Collateral projections of trigeminal sensory neurons to both cerebellar paramedian lobules in the rabbit: demonstration by fluorescent double labeling study

Neurons in the trigeminal sensory nuclei (TSN) were indicated to project si multaneously to the cerebellar paramedian lobule (PML) of both sides by way of axon collaterals in the rabbit. Injections of Fast Blue and Diamidino Y ellow made in twelve rabbits into the regions of the left and right PML cor tex, respectively, resulted in retrograde double labeling of neurons in the principal trigeminal (Vp) and spinal trigeminal nuclei including subnucleu s oralis (Vo), and rostral (Vir) and caudal (Vic) subnucleus interpolaris. Sparse labeling was seen among the root fibers of the facial nerve immediat ely adjacent to Vp. The data indicated that out of the total population of ipsilateral and contralateral single labeled neurons in TSN subdivisions, d ouble labeled neurons constituted about 11% in Vo, 2% in Vp and Vir, and 1% in Vic. Thus. it seems that out of TSN neurons sending collateral projecti ons to PML of both hemispheres, those in Vo project more frequently than th ose in Vp, Vir and Vic. The homotopic (corresponding) middle (c and d) PML sublobules receive collateral projections from neurons of all these TSN sub divisions, whereas the caudal (a and b) and rostral (e and f) sublobules se em to be targets of the projection from Vp and Vir. The findings suggest th at some TSN neurons may exert simultaneous bilateral modulatory influences upon the regions in various sublobules of both PML.