The organization of nerve fibers in the spiral tunnel bundle (STB) of the guinea pig organ of Corti has been studied by light and electron microscopy. In the basal turn STB fibers were found to run toward the base or apex in about equal numbers while in the upper turns they ran predominantly in an apical direction. Long spiral fascicles of 100 microns or more in length were abundant in the third and fourth turns, but were rarely seen in the lower turns. No synaptic contacts between tunnel bundle fibers could be found and no afferent nerve fibers were seen in the bundle. Furthermore, interruption of the efferent nerve supply resulted in complete degeneration of the STB. The tunnel bundle does not, therefore, appear to be a site of synaptic interaction between afferent and efferent neural elements.
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