Convergent projections of trigeminal afferents from the principal nucleus and subnucleus interpolaris upon rat ventral posteromedial thalamic neurons.

Synaptic boutons originating from the trigeminal principal sensory nucleus (PrV) and the subnucleus interpolaris (SpI) which contact rat ventral posteromedial (VPM) neurons are similar in appearance. They are large boutons and contain a moderate packing density of round synaptic vesicles and established asymmetric (Gray type I) synaptic contacts principally on dendrites occasionally on somata. These boutons are similar to RL boutons of dorsal column nuclei and spinothalamic tract origin found in the ventral posterolateral thalamic nucleus. The boutons of PrV and SpI origin have an overlapping distribution on proximal dendrites of VPM neurons. Double labeling using degeneration and WGA-HRP shows that boutons from PrV and SpI contact the same VPM neurons confirming there is convergence of trigeminal afferents in VPM.