Final answer:
The stereocilia of the hair cells bend due to mechanical movement, causing protein fibers to stretch and gated ion channels to open, leading to cell depolarization and signal transmission to the cochlear nerve.
Step-by-step explanation:
When the bundles of stereocilia of the hair cells in the organ of Corti tilt, the protein fibers (also known as tip links) connecting them are stretched, opening gated ion channels. The stereocilia are mechanoreceptor structures on the apical surfaces of the hair cells that transduce mechanical movement into electrical signals. They are arranged from tallest to shortest and are tethered together so that when they bend towards the tallest member of the array, the tension on these protein fibers increases. This increased tension opens the ion channels, leading to depolarization of the hair cell membrane and the initiation of a signal that is transmitted to the cochlear nerve. Conversely, bending towards the shortest stereocilia results in slackening of these tethers, closing the ion channels and stopping the depolarization.