Quantitative analysis of cochlear structures in the house mouse in relation to mechanisms of acoustical information processing

Abstract

1. Soft surface preparations of the cochlea were used to determine the density of inner and outer hair cells along the basilar membrane. A total number of 2526 outer hair cells (in three rows) and 765 inner hair cells (in one row) were counted. The density of hair cells is significantly higher in the middle part of the cochlea (3.5–4 mm from the apex, total length 6.84 mm) than near apex and base. 2. A special technique was developed for cutting the cochlea on an ultramicrotome radially (2 μm or 3 μm thick sections) throughout the total length. The evaluation of the radial sections showed a) two significant maxima of density of habenular openings (places where nerve fibres enter the organ of Corti), which can be assumed to represent maxima of innervation density, b) a significant increase of thickness of the basilar membrane from the apex (1μm) up to 5.8 mm from the apex (14.5 μm), followed by a plateau up to 6.1 mm and a rapid decrease toward the base (4.5 μm), c) a significant decrease in width of the basilar membrane from 1.5 mm from the apex (160 μm) to the base (40 μm), d) a significant reduction in height of the tunnel of Corti from 1 mm from the apex (40 μm) to the base (18 μm). 3. The area of the helicotrema amounts to 0.047 mm2. 4. The present data on hair cell and innervation (habenular opening) density are closely related to the frequency region of best hearing (15 kHz) and to features of frequency and intensity discrimination. 5. From the data on width and thickness a function for changing stiffness (z) along the basilar membrane was calculated (z = 0.0032·100.206c;c=distance in mm from apex). 6. The most probable mode of displacement of the basilar membrane and of excitation of hair cells was derived from pictures of radial sections.