An experimental light and electron microscopic study of cerebellorubral projections in the opossum, Didelphis marsupialis virginiana

Abstract

The origin, course and distribution of cerebellorubral fibers was studied in the opossum by employing the Fink-Heimer technique. Many of the cerebellorubral fibers may be collaterals of an axon which continues to the thalamus. The majority of cerebellorubral fibers arise in the nucleus interpositus and distribute throughout the red nucleus, but they are most numerous in the caudal one-third. The interpositorubral fibers appear to be topographically organized. The only other cerebellar input to the red nucleus takes origin within the lateral cerebellar nucleus and distributes exclusively to a small dorsal rostral portion. Electron microscopic analysis of the red nucleus following either cerebellectomy, hemicerebellectomy or stereotaxic lesions in nucleus interpositus and nucleus lateralis, reveals that axon terminals of cerebellorubral fibers mainly contact the somata and proximal dendrites of giant and large-medium nerve cells. Many of the terminals on the somata and proximal dendrites reside in depressions of the plasma membrane. In addition to direct axosomatic contacts, these large boutons also synaptically contact small appendages which arise from the cell body. The majority of the terminals are large and ovoid (2–4 μm × 5–10 μm) or elongate (1 μm × 10–12 μ) and, after lesions that encroach upon the interpositus nucleus, undergo a filamentous type of degeneration followed by electron dense degeneration and glial investment. Lesions restricted to the lateral nucleus result in electron dense degeneration of small (1–3 μm) axon terminals which primarily contact the somata and proximal dendrites of large-medium neurons.