Inner Ear and Cochlear Function

The cochlea is the spiral, fluid-filled organ of the inner ear that converts sound vibration into neural signals. As the stapes drives the oval window, a travelling wave moves along the basilar membrane and reaches its peak at a position determined by frequency, separating sound into its components. Hair cells of the organ of Corti transduce this motion into electrical signals, with outer hair cells actively amplifying the response and inner hair cells driving the auditory nerve.

Definition

The cochlea is the auditory part of the inner ear, a coiled fluid-filled duct in which the basilar membrane and organ of Corti perform frequency analysis and convert sound-induced vibration into neural signals via mechanotransducing hair cells.

Scope

This topic covers the anatomy of the cochlea and organ of Corti, the travelling wave and tonotopic place coding, hair-cell mechanotransduction, and the cochlear amplifier. It is a reference account of normal inner-ear hearing function and does not address the clinical management of inner-ear disorders.

Core questions

• How does the cochlear travelling wave separate sound by frequency along the basilar membrane?
• How do hair cells convert mechanical motion into electrical signals?
• What is the cochlear amplifier and what role do outer hair cells play?
• How do inner hair cells encode sound for the auditory nerve?

Key concepts

• Scala vestibuli, scala media, scala tympani
• Basilar membrane and travelling wave
• Tonotopy (place coding of frequency)
• Organ of Corti
• Inner and outer hair cells
• Hair-bundle mechanotransduction
• Endocochlear potential and endolymph
• Outer-hair-cell electromotility

Key theories

Place (tonotopic) theory of frequency coding

Each frequency produces a travelling wave that peaks at a specific place along the basilar membrane, so frequency is encoded by which cochlear location and which set of nerve fibres are most strongly stimulated.

Cochlear amplifier (active process)

Outer hair cells add mechanical energy to the travelling wave, boosting and sharpening the response to soft sounds and accounting for the cochlea's high sensitivity, sharp tuning, and nonlinear behaviour.

Mechanisms

Stapes movement at the oval window sets up a pressure difference across the cochlear partition, launching a travelling wave along the basilar membrane that grows then decays, peaking nearer the base for high frequencies and nearer the apex for low frequencies. This displacement deflects the stereocilia of hair cells in the organ of Corti, opening mechanotransduction channels and changing the cells' membrane potential; the high potassium concentration and positive endocochlear potential of the endolymph drive this transduction current. Outer hair cells change length in response to their receptor potential (electromotility), feeding energy back into the partition to amplify and sharpen the local motion. Inner hair cells, the principal sensory receptors, release transmitter onto auditory-nerve fibres, encoding the frequency, level, and timing of sound.

Clinical relevance

The cochlea is where sensorineural hearing depends; loss or dysfunction of hair cells, especially outer hair cells, reduces sensitivity and tuning, the basis of sensorineural hearing loss. This entry describes normal cochlear function as reference material and does not provide diagnostic or treatment advice.

History

Georg von Békésy's direct observations of the cochlear travelling wave established the place principle of frequency analysis and earned the 1961 Nobel Prize. Subsequent work revealed that the living cochlea is not passive: the discovery of otoacoustic emissions and of outer-hair-cell motility led to the concept of the cochlear amplifier, an active process that explains the ear's extraordinary sensitivity and frequency selectivity.

Key figures

• Georg von Békésy
• A. James Hudspeth
• Mario Ruggero
• Robert Fettiplace

Seminal works

• bekesy-1960
• robles-ruggero-2001
• fettiplace-hackney-2006

Frequently asked questions

How does the cochlea tell apart different frequencies?

Different frequencies peak at different places along the basilar membrane, so the location of maximal stimulation and the nerve fibres activated there encode the frequency of a sound.

What do outer hair cells do?

They act as a biological amplifier, changing length to add mechanical energy to the travelling wave, which boosts sensitivity to soft sounds and sharpens the cochlea's frequency tuning.