[Electron microscopic studies on the medial septal nucleus of the rat: neuronal somata and axosomatic contact].

At least 4 categories of neurons were found in this region, whose somata differ in ultrastructural features. 1. Large cells with a broad rim of cytoplasm (nucleus cytoplasm-index; 3.07 +/- 0.67), RER is densely arranged throughout the whole perikaryon. The nuclear invagination index (1.34 +/- 0.19) is low. Few axosomatic contacts are present. 2. Large to medium-sized cells with a broad rim of cytoplasm (nucleus/cytoplasm-index 2.58 +/- 0.36 and 2.08 +/- 0.42). The RER is clustered in voluminious staples. There are differences in the nuclear invagination indices between subpopulations. In large cells it is low (1.36 +/- 0.24), but it is high in medium-sized cells (1.70 +/- 0.5). Axo-somatic contacts are more numerous in this type than in others. 3. Large, medium-sized and small cells with a broad rim of cytoplasm (nucleus/cytoplasm-index; 2.50 +/- 0.32, in large and medium-sized 2.25 +/- 0.5 and 2.24 +/- 0.48 in small cells). Single RER elements tends to be arranged in loops. The cells are characterized by a high nuclear invagination index. It is 1.68 +/- 0.38, in the large cells, 1.56 +/- 0.44 in the medium-sized and 1.55 +/- 0.34 in the small ones. In this neuron category the number of axo-somatic contacts is very low. 4. Medium-sized to small cells with a narrow cytoplasmic rim (nucleus/cytoplasm-index; 1.78 +/- 0.23 and 1.89 +/- 0.39). RER is nearly absent. The nuclear invagination index of medium-sized cells (1.11 +/- 0.07) and of small cells (1.14 +/- 0.12) is lower than in other categories. These cells exhibit a very low number of axo-somatic contacts. Comparison with findings of immunostained cells lead to the conclusion that category 2 corresponds to GABA-ergic and category 3 to cholinergic neurons of the medial septal region.