An electron microscopic study of the cerebellorubral connections after neonatal lesions in the sensorimotor and adjacent cortex in the albino rat.

After a single right deep cerebellar nuclear (DCN) lesion in the dentate and adjacent interposed nuclei in adult rats, most degenerating cerebellorubral terminals in the rostral parvicellular portion of the contralateral red nucleus make contact with medium-sized and large dendrites. A few of them make contact with small dendrites, somata and other axonal terminals. The degenerating axons and terminals show the dense type of reaction. After left neonatal sensorimotor and adjacent cortical lesions followed by a right DCN lesion 2-10 months later, the pattern of most synaptic contacts resembles that in normal adult rats receiving only DCN lesions. There are, however, noteworthy differences. The incidence of axodendritic contact on dendritic spines is greatly increased. There is also an increase in the incidence of two terminals in close proximity and in contact with other neuronal profiles, and of one terminal in contact with more than one dendritic profile. The incidence of axodendritic contact on medium-sized and large dendrites is slightly reduced. The possibility of a taking over of cerebrocorticorubral projections by cerebellorubral fibers and of axonal and dendritic sprouting is suggested. In animals receiving double lesions, the filamentous type of degeneration could be seen. Degenerating dendrites occur in animals receiving single or double lesions.