Richard Dom

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.

Spino-cerebellar fibers of the opossumDidelphis marsupialis virginiana

Abstract Complete and partial spinal cord hemisections, one lesion per animal, were performed at cervical, thoracic, lumbar or sacral spinal cord levels in a series of 12 adult opossums. Following a survival time of 1–3 weeks, the brains and spinal cords were removed and stored in buffered formalin. The cerebella and brain stems were cut in the transverse, the sagittal or horizontal plane and subjected to either the Nauta-Gygax method or Fink-Heimer technique. In addition, the extent of each lesion was verified histologically. After cervical and thoracic hemisections, abundant fiber degeneration was present bilaterally in fairly distinct longitudinal rows within both the subcortical white matter and the adjacent granular layer of the anterior lobe. Also, degenerating fibers were present bilaterally within the granular layer of the posterior portion of the paramedian lobules and pyramis. The fibers which distributed to these areas were most abundant on the side of the lesion. Although lumbar and sacral lesions resulted in fiber degeneration within the same regions of the cerebellum, the fibers were fewer in number. This was particularly true in the pyramis and paramedian lobules. In all specimens degenerating fibers of fine diameter, judged to be collaterals from fibers passing into the granular layer, entered the nucleus fastigii, the nucleus interpositus and the medial portion of the nucleus dentatus. However, these were much more numerous after cervical and thoracic hemisections than after lumbar and sacral lesions.

The motor cortex and corticospinal tract of the Armadillo (Dasypus novemcinctus)

The neocortex of the armadillo, Dasypus novemcinctus, was explored by electrical stimulation and the motor area determined to be caudal to the supraorbital sulcus. Movements of the tongue, lips, pinna, scalp, shoulder, back, fore-limb and, to some extent, hind-limb could be elicited. The motor area for the lips and tongue was particularly large and it had the lowest threshold to stimulation. The motor cortex was characterized by deeply-staining pyramidal cells and the lack of a well-developed internal granular layer. The pyramidal cells were largest and most numerous in the limb area. Ablation of the motor cortex or part of it resulted in fiber degeneration within the contralateral ventral and lateral funiculi of the spinal cord. Degenerating fibers distributed extensively to the medial parts of laminae IV through VI and, to a much less extent, to the medial part of lamina III and the lateral parts of laminae V and VI. Degenerating fibers were not observed in the spinal cord after presupraorbital lesions or after ablations of the caudal or ventrocaudal neocortex.