[The spiral ganglion of the rhesus monkey]

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.