The posteromedial ventral nucleus of the thalamus (VPM) of the cat: direct ascending projections to the cytoarchitectonic subdivisions.

The posteromedial ventral nucleus (VPM) of the cat is divided cytoarchitectonically into the magnocellular (VPMmc), lateral parvocellular (VPMpcl), and medial parvocellular (VPMpcm) divisions. Cell bodies of neurons in the VPMpcm are small, while those in the VPMpcl are small to medium-sized. The VPMmc contains large neurons. Direct projections from the lower brain stem structures to each of the three divisions of the VPM were examined by the retrograde horseradish peroxidase (HRP) method. When HRP injection was done into the VPMmc, labeled neurons were mainly located contralaterally in the ventral division of the principal sensory trigeminal nucleus (Vp), in the rostral part of the oral subnucleus in the spinal trigeminal nucleus (Vsp), and in the interpolar subnucleus of the Vsp; a few labeled neurons were also found contralaterally in lamina I of the caudal subnucleus of the Vsp. When HRP injection was restricted to the VPMpcl or VPMpcm, HRP-labeled neurons were mainly observed ipsilaterally, respectively, in the dorsal division of the Vp, or in the parabrachial nucleus (PBN) regions dorsomedial and ventromedial to the brachium conjunctivum. After HRP injection into the parvocellular part of the VPM (VPMpc), labeled neurons were also seen contralaterally in the Vsp, but these were far less numerous than those seen after HRP injections into the VPMmc. Thus, each of the three divisions of the VPM receives main ascending afferent fibers from different brain stem structures; the VPMpcm, VPMpcl, or VPMmc receives afferent fibers, respectively, from the PBN ipsilaterally, from the dorsal division of Vp ipsilaterally, or from the ventral division of the Vp and the Vsp contralaterally.