
facilitate experimental correlation of tactile functions with thalamic regions.