Somatosensory Trigeminal Projections to the Inferior Olive, Cerebellum and other Precerebellar Nuclei in Rabbits.

We have analysed the pathways through which somatosensory information from the face reaches the inferior olive and the cerebellum in rabbits. We used wheatgerm agglutinin - horseradish peroxidase (WGA - HRP) to trace projections from all parts of the somatosensory trigeminal system to the olive, cerebellar cortex, the cerebellar deep nuclei and the pontine nuclei. Projections to the cerebellar cortex and inferior olive were verified using retrograde transport of WGA - HRP. Two regions of the inferior olive-the medial dorsal accessory olive and the ventral leaf of the principal olive-receive inputs from pars interpolaris (Vi) and rostral pars caudalis (Vc) of the spinal trigeminal nucleus and from the principal trigeminal nucleus (Vp). Another area in the caudal medial accessory olive receives inputs from rostral Vo (pars oralis of the spinal trigeminal nucleus), caudal Vi and Vc. There are trigemino-olivo-cortical inputs to lobule HVI via all these olivary areas and to the paramedian lobe via the principal olive only. Cerebellar cortex-lobules HVI, crus I and II, paramedian lobe and IX-receives direct mossy fibre inputs from Vp, Vo and rostral Vi. The pontine nuclei receive an input only from rostral Vi. We saw no trigeminal projections to other precerebellar nuclei or to the deep cerebellar nuclei. The concentration of face somatosensory cortical inputs, via several pathways, upon lobule HVI may underlie its important role in the regulation of learned and unlearned eyeblinks.