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 thc gerbil cochlea fall into morphologically and spati ally classified groups. In cats and guinea pigs, these groups. based on siz e, location oil inner hail cell (IHC) and stratification within the osseous spiral lamina, have been found to correlate with spontaneous rate, thresho ld sensitivity and projection pattern to the cochlear nucleus. Thus, there may be anatomical data to suggest mechanisms for intensity coding of differ ent frequencies of sound. Afferent nerve terminals contacting IHCs in the g erbil cochlea were analyzed with regard to size and location. Data were obt ained from serial thin sections (700 for each THC) cut perpendicular to the lung axis of eight IHCs (two apical and two basal IHCs from two cochleas), observed and photographed using a transmission election microscope. Result s indicate tart the percentage of modiolar versus pillar-side terminals aro und each IHC varies from cell to cell. In some cases, the smallest fibers w ere located on the modiolar side, but a consistent distribution of the smal lest fibers on this side of the cell was not characteristic. While a size-b ased segregation of terminals does not appear around the perimeter of the I HC, modest size-based segregation of nerve fibers is found in the osseous s piral lamina. Perimeter measurements were made from myelinated fibers cut i n cross-section, obtained from semi-thin sections in the distal (near the I HCs) 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 lympani/scala vestibuli (SV) axis of the nerve bundles within the osseous spiral lamina such that more of the s maller perimeter fibers are located on the SV side and more of the larger p erimeter fibers are located on the ST side. Our data for terminals at the I HC are different from those seen in the cat. our data for nerve fibers in t he osseous spiral lamina support those seen in the cat and guinea pig. (C) 2000 Elsevier Science B.V. All rights reserved.