Ultrastructure of neurons and their synaptic contacts in the medial septal nucleus of the rat.

The medial septal nucleus of non-experimentally treated albino rats was investigated electron microscopically. According to former light microscopical findings most of the dendrites in this region are spineless but conspicuously varicose. Besides a striking diminution of the packing density of neurotubules inside the varicosities--compared with the narrow portions of varicose dendrites--some organelles like multivesicular bodies, lamellated bodies and cisterns of the smooth endoplasmic reticulum were found to be characteristic of them. Large neuronal somata rich in RER with a broad rim of cytoplasm around the nucleus and numerous lipofuscin-like granules occur. But there are numerous small cells exhibiting a narrow rim of cytoplasm and pourly developed RER, too. At least six classes of synaptic junctions can be distinguished, three of the asymmetric and three of the symmetric type. Most of axo-somatic contacts are of the symmetric type, at the dendrites asymmetric contacts dominate. Presynaptic profiles with large dense-core vesicles form symmetrical contacts with varicose and non-varicose dendritic profiles.