Step by Step
Mech
From sound wave to mechanical vibration
Sound waves enter the ear canal, vibrating the tympanic membrane. This vibration transmits through the ossicles (malleus, incus, stapes), which amplify it roughly 20-fold, and the stapes pushes on the oval window.
Fluid
Fluid waves and the basilar membrane
The stapes pushing on the oval window creates pressure waves in the cochlear fluid (perilymph, then endolymph), which cause the basilar membrane to vibrate.
Hair
Hair cells — converting mechanical energy to a signal
Hair cells on the organ of Corti bend against the overlying tectorial membrane as the basilar membrane vibrates. This mechanical bending opens ion channels, depolarizing the hair cells and triggering neurotransmitter release.
CN8
CN VIII and tonotopic organization
The signal stimulates the cochlear branch of CN VIII (the vestibulocochlear nerve), traveling to the brainstem and auditory cortex. Pitch is coded by which specific location along the basilar membrane vibrates most — high frequencies at the base, low frequencies at the apex — a pattern called tonotopic organization.
The basilar membrane's tonotopic organization means that a high-pitched sound and a low-pitched sound activate hair cells at entirely different physical locations along the membrane — high frequencies stimulating the base, and low frequencies stimulating the apex — giving the brain a direct spatial code for pitch.
Applied Walkthrough
1
An audiologist explains that damage to hair cells at one specific location along the basilar membrane causes hearing loss for one specific range of pitches, while leaving other pitches unaffected.
2
Ask: why would localized damage produce pitch-specific hearing loss, rather than a general reduction in hearing across all frequencies? Because the basilar membrane is tonotopically organized — different physical locations along its length respond most strongly to different sound frequencies, with high frequencies concentrated near the base and low frequencies near the apex.
3
This means damage confined to hair cells at the base of the basilar membrane would specifically impair high-frequency hearing, while hair cells elsewhere (responsible for lower frequencies) would remain unaffected — producing a very specific, frequency-limited pattern of hearing loss rather than a uniform reduction.
4
This tonotopic mapping is exactly why audiologists can use frequency-specific hearing tests to localize likely damage along the basilar membrane, based on which pitches are and aren't affected.
Exam Application
Exams test the correct sequence of the hearing pathway (sound wave → tympanic membrane → ossicles → oval window → cochlear fluid → basilar membrane → hair cells → CN VIII), how hair cells convert mechanical bending into a neural signal, and tonotopic organization (base = high pitch, apex = low pitch).
⚠ Common Trap
The most common trap is forgetting that pitch is encoded by location along the basilar membrane (tonotopic organization) rather than by signal strength or firing rate alone — a specific physical location vibrating most strongly is what tells the brain which pitch was heard.
✓ Quick Self-Check
1. What happens to the ossicles when the tympanic membrane vibrates?
They transmit and amplify (roughly 20-fold) that vibration, with the stapes ultimately pushing on the oval window.
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2. What happens to the basilar membrane as a result of pressure waves in the cochlear fluid?
It vibrates.
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3. How do hair cells convert mechanical vibration into a neural signal?
Bending against the tectorial membrane opens ion channels, depolarizing the hair cells and triggering neurotransmitter release.
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4. What cranial nerve carries the auditory signal to the brain, and what is this nerve called?
CN VIII, the vestibulocochlear nerve (specifically its cochlear branch for hearing).
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5. What is tonotopic organization, and where are high versus low frequencies detected along the basilar membrane?
The mapping of sound frequency to a specific location along the basilar membrane; high frequencies are detected at the base, low frequencies at the apex.
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