Diverging projection of the trigeminocerebellar fibers in the rabbit paramedian lobule.

Abstract

The present study has been attempted to investigate the issue of intralobular branching of cerebellar afferent axons arising from neurons in TSN and terminating in rPML and cPML sublobules, known to be the face-forelimb and hindlimb receiving areas, respectively. In this aim the double fluorescent retrograde technique was employed in the rabbit. No other reports have addressed this question. Non-overlapping unilateral injections of the cytoplasmic tracers FB and the nuclear dye DY into rPML and cPML, respectively, resulted in numerous single FB or DY labeled neurons and small number of double FB + DY ones in Vp, Vo, Vir and Vic bilaterally, with a very clear ipsilateral predominance. No evidence has been disclosed for projection from Vmes and Vc. Distribution pattern of single labeling allows to assume that projection exhibits some degree of topographical organization. Thus, there are populations of TSN neurons projecting independently to rPML and cPML and, to a larger extent, populations of neurons whose projection areas more or less overlap. Profuse projection arises from Vir and less numerous fibers originate from Vp and the rostral part of Vic. Neurons in Vo, mainly in the caudal regions, participate in a relatively moderate degree to this projection. Double labeled neurons recognized herein indicate that TSN projections reaching the two non-homologous PML regions may be collaterals of the same axons. The cells of origin for such projections are distributed in defined regions of Vir (n = 214), Vic (n = 107), Vp (n = 73) and Vo (n = 25). Considering small percent of neurons with divergent axons (about 3% in Vic and Vo, and 2% in Vir and Vp) it can be concluded that trigeminal inputs to rPML and cPML correspond to a larger extent to separate rather than collateral projection. In spite of this the findings indicate that functionally different PML regions are linked. The present results are discussed with those of earlier studies and commented on possible functional meaning of the projection by way of axonal branchings.