Hearing Mechanism: Hair Cells, Cochlea, and Sound Transduction

Hearing relies on mechanotransduction by cochlear hair cells—the sensory receptors that convert mechanical sound vibrations into neural signals.

Auditory Pathway: Step by Step

1. Sound waves enter the pinna → ear canal → vibrate the tympanic membrane.
2. Ossicles (malleus, incus, stapes) amplify and transmit vibrations to the oval window.
3. Perilymph wave travels through the cochlea → basilar membrane vibrates in a tonotopic pattern (high frequencies at base, low at apex).
4. Inner hair cells (IHCs): Stereocilia deflection opens K⁺ channels → depolarization → glutamate release → afferent auditory nerve (CN VIII) firing.
5. Outer hair cells (OHCs): Exhibit electromotility (driven by prestin) → act as a cochlear amplifier → enhance frequency selectivity and sensitivity.

Key Facts

• ~15,000 hair cells per ear (3,500 IHCs, 12,000 OHCs).
• Non-regenerative in mammals; loss = permanent sensorineural hearing loss.
• Major threats: Noise exposure, ototoxic drugs (aminoglycosides, cisplatin), aging (presbycusis), genetics.
• Cochlear implants bypass damaged hair cells, directly electrically stimulating the auditory nerve.