Auditory nerve fibers in young and quiet-aged gerbils: morphometric correlations with endocochlear potential

The number, size and distribution of myelinated nerve fibers were analyzed in the osseous spiral lamina (OSL) of young and old gerbils raised in a quiet environment. Because decreased endocochlear potentials (EPs) play a significant role in age-related hearing loss in the gerbil, we correlated morphometric and topographical data for nerve fibers with EP measurements in the same ear. Fibers were analyzed at the 2 and 10 kHz locations. The number of fibers at the 2 kHz location ranged from 12 to 47% greater than at the 10 kHz place in both young and aged specimens. No significant correlation was found between the number of fibers and the EP. Nerve fibers in gerbil tend to be distributed vertically by size within the OSL [Slepecky et al. (2000) Hear. Res. 144, 124-134], a result also found in cats and guinea pigs. Smaller fibers are more often found towards the scala vestibuli side of the OSL, whereas larger fibers are concentrated towards the scala tympani. The present data confirmed this distribution in young gerbils; however, in aged ears the distribution often became more uniform. Moreover, fiber distribution and ganglion cell size were highly correlated with EP. As EP declined, the fiber size distribution in the OSL became more uniform and the mean cross-sectional area of spiral ganglion cells and fiber diameter decreased. Thus, for whatever reason, certain indices of auditory nerve fiber morphometrics appear to be associated with the EP.