The inferior olivary complex of guinea pig: cytoarchitecture and cellular morphology.

The inferior olivary complex (I.O.C.) of the guinea pig can be divided into three primary subdivisions: the principal olive (PO), the medial accessory olive (MAO), and the dorsal accessory olive (DAO). In Nissl-stained preparations, the PO possessed darker staining cells than did the MAO and DAO and was the most densely populated with cells. All neuronal somata in the I.O.C. were oblique-spheroid in profile (mean size: coronal = 18.3 microns, parasagittal = 15.8 microns). Based on Golgi impregnations, it was apparent that inferior olive cells were of two unique radiate-cell types (I and II). Type I neurons had relatively diffuse, sparsely branched dendritic arbors, whereas type II cells had dendrites which were highly branched and massed about the cell body, at times creating complex spirals. Type II cells were further categorized into types IIa and IIb based on geometric variations of the type II dendritic arbors. Indices of branching and tortuosity, together with estimates of dendritic arbor volume, were quite helpful in distinguishing cell types. The cell types were differentially distributed across the subdivisions with type I neurons being encountered in the MAO while type II cells were found in all three subdivisions. Within the neuropil of the I.O.C., three different afferent axonal arbors were identified, as was the presence of dendrites from surrounding reticular formation cells. Neuronal aggregates creating a possible electrical syncytium within the I.O.C. are consistent with the dendroarchitectonics of the cells.