Final answer:
Sodium ions (Na+) enter the cell through mechanosensitive ion channels when the stereocilia in the ear are bent by sound vibrations, causing membrane depolarization and initiating the hearing sensation.
Step-by-step explanation:
The opened channels allow sodium ions (Na+) to pass into the cell, causing an ion current and initiating the hearing sensation. This process begins in the stereocilia of the ear, which are mechanoreceptors connected to mechanosensitive ion channels. When vibrations from sound waves bend the stereocilia, these channels open, allowing sodium ions to enter the hair cells and depolarize the membrane. This depolarization generates a signal that is transmitted to the cochlear nerve and eventually to the brain, where it is interpreted as sound. The intensity or volume of the sound is determined by the number of hair cells that are stimulated at any particular location within the ear.