Origin and trajectory of the cerebello-olivary projection: an experimental study with radioactive and fluorescent tracers in the cat.

It was demonstrated with autoradiographic methods that a tract of thin fibers, which is for the most part distinct from the brachium conjunctivum and its crossed descending limb, can be followed from the cerebellar nuclei to the inferior olive. Neurons from all regions of the interposed and dentate nuclei contribute to this tract. With two fluorescent tracers, Fast Blue and Diamidino Yellow Dihydrochloride, the perikarya of neurons in the cerebellar nuclei that project to the inferior olive or thalamus were identified and the possibility of collateral projections from single neurons to both regions was investigated. Numerous retrogradely labeled neurons were seen in the contralateral interposed and dentate nuclei but no double-labeled cells were ever encountered. The mean diameters of the cells projecting to the inferior olive were between 15 and 20 microns while the mean diameters for those projecting to the nucleus ventrolateralis of thalamus were between 25 and 35 microns. Differences in the morphology of the two cell populations were noted. It was also found that the fastigio-olivary projection is small, terminating mainly contralaterally, in the caudal medial accessory olive and subnucleus beta. It is concluded that cerebello-olivary fibers are not collaterals of the cerebellothalamic projection and that there are two different populations of neurons in the cerebellar nuclei that give rise to these distinct efferent projections.