Dual efferent projections of the trigeminal principal sensory nucleus to the thalamic ventroposteromedial nucleus in the squirrel monkey

Anterograde degeneration methods demonstrated two efferent components from the trigeminal principal sensory nucleus (PrV) to the thalamic ventroposteromedial nucleus (VPM) in the squirrel monkey: (a) fibers from the dorsal PrV coursed within the central tegmental tract and terminated in a dorsoventromedial strip of the ipsilateral VPM; (b) fibers from the ventral PrV mainly decussated caudal to the interpeduncular nucleus and terminated in the contralateral VPM exclusive of the sector receiving the dorsal PrV component, contralaterally. Adjacent Nissl sections showed an apparent increase in glial profiles accompanying an intense somal staining among the deafferented neuronal population in the VPM, coextensive with those regions in the VPM exhibiting terminal field degeneration.