The organization of nerve fibers in the vicinity of the inner hair cells of the guinea pig cochlea was studied in silver-stained whole-mount specimens by light microscopy. In all turns of the organ of Corti the inner spiral bundle (ISB) was found to contain both short and long spiral fascicles coursing toward either the base or the apex. The ISB is largest and its organization is most complex in the upper basal and lower second turns. It becomes markedly reduced in size in the apical and lower basal turns. Many ISB fibers cross the tunnel of Corti to the outer hair cells. Surgical interruption of the efferent nerve supply in the brain stem resulted in degeneration of all ISB fibers. After elimination of the efferents it could be seen that the majority of afferent fibers end on the inner hair cells.
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