Nerve Fiber Synapses on Spiral Ganglion Cells in the Human Cochlea

Abstract

Nerve fiber synapses were observed on the small myelinated and unmyelinated spiral ganglion cells of the human cochlea. The nerve fibers penetrated the junctions of the ensheathing satellite cells and myelin lamellae, and partly invaginated into the perikarya to establish axosomatic synapses. One small myelinated neuron with a nerve fiber synapse demonstrated multipolar cell processes. However, most small neurons with identical ultrastructural characteristics did not show these synapses. Axodendritic synapses were also seen on the dendritic processes of the small neurons and between varicose nerves and unmyelinated nerve fibers some distance from the small neurons. While these observations conform in some aspects with the concept of parasympathetic nerve fibers and neurons in the cochlea, they are also compatible with the idea that the small neurons may have an auditory function influenced by the synaptic contacts of efferent fibers from the olivocochlear bundle.