Density of myelinated nerve fibers in the chinchilla cochlea.

Baseline values for the density of myelinated nerve fibers at 11 locations in the chinchilla cochlea were obtained as a prerequisite for studying the relationship between sensory cell degeneration and spiral ganglion cell loss. Myelinated nerve fibers were counted within the osseous spiral lamina near the habenulae perforata. The data were expressed as myelinated nerve fibers per inner hair cell (MNF/IHC). Since the vast majority of the afferent fibers innervate the inner hair cells and since the percentage of myelinated efferent fibers at the level of the osseous spiral lamina is estimated to be quite small, MNF/IHC is a useful value which will permit the comparison of data from normal and damaged specimens. An average density of 9.4 MNF/IHC was found at the apex. This rose to a peak of 18.8 MNF/IHC in the first turn and then decreased to 14.2 MNF/IHC in the round window region. The standard deviations at the different locations ranged from 0.8 to 1.5 MNF/IHC.