Study of afferent nerve terminals and fibers in the gerbil cochlea: distribution by size

The purpose of the present study was to determine if the synaptic terminals and nerve fibers in the gerbil cochlea fall into morphologically and spatially classified groups. In cats and guinea pigs, these groups, based on size, location on inner hair cell (IHC) and stratification within the osseous spiral lamina, have been found to correlate with spontaneous rate, threshold sensitivity and projection pattern to the cochlear nucleus. Thus, there may be anatomical data to suggest mechanisms for intensity coding of different frequencies of sound. Afferent nerve terminals contacting IHCs in the gerbil cochlea were analyzed with regard to size and location. Data were obtained from serial thin sections (700 for each IHC) cut perpendicular to the long axis of eight IHCs (two apical and two basal IHCs from two cochleas), observed and photographed using a transmission electron microscope. Results indicate that the percentage of modiolar versus pillar-side terminals around each IHC varies from cell to cell. In some cases, the smallest fibers were located on the modiolar side, but a consistent distribution of the smallest fibers on this side of the cell was not characteristic. While a size-based segregation of terminals does not appear around the perimeter of the IHC, modest size-based segregation of nerve fibers is found in the osseous spiral lamina. Perimeter measurements were made from myelinated fibers cut in cross-section, obtained from semi-thin sections in the distal (near the IHCs) and proximal (near the spiral ganglion) regions of the osseous spiral lamina. Best-fit line analysis indicates there is a modest nerve fiber size/vertical organization along the scala tympani/scala vestibuli (SV) axis of the nerve bundles within the osseous spiral lamina such that more of the smaller perimeter fibers are located on the SV side and more of the larger perimeter fibers are located on the ST side. Our data for terminals at the IHC are different from those seen in the cat; our data for nerve fibers in the osseous spiral lamina support those seen in the cat and guinea pig.