The organization of projection neurons in the opossum red nucleus

Abstract

The opossum red nucleus is populated by neurons encompassing a considerable size range. The largest neurons (giant neurons, 45–70 μm) are restricted to its caudal, medial third, whereas those in the large-medium category (25–40 μm) are located throughout the nucleus. The smallest neurons (less than 20 μm) are relatively achromatic and few in number, but are also scattered throughout the nucleus. Evidence from both retrograde and orthograde degeneration studies shows that rubrospinal fibers arise from both giant and large-medium neurons in the caudal third of the nucleus and from large-medium neurons in its rostral two-thirds (mainly the ventral part). Neurons in the medial part of the caudal red nucleus (giant neurons particularly) contribute relatively few fibers to contralateral brain stem nuclei, whereas, large-medium neurons residing in its rostral two-thirds and in the lateral extreme of its caudal third project more extensively to such areas and appear to be the main source of fibers to the chief sensory and spinal trigeminal nuclei, the facial nucleus and the parvicellular reticular formation. Some of these rubrobulbar fibers are likely collaterals of spinal axons. The experimental results further suggest that (1) rubrocerebellar axons arise from both caudal and rostral areas of the nucleus, (2) some large-medium neurons project only to the contralateral brain stem and/or cerebellum, and (3) the ipsilateral rubrobulbar bundle arises from large-medium neurons which are located within the rostral red nucleus. Previous experimental light and electron microscopic studies, together with observations made from Golgi impregnated sections, provide evidence that the small neuron is intrinsic to the nucleus. The organization of the opossum red nucleus revealed by the origin of the various descending projections is generally reflected by its cortical and cerebellar inputs and by its histochemistry.