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Mechanisms of hearing

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Mechanism of hearing

Hearing converts sound waves into electrical signals that the brain recognizes as sound.

1. Collection and conduction of sound

  1. Pinna (auricle) collects sound waves and directs them into the external auditory canal.
  2. Waves strike the tympanic membrane, causing it to vibrate.
  3. Vibrations pass through the ossicles:
    malleus → incus → stapes.
  4. The stapes moves at the oval window.
The ossicles increase pressure at the oval window by impedance matching, allowing efficient transfer of sound from air to cochlear fluid. The tensor tympani and stapedius muscles can dampen transmission during loud sounds.
Guyton and Hall Textbook of Medical Physiology, p. 657.

2. Hydraulic wave in the cochlea

The stapes movement generates pressure waves in the perilymph of the scala vestibuli. The wave travels through the cochlea, displaces the basilar membrane, and is released through the round window into the scala tympani.
The cochlea has three fluid-filled chambers:
  • Scala vestibuli: perilymph
  • Scala media (cochlear duct): potassium-rich endolymph
  • Scala tympani: perilymph
The organ of Corti, which contains the sensory hair cells, sits on the basilar membrane in the scala media.
Guyton and Hall Textbook of Medical Physiology, p. 657.

3. Frequency analysis: place theory

Different sound frequencies produce maximal displacement at different locations on the basilar membrane:
  • High-frequency sounds: base of cochlea, near the oval window. This region is narrow and stiff.
  • Low-frequency sounds: apex of cochlea, near the helicotrema. This region is broader and more compliant.
This spatial coding is called tonotopy and remains organized throughout the auditory pathway.
Neuroanatomy through Clinical Cases, 3rd ed., p. 547.

4. Mechanoelectrical transduction by hair cells

Movement of the basilar membrane creates a shearing force between the tectorial membrane and hair-cell stereocilia.
  • Deflection of stereocilia toward the tallest stereocilia tightens tip links.
  • This opens mechanically gated cation channels.
  • Because endolymph is rich in K⁺ and has a positive endocochlear potential, K⁺ enters the hair cell, causing depolarization.
  • Depolarization opens voltage-gated Ca²⁺ channels.
  • Ca²⁺ entry triggers release of glutamate at ribbon synapses onto cochlear nerve afferents.
  • Action potentials are generated in neurons of the spiral ganglion.
Deflection in the opposite direction closes transduction channels, hyperpolarizes the hair cell, and decreases transmitter release.

5. Roles of inner and outer hair cells

  • Inner hair cells are the main sensory receptors. They transmit most auditory information to type I spiral ganglion neurons.
  • Outer hair cells act as a cochlear amplifier. Through electromotility, mainly mediated by prestin, they contract and elongate with changing membrane voltage. This increases basilar-membrane motion, improving sensitivity and frequency discrimination.
Outer hair-cell function can be assessed through otoacoustic emissions. NCBI's cochlear physiology overview describes the hair-cell transduction process and cochlear frequency mapping.

6. Central auditory pathway

The neural signal travels through:
Inner hair cell → spiral ganglion → cochlear division of CN VIII → cochlear nuclei → superior olivary complex → lateral lemniscus → inferior colliculus → medial geniculate body of thalamus → primary auditory cortex
The primary auditory cortex is located in Heschl gyri of the superior temporal lobe.
Important features:
  • Fibers project bilaterally after the cochlear nuclei.
  • The superior olivary complex compares timing and intensity differences between ears, helping localize sound.
  • Because of bilateral representation, a unilateral central auditory lesion above the cochlear nuclei usually does not produce complete deafness in one ear.

Flow summary

Sound wave → tympanic membrane → ossicles → oval window → cochlear fluid wave → basilar membrane motion → stereocilia deflection → K⁺ influx into hair cells → Ca²⁺ influx and glutamate release → cochlear nerve → auditory cortex.
A recent systematic-review search did not identify evidence that changes this established physiological mechanism.

Mechanism of hearing

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NCBI Bookshelf cochlear function hair cell mechanoelectrical transduction auditory pathway

Mechanism of Hearing

1. Sound conduction through the outer and middle ear

  • The pinna collects sound waves and directs them through the external auditory canal.
  • Sound waves vibrate the tympanic membrane.
  • Vibrations pass via the auditory ossicles:
    malleus → incus → stapes.
  • The stapes footplate moves in and out at the oval window.
The ossicles concentrate the sound energy from the large tympanic membrane onto the much smaller oval window. This impedance-matching action enables efficient sound transmission from air into cochlear fluid.

2. Formation of a travelling wave in the cochlea

Movement of the stapes at the oval window produces pressure waves in cochlear fluid:
Oval window → scala vestibuli → cochlear structures → scala tympani → round window
The cochlea contains:
  • Scala vestibuli: perilymph
  • Scala media/cochlear duct: potassium-rich endolymph
  • Scala tympani: perilymph
The wave causes vibration of the basilar membrane, on which the organ of Corti rests.
Guyton and Hall Textbook of Medical Physiology, p. 657.

3. Frequency discrimination

Each frequency vibrates a particular location on the basilar membrane:
  • High-frequency sounds produce maximum vibration near the base of the cochlea, close to the oval window.
  • Low-frequency sounds produce maximum vibration near the apex, close to the helicotrema.
This location-dependent frequency coding is called tonotopic organization.

4. Transduction by hair cells

The organ of Corti contains inner and outer hair cells. Vibration of the basilar membrane bends the hair-cell stereocilia against the tectorial membrane.
When stereocilia bend toward the tallest stereocilia:
  1. Tip links pull open mechanically gated cation channels.
  2. K⁺ enters the hair cell from potassium-rich endolymph.
  3. The hair cell depolarizes.
  4. Voltage-gated Ca²⁺ channels open.
  5. Ca²⁺ triggers release of glutamate at the hair cell's ribbon synapse.
  6. Nearby cochlear nerve fibers are activated.
Bending in the opposite direction closes these channels, hyperpolarizes the cell, and reduces neurotransmitter release. This direct mechanical channel opening makes auditory transduction extremely rapid, as described in NCBI's review of hair-cell transduction.

5. Functions of the two hair-cell types

  • Inner hair cells: the true sensory receptors. They convert cochlear vibration into neural information and transmit most of it to the auditory nerve.
  • Outer hair cells: act as a cochlear amplifier. Their electromotility magnifies basilar-membrane movement, improving hearing sensitivity and sharp frequency discrimination.

6. Auditory neural pathway

The signal reaches the brain through:
Inner hair cell → spiral ganglion → cochlear division of CN VIII → cochlear nuclei → superior olivary complex → lateral lemniscus → inferior colliculus → medial geniculate body of thalamus → primary auditory cortex in Heschl gyri
The superior olivary complex compares differences in sound timing and intensity between both ears, enabling sound localization.

One-line flowchart

Sound waves → tympanic membrane → ossicles → oval window → cochlear travelling wave → basilar membrane vibration → stereocilia deflection → K⁺ influx and hair-cell depolarization → glutamate release → auditory nerve → auditory cortex.
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