A detailed parcellation of the entire cochlear nucleus of the cat was prepared with the Nissl and Protargol methods in the form of a cytoarchitectonic atlas. Neuronal cell types were characterized in rapid Golgi impregnations. Golgi impregnations were used to define the specific features of the mature neuronal types. Neurons from the Golgi preparations were systematically mapped according to type and location in serial sections. These neurons were then plotted in corresponding positions in the cytoarchitectonic atlas. This combined GolgiNissl approach provides a scheme in which neuronal types, defined in terms of a number of functionally significant features, can be precisely localized within the cochlear nucleus.The cochlear nucleus provides the first opportunities for recoding of signals from the auditory nerve. So the neuronal architecture of the cochlear nucleus must play a key role in defining the functional organization of the ascending auditory system. The structural organization of the cochlear nucleus is complicated by the fact that it is not a single nucleus but a complex of distinct neuronal populations. Previous investigators, using several histological techniques, have attempted to identify and describe types of neurons and to subdivide the nucleus. Most schemes of parcellation have been based on cell body stains, especially the Nissl method, whereas neuronal cell types have been characterized with a variety of techniques, including Nissl, reduced silver, and Golgi methods (e.g., in the cut: Ram6n y Cajal,

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This report describes some observations of the synaptic organization of one region of the cat dorsal cochlear nucleus (DCN). The large "fusiform cell" and its innervation from the cochlea are emphasized. The morphology of the mature fusiform cell and its postnatal development are described in rapid Golgi impregnations of perfusion-fixed littermate cats. The mature features are correlated with profiles of fusiform cell bodies, apical dendrites, and basal dendritic trunks in electron micrographs from adult cat brains. Small neurons and granule cells are also identified in electron micrographs. In Golgi impregnations, axons of small cells and granule cells may terminate upon fusiform cells. Six classes of axons can be distinguished in rapid Golgi impregnations of the DCN. Two classes are of cochlear origin. One axonal class arises from small cells. The sources of the remaining axonal classes have not been identified in this study. Primary afferents can terminate as large, mossy endings in the DCN neuropil. They can also participate in axonal nests along with axons and dendrites of small cells. In electron micrographs, four synaptic endings can be distinguished. Primary cochlear fibers end in large terminals with asymmetrical synaptic complexes and round, clear vesicles. Primary axons can end in glomeruli, resembling those of the cerebellum, or in synaptic nests which are conglomerates of neuronal processes including other types of endings. The origins of the other synaptic types are not yet known. According to this study, primary afferent input could influence fusiform cells directly or indirectly, via small cells and granule cells.

Study of the caudal cochlear nucleus of the cat confirms the cochlear origin of synaptic terminals, identified in correlated rapid Golgi and electron microscopic preparations of the octopus cell area (OCA) and the dorsal cochlear nucleus (DCN) in normal cats. Type 1 and type 2 endings on octopus cell somas and basal dendrites, as well as type 1 and type l a endings of the outer DCN, degenerate following complete ipsilateral cochlear ablations and short survival periods (12, 24, 48, 96 hours). Two distinct patterns of synaptic degeneration occur after short survival times; "dense degeneration" occurs in type 1 endings on octopus cells and several endings of the DCN. Dense terminals that contain tightly packed, but intact vesicles, occur most frequently after a 48-hour survival period. A second type of degeneration, called "flocculent degeneration" occurs in type 1 and type 2 endings of the OCA and in type 1 and type l a DCN terminals. Between 12 and 48 hours after ablation, the flocculent degeneration involves a continuous breakdown of organelles. Evidence for transneuronal degeneration of octopus cells and DCN granule cells is presented.