Cochlear Transduction
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Direct answer
From 20 to 20,000 hertz, every audible frequency has a place on the basilar membrane: high frequencies vibrate the stiff, narrow base and low frequencies the floppy, wide apex — von Bekesy's travelling wave, the physical basis of tonotopy. Deflection of hair-cell stereocilia toward the tallest cilium opens mechanically gated channels, and potassium floods in from the potassium-rich endolymph, whose positive 80 millivolt endocochlear potential combines with the cell's negative interior to create a roughly 140 millivolt driving force — the largest transduction gradient in the body. The 3,500 inner hair cells in a single row carry about 95% of the afferent fibres and do the actual hearing; the 12,000-20,000 outer hair cells in three rows are a cochlear amplifier, changing length through the motor protein prestin, sharpening tuning and generating otoacoustic emissions — the basis of newborn hearing screening. The middle ear matches air-to-fluid impedance about 22-fold through its 17:1 area ratio and 1.3:1 lever.
What you must remember
- Impedance matching: tympanic membrane to oval window area ratio 17:1 plus ossicular lever 1.3:1 gives about 22-fold pressure gain; the acoustic reflex (stapedius, facial nerve; tensor tympani, trigeminal) attenuates loud sound.
- Endocochlear battery: stria vascularis maintains endolymph potassium near 150 mmol/L at plus 80 mV; hair cell resting potential about minus 45 mV — together a 140 mV drive through open transduction channels.
- Inner versus outer hair cells: about 3,500 inner hair cells with 95% of afferents are the transducers; 12,000-20,000 outer hair cells, mostly efferently driven (olivocochlear bundle, acetylcholine), amplify via prestin.
- Mechanotransduction: tip links between stereocilia (cadherin-23, protocadherin-15) pull channels open on deflection toward the tallest row; potassium and calcium inflow depolarise the cell and release glutamate.
- Otoacoustic emissions: outer hair cell amplification leaks sound back out — click-evoked otoacoustic emissions are the newborn screening tool; their absence refers the baby for auditory brainstem response testing.
- Tonotopy pays clinically: noise damage and early presbycusis hit high frequencies — the 4 kHz audiometric notch is the fingerprint of noise-induced hearing loss because basal, stiff turns are most exposed.
- Indian programme link: hearing screening of newborns is part of Rashtriya Bal Swasthya Karyakram screening under the National Health Mission, with otoacoustic emissions as the first-line bedside test.
From tuning fork to audiogram
A 45-year-old factory worker complains others mumble. Rinne is positive bilaterally and Weber is midline — sensorineural, not conductive. The audiogram shows a notch at 4 kHz bilaterally, recovering at 8 kHz: the cochlear base, stiffest and first in the travelling wave's path, has taken years of acoustic trauma; the outer hair cells at that turn are the most metabolically vulnerable element of the amplifier. As damage progresses, recruitment appears — abnormal loudness growth — precisely because the amplifier's compression is lost. The clinical logic runs straight back to mechanics: place of maximum vibration determines which cells die, and the cells that die determine the audiogram shape. Contrast a patient with otosclerosis: stapes fixation removes the 22-fold impedance match, bone conduction outruns air (negative Rinne), and the loss is conductive with an intact cochlea.
Where students slip
The question "which hair cell hears?" reliably separates candidates: inner hair cells transduce, outer hair cells amplify — reversing them mangles every downstream answer, including why otoacoustic emissions screen cochlear (outer hair cell) function but not neural function, so an infant with auditory neuropathy can pass screening. The second slip is the potassium story: endolymph is the odd extracellular fluid, high in potassium, so hair-cell depolarisation is potassium influx, not sodium — the stria vascularis exists to maintain this strange battery, and Meniere disease bloating the endolymphatic space disturbs both hearing and balance. Third, the driving force: candidates quote plus 80 mV alone; the full answer is 80 mV endocochlear potential plus the cell's minus 45 mV interior, roughly 140 mV across the transduction channel.
Frequently asked questions
Which hair cells actually transduce sound into nerve impulses?
The inner hair cells — a single row of about 3,500 cells carrying roughly 95% of the cochlear afferent fibres; outer hair cells amplify rather than report.
What is the endocochlear potential and what maintains it?
A positive 80 millivolt potential of the potassium-rich endolymph, generated by the stria vascularis, providing about a 140 millivolt driving force across the hair-cell apical membrane.
How does the basilar membrane encode frequency?
Tonotopy: high frequencies maximally displace the stiff basal turn and low frequencies the compliant apex, as described by von Bekesy's travelling wave theory.
What are otoacoustic emissions used for?
Sounds generated by prestin-driven outer hair cell motility, measured as a non-invasive newborn hearing screening tool; their absence indicates cochlear outer hair cell dysfunction.
Why does noise-induced hearing loss show a 4 kHz notch?
The basal cochlear turn, maximally stressed by intense sound and stiff enough to resonate near 4 kHz, loses outer hair cells first, producing the characteristic audiometric notch.