Ultrastructure of Golgi-impregnated and gold-toned neurons in the central nucleus of the inferior colliculus in the cat.

The combined Golgi/electron microscope technique was used to analyse the cytoarchitecture and the fine structure of the central nucleus of the inferior colliculus in the cat. The analysis of Golgi-impregnated sections discriminates three major neuronal types, according to somato-dendritic morphology, and to presence or absence of dendritic spines. Two major types (spiny and aspiny) might be further subdivided into large, medium-large, medium-small and small, whilst the third ("mixed") type is subdivided into large and medium-large types. The large, medium-large and medium-small cells of the major types appear to be efferent (relay) neurons, whilst the small spiny, and especially the small aspiny cells are interneurons. In agreement with previous data, the subdivision of the relay neurons in disc-shaped and stellate cells is confirmed but the disc-shaped neurons are further subdivided in typical and atypical. The dendritic fields of the latter neurons correspond greatly but not exclusively to the fibrodendritic laminae of the central nucleus. In addition to the axonal ramification of the local circuit neurons, the axons of most (if not all) types of relay cells emit a moderate to scant, rarely--a substantial number of collaterals. The collaterals of the large spiny neurons (atypical disc-shaped cells) occasionally innervate also the cell of origin. Parallel to the light microscopic discriminations of the different neuronal types, the electron microscopic observations confirm that the ultrastructural characteristics might be very distinct. Especially evident are the differences between the large neuronal types, concerning the amount and arrangement of the granular endoplasmic reticulum, and the mode of the perikaryal, dendritic, and axonal innervation by various synaptic bouton types. Along with the unequivocal discrimination of the neurons in the central nucleus of the inferior colliculus according to the dendritic orientation, we suggest also a more detailed classification of the neuronal types according to the perikaryal size, fine dendritic morphology, and ultrastructural characteristics. Further hodological experiments, combined with the presently explored technique, will help to clarify the complicated synaptic events in the central nucleus of the inferior colliculus.