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Four temporal bones of rhesus monkeys were investigated in respect of composition and morphology of the spiral ganglion cells. Using light and electron microscopic technique two main types of ganglion cells could be distinguished, namely 90% large (type I) and 10% small (type II) cells. Both small and large ganglion cells are surrounded by myelin sheaths: 90% of the large neurons are surrounded by myelin sheaths, whereas only 15% of the small neurons have a myelin sheath. The small neurons are exclusively connected with unmyelinated nerve-endings forming synaptic contact with the cell surface. These nerve endings contain vesicles similar to synaptic vesicles; the cytoplasm of the adjacent small ganglion cells contains dense core vesicles. Similar dense core vesicles are, however, also found in the cytoplasm of the large ganglion cells and in the axoplasm of the corresponding nerve axons. It is only in the cytoplasm of the small ganglion cells that dense bundles of neurofilaments are present. It seems that the nerve endings resembling synapses and contacting the small ganglion cells belong to the efferent, olivo-cochlear spiral bundle.

Four temporal bones of human newborns were studied light- and electron-microscopically. Different parts of the spiral ganglion were examined but absolutely no myelinization of the large (type I) and small (type II) ganglion cells could be seen. Between the ganglion cells dense bundles of mixed nerves are detected, some of the unmyelinated nerves show synaptic-like contact zones to the surface of type II-cells. The small type II cells are mainly located at the periphery of the ganglion cell complex. Between the spiral ganglion and the osseous spiral lamina large bundles of unmyelinated nervecomplexes are crossing the mainly myelinated radial fibers. The nerve fibers running from the modiolus to Rosenthal's channel contain bout 12% unmyelinated nerves whereas the nerve fascicles crossing the osseous spiral lamina reveal a population of 22-25% unmyelinated nerve fibers. About 2% of the small unmyelinated nerve fibers (diameter 200-500 nm) which run between the ganglion cells contain dense core vesicles.