Pathophysiological Mechanisms Of Hearing Loss

An understanding of auditory transduction in the ear can contribute to a better comprehension of the pathophysiological mechanisms which give rise to hearing loss. The incoming sound sets up a mechanical traveling wave which begins at the base and progresses along the basilar membrane, reaching a point of maximal displacement. The region of maximal displacement is a function of stimulus frequency. The mechanical displacement, by directly opening ion channels in the stereocilia of the hair cells, induces changes in the electrical potential of the hair cells. This initial stage is called mechano-electrical transduction, and in the normal ear, is followed by a stage of electro-mechanical transduction based on the ability of the outer hair cells to respond to the electrical changes induced in them with a change in their length. This “electromotility” presumably provides mechanical feedback to the basilar membrane, augmenting its mechanical displacement. This is called the cochlear amplifier, providing the ear with improved sensitivity and frequency discrimination. Most forms of sensori-neural hearing losses (affecting the inner ear) are due to a lesion to some part of this cochlear amplifier (e.g. noise induced hearing loss, ototoxic drugs) and are therefore characterized by auditory threshold elevations and poorer frequency discrimination.