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Inner Hair Cells

  1. Detect the sound (sound transduction)
  2. In a single row along the length of the cochlea
  3. About 3000 in number
  4. Innervated by type I nerves (about 27000 of them)
  5. 90 to 95% of the afferent neurons arise from the inner hair cells; only 5-10% arise from the outer hair cells (In contrast, most efferent neurons [the olivocochlear bundle] end on the outer hair cells). The cell bodies of the afferent neurons are in the spiral ganglion.

Mechanotransduction – is by the bending of the stereocilia which has ‘ tiplinks’ leads to opening of K+ Channels leading to depolarization of Hair cells and release of NT.

(Note that the scala media is electropositive with respect to the scala vestibuli and scala tympani)

Auditory pathway

Auditory (cochlear) division of vestibulo cochlear nerve

Cochlear nuclei in the medulla

Inferior colliculi (the centers for auditory reflexes)

Medial geniculate body

Auditory cortex (Area 41) (In superior portion of the temporal lobe in the sylvian fissure).

The efferent olivocochlear bundle arises from the superior olivary complex and ends primarily in the outer hair cells of the organ of cortex.

Loudness/ pitch/ timbre

Loudness is related to the amplitude

Pitch is related to frequency

Timbre (quality) is related to overtones (the number of harmonic vibrations)

Loudness is measured in decibels (dB)



Note that the average auditory threshold for humans is zero decibel.


The frequency range for hearing is from 20-20000 Hz. The greatest sensitivity lies between 1000- 4000 Hz. The

pitch discrimination is between 1000-3000Hz

Tympanic reflex (attenuation reflex) – Loud sounds cause reflex contraction of the tensor tympani and stapedius

muscles, decreasing sound transmission.

Theories of Hearing

Place theory – Frequency discrimination is dependent on the exact place on the basilar membrane. Which is most stimulated. (It has been known from Von Be’ Ke’s ys travelling wave theory that high- pitched sounds reach maximum height near the base of the cochlea and low- pitched sounds reach maximum height near the apex of the cochlea) Place theory helps to explain high frequency discrimination.

For explaining pitch discrimination in the low- frequency range (say 20 to 2000), there is the volley or the frequency principle. That is, low frequency sounds can cause volleys of impulses synchronised at the same frequencies and these volleys are transmitted by the cochlear nerve into the cochlear nuclei of the brain.

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