Light and electron microscopic observations of the oculomotor nucleus in the frog, Rana ridibunda

The oculomotor nucleus of the frog Rana ridibunda was studied with both light and electron microscopy. This nucleus shows two different motoneuronal groups, ventromedial and dorsolateral. As demonstrate in Golgi impregnations and Nissl stained material there are two cell types of oculomotor neurons, large and small, which range from 16 to 20 micrometers and 11 to 15 micrometers in diameter of cell body respectively. Examination of the large cells with the electron microscope reveals within their cytoplasm, as an impressive feature, a high concentration of granular endoplasmic reticulum which displays layered arrays and a well-developed Golgi apparatus scattered throughout. On the contrary, small cell cytoplasm is just a thin perinuclear rim and the scarce cell organelles are scattered and the rough endoplasmic reticulum do not form a layered arrangement. Two different sort of synapses were found either on the perikaryon or the dendritic trunks of both cell types. Both of them show a big amount of clear presynaptic isodiametric vesicles, and there are only few dense core or flattened vesicles. But a part of these contacts have prominent subsynaptic cisterns. Electrical coupling in the oculomotor nucleus lacks evidences.