Quantitative analysis of nerve fibre densities in the cochlea of the house mouse (Mus musculus).

Transverse sections of the cochlear nerve, silver-stained surface preparations of the cochlea, and silver- and osmium-stained tangential sections of the cochlea of the house mouse were made to determine the number and density of nerve fibres entering the organ of Corti and their distribution to inner and outer hair cells along the cochlear spiral. A number of 12,578 +/- 819 fibres was counted within the cochlear nerve, which is almost equal to the total number of 12,350 +/- 810 fibres entering the organ of Corti. The 12,350 fibres divide into 9,780 (= 79%) fibres running to the inner hair cells, 703 (= 6%) basilar (afferent) fibres and 1,867 (= 15%) upper tunnel radial (efferent) fibres innervating the outer hair cells. About 93% of all afferent fibres are connected to the inner hair cells, and only 7% innervate outer hair cells. The density of fibres running to the inner hair cells varies considerably and has a significant (p less than 0.01) absolute and relative maximum 3.7 mm and 2.9 mm from the apex respectively (total length: 6.84 mm) and decreases toward apex and base. The density of afferent fibres running to the outer hair cells shows a slow increase up to 2 mm from the apex, and remains on a constant low level (5.5 fibres per 40 micrometer) down to the base. The density of efferent fibres running to the outer hair cells increases linearly up to about 2 mm from the apex, remains rather constant (10 fibres per 40 micrometer) from 2 mm to 4 mm, and then decreases toward the base. Generally, the outer hair cells show a convergent innervation pattern, the inner hair cells a divergent one. The significance of the present measurements is discussed in relation to respecitve results from other mammals and in relation to auditory thresholds.