Collateral projections from trigeminal sensory nuclei to ventrobasal thalamus and cerebellar cortex in rats

The retrograde fluorescent labeling technique reveals that trigeminal projections to the ventroposteromedial nucleus of the thalamus (VPM) of the rat originate from the main sensory nucleus (MSN) of the trigeminal and subnuclei interpolaris (V1) and caudalis (Vc) of the spinal trigeminal nucleus. These projections are predominantly contralateral; however, the presence of a few ipsilateral labeled cells in MSN suggests an uncrossed trigeminothalamic pathway. Trigeminocerebellar fibers projecting to the paramedian lobule (PML) of the cerebellar cortex are located in Vi and caudal subnucleus oralis (Vo). This is principally an ipsilateral pathway, but several bisbenzimide-labeled cells are present in contralateral Vi. The most notable finding occurred after paired injections of Evans Blue into VPM and bisbenzimide into PML, demonstrating neurons in Vi with divergent projections to both structures. The presence of this type of projection was not found in mice (Steindler: J. Comp. Neurol. 237:155-175, 1985) and has not been reported in other species.