The inferior olivary nucleus of the rat: A light and electron microscopic study

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Abstract
This study provides basic data on the normal structure of the inferior olivary nucleus of the rat at both the light and electron microscopic level. The cytoarchitecture of the nucleus was mapped in serial sections cut in the transverse plane and stained with cresyl violet. The bulk of the inferior olive is composed of the princpal olive and the dorsal and medial accessory olives. The dorsal cap, nucleus β, ventrolateral outgrowth and dorsomedial cell column are minor subgroups of cells which are clearly represented in the nucleus.Golgi preparations revesled two types of dendritic pattern both of which are found in the principal olive and in the accessory olives. Portions of the dendritic tree are varicose in appearance and bulbous expansions, often bearing spines, are pendage may be observed; namely, simple spines, pedunculatd club‐shaped spines, and racemose appendages. Appendages are most numerous in association with distal portions of th dendritic tree.Analysis of the principal olive of the principal olive and of the accessory olives with the electron microscope revealed the following characteristics: Round vesicle terminals which are the predominat type found in the nucleus range in size from 0.3 μm. to 4.0 μm. Pleomorphic vesicle terminals range in size from 0.5 μm to 3.5 μm. Both types of terminal are ubiquitous in their distribution with regard to the dendritic tree of olivery neurons but are more numerous in relation to dendritic elements measuring less than 2 μm in diameter. Axosomatic synapses and synapses on proximal dendrites are few in number and are made by both types of terminal. A third type of terminal which contains up to 25% dense core vesicles is found infrequently in all three major subnuclei. Much of the surface of the soma of olivary neurons is covered with glial cell processes or by dendritic elements. No membrane specialization is found between these apposed surfaces.Dendritic spines, some of which contain a well marked spine apparatus, are commonly seen and appear to correspond to the pendunculated club‐shaped spines seen in the Golgi preparations. Other large dendritic profiles which are packed with mitochondria are considered to possibly correspond to the varicose dilations of the dendrites or to “bleb‐like” protrusions of dendritic shafts seen in Golgi preparationsGlomerulus‐like structures are seen in the neuropil and have been shown to possess a core of small dendritic elements which are contacted at their pertially surrounded by glial cell processes and has been termed a synaptic cluster. Such axo‐dendritic complexes are largest and most numerous in the principal olive and rostral half of the medial accessory olive. In some planes of section, it is possible to trace the dendritic core of the synaptic cluster back to a larger dendritic shaft. It is suggested that such dendritic cores correspond to the racemose appendages which are seen in Golgi preparations.Gap junctions and attachment plaques are frequently seen between dendritic elements in synaptic clusters and also between dendritic elements which are not part of synaptic clusters, in all three major subnuclei. In the medial accessory olive, several examples of two gap junctions arranged in series between three small dendritic elements have been observed.