Which inner-ear structure changes sound vibrations into nerve impulses?

• A. Semicircular canals
• B. Vestibule
• C. Cochlea
• D. Eustachian tube

Answer: (C)

Sound vibrations become nerve signals inside the cochlea, where mechanotransduction happens in the hair cells of the organ of Corti. When the stapes’ movement sets up fluid motion in the cochlea, a traveling wave forms on the basilar membrane; its peak location depends on the sound frequency, giving the cochlea its frequency-specific response. The inner hair cells, the true receptors, bend their stereocilia as the basilar membrane and tectorial membrane move relative to one another, opening ion channels and triggering a receptor potential that causes neurotransmitter release onto spiral ganglion neurons. Those neurons form the auditory nerve and carry the signal to the brain. Outer hair cells help boost sensitivity and sharpen tuning, but the conversion from mechanical sound to neural impulses occurs in the inner hair cells of the cochlea. Structures like the semicircular canals and vestibule govern balance, while the Eustachian tube mainly equalizes middle-ear pressure.