A cytoarchitectonic and Golgi study of the red nucleus in the rat

The cytology of rubral neurons was investigated using both paraffin and 1 mu thick Epon sections. The neurons were divided into four size categories which form a continuum with regard to cellular characteristics. Giant neurons (greater than 40 mu) and large neurons (26-40 mu) predominate in the caudal one-third of the nucleus. Large neurons extend up the ventral, medial and lateral borders of the nucleus into its middle one-third. The caudal one-third of the nucleus together with this extension of large cells is designated as the magnocellular part of the nucleus. The more rostral part of the nucleus contains predominantly small (less than 20 mu) and medium size (20-25 mu) neurons, and this constitutes the parvocellular part of the nucleus. The characteristics of each cell type are described. Camera lucida drawings of the rubral neurons were made in three different planes of section, and the extent of the magnocellular and parvocellular portions of the nucleus was determined. Dorsomedial and ventrolateral subgroups of the magnocellular part of the nucleus are found 300-400 mu from the caudal pole. A lateral horn of small and medium size neurons with the addition of a few large neurons extends from the lateral part of the nucleus 500-700 mu from the caudal pole. Golgi preparations of the red nucleus were examined in three planes of section. Giant and large neurons display short spines on the soma and also, along the entire length of the dendrites. Inaddition, more elongate spinous processes are seen on these dendrites and are frequently aggregated into tufts at loci on the dendrites or at their terminations. The dendrites of these neurons radiate in all directions from the soma but remain within the confines of the nucleus. Medium size neurons demonstrate radially arranged dendrites. Sparsely positioned spines are seen along the length of the dendrites but are absent on the soma. A number of such neurons demonstrate tufts of elongate spines on their dendrites. The dendrites of small neurons branch infrequently and remain within the confines of the nucleus. A few dendritic but no somatic spines are seen in association with these neurons.