Coordinated movement of the three rows of outer hair cells is essential for cochlear amplification

Abstract

Abstract. The process known as cochlear amplification is realized by coordinated movement of the outer hair cells (OHCs) in response to changes in their membrane potential. In this process, the displacement amplitude of the basilar membrane (BM) is thought to be increased, thereby leading to the high sensitivity, wide dynamic range and sharp frequency selectivity of our hearing. Unfortunately, however, OHCs are vulnerable to noise exposure, ototoxic acid, aging and so on. Previous studies have shown that exposure to intense noise causes functional loss of OHCs from the innermost row (i.e., close to the modiolus) to the outermost row (i.e., close to the cochlear wall). On the contrary, by other traumatic stimuli such as ototoxic acid, aging and ischemia, such loss of OHCs has been reported to occur from the outermost row toward the innermost row. However, how the cochlear amplification changes when coordinated movement of OHCs is impaired, that is when the OHCs in one, two or all three rows have become dysfunctional, remains unclear. In the present study, therefore, a finite element (FE) model of the gerbil cochlea, which takes the motility of OHCs into account, was developed based on our previous FE model. Using this model, changes in the displacement amplitude of the BM due to the functional loss of OHCs in one, two or all three rows were investigated and the effects of incoordination of the three rows of OHCs on cochlear amplification were estimated. Results showed that the displacement amplitude of the BM significantly decreased when either the innermost row or the outermost row of OHCs lost its function, suggesting that all three rows of OHCs are required for cochlear amplification.

Change of cochlear micromechanics due to different types of hearing loss

“…The whole organ of Corti moves in phase and the BM displacement increases linearly from the feet of the inner hair cell to the feet of the outer pillar cells and the amplitude is maximized close to the second row of the OHCs. This is consistent with the observations of Fridberger and Boutet de Monvel [13], in which motions within the organ of Corti are measured at a high level excitation and a drug is used to block the OHCs electromotility. A clear shearing motion of the stereocilia between the reticular lamina and tectorial membrane can also be seen to move outwards, which would cause depolarization (contraction) of the OHCs, if OHCs electromotility was included.…”

“…In the case of ototoxic acid, which leads to OHCs function loss from the outermost row, the reductions are 58% (third row removed), 86% (second and third rows removed) and 100% (all three rows removed). The level of amplification reduction from this human cochlear model is smaller compared with those from a gerbil cochlear model [13], but the conclusion that the third row of the OHC plays the most important role in the mechanical amplification is the same with the gerbil model. …”