Hearing and Cochlear Physiology
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Direct answer
Hearing converts airborne sound into neural discharges through three stages: the external canal resonates (best near 3-4 kHz), the middle ear matches the impedance of air to the impedance of cochlear fluids by amplifying pressure roughly 22-fold (about 17 times from the tympanic-membrane-to-stapes-footplate area ratio multiplied by a 1.3-fold ossicular lever), and the cochlea performs frequency analysis. Von Békésy's travelling wave moves from base to apex with its peak at the base for high frequencies and near the apex for low frequencies. Deflection of hair-cell stereocilia towards the tallest cilium opens mechanotransducer channels, and the 140 mV driving force between the +80 mV endocochlear potential of scala media and the hair-cell interior drives the receptor current.
What you must remember
- Ossicular impedance matching: effective tympanic membrane area to stapes footplate area ratio about 17:1, lever ratio about 1.3:1, together roughly 22:1 or about 25 dB of gain; the attenuation reflex (stapedius, nerve VII) protects against sustained loud sound.
- Travelling wave: von Békésy — the envelope peaks basally for high frequencies (up to 20 kHz) and apically for low (down to 20 Hz); the basal end is stiff and narrow, the apex floppy and wide.
- Endocochlear potential: +80 mV in scala media generated by the stria vascularis; with the hair cell at about −45 to −60 mV, the transduction driving force across the apical membrane is about 140 mV.
- Hair cells: inner hair cells (one row, about 95% of afferent fibres) are the true sensory transducers; outer hair cells (three rows) provide the cochlear amplifier via prestin, sharpening tuning and generating otoacoustic emissions.
- Cochlear potentials: endocochlear (DC, +80 mV), cochlear microphonic (follows the sound waveform, chiefly from outer hair cells), summating potential (DC shift), and compound action potential (from the auditory nerve).
- Efferent control: the olivocochlear bundle (lateral and medial systems) modulates outer hair cells and tunes attention to sound in noise.
- Clinical correlates: 512 Hz tuning fork for Rinne and Weber tests; presbycusis hits high frequencies first; noise notch at 4 kHz on audiometry.
How to work through a hearing loss case
A 45-year-old with slowly progressive hearing loss and a history of childhood ear discharge needs localisation before anything else. On Rinne test with a 512 Hz fork, air conduction greater than bone conduction in both ears, but Weber lateralises to the quieter ear — the pattern of sensorineural loss. Audiometry shows a high-frequency sloping loss with a 4 kHz dip: noise-induced cochlear damage, because the travelling wave's high-frequency basal peak plus the ear canal's 3-4 kHz resonance concentrate acoustic energy at the basal cochlea, where 4 kHz-sensitive cells sit.
Contrast a conductive pattern: otosclerosis fixing the stapes gives bone conduction greater than air on Rinne and Weber lateralising to the affected ear. The physiology explains the tuning fork logic — in conductive loss, the blocked air route removes the middle-ear amplification, so bone routes the signal directly; in sensorineural loss, the damaged cochlea on the bad side hears the bone-conducted vibration worse, so the fork is heard in the better ear. Otoacoustic emissions testing in newborns (part of universal screening programmes) exploits the outer hair cell amplifier: present emissions mean the cochlea works, and failure then points to auditory neuropathy when the brainstem response is abnormal.
Where students slip
Two errors dominate. Students often assign the sensory role to outer hair cells because they are numerous; the inner hair cells carry about 95% of the afferent information, while outer hair cells electromotilely feed energy back into the basilar membrane — the cochlear amplifier — which is why outer hair cell loss gives only moderate loss with poor speech-in-noise discrimination. The second error is drawing the travelling wave peaking apically for high tones. Remember the mechanical logic: stiff, narrow, tightly-anchored basilar membrane at the base resonates with high frequencies; loose and wide at the apex for low ones — and place coding is sharpened by the amplifier so human frequency discrimination reaches about 0.2% (a few hertz at 1 kHz), far better than the broad passive envelope.
Frequently asked questions
How does the middle ear match air to cochlear fluid impedance?
By amplifying pressure about 22 times: a 17-fold area ratio between tympanic membrane and stapes footplate and a 1.3-fold ossicular lever advantage, recovering about 25-30 dB that would otherwise be lost at the air-fluid interface.
What generates the +80 mV endocochlear potential?
The stria vascularis, via its marginal cell intermediate-cell potassium circulation; it maintains the high potassium of endolymph (about 150 mmol/L) and the positive DC potential essential for the large transduction driving force.
Which cochlear potential follows the sound wave form?
The cochlear microphonic, an AC potential generated chiefly by outer hair cell receptor currents, which mirrors the stimulus frequency and phase.
Why are otoacoustic emissions used in newborn screening?
They are by-products of outer hair cell electromotility; presence indicates a functioning cochlear amplifier and thus normal outer hair cells, making the test a quick objective screen before behavioural audiometry is possible.
Why does early presbycusis and noise damage impair hearing at 4 kHz first?
The basal cochlea handles high frequencies and bears the peak of the travelling wave for the resonant 3-4 kHz band of the ear canal, so metabolic and noise injury concentrates there.