Inner Ear and Cochlear Anatomy

The cochlea is the spiral, fluid-filled organ of the inner ear that performs the auditory system's frequency analysis and converts sound into neural signals. Its three fluid compartments and the organ of Corti are arranged so that a travelling wave along the basilar membrane peaks at different places for different frequencies, giving the cochlea a tonotopic map from high frequencies at its base to low frequencies at its apex.

Definition

The cochlea is the coiled inner-ear structure containing the scala vestibuli, scala media, and scala tympani, the basilar membrane, and the organ of Corti, where sound-induced fluid motion is analysed by frequency and transduced into neural activity.

Scope

This topic covers the structural plan of the cochlea, the three scalae and their fluids, the basilar membrane and organ of Corti, the tonotopic travelling wave, and the active amplification that sharpens cochlear tuning. It is the sensory bridge between middle-ear input and the auditory nerve; the molecular transduction step is developed in the hair-cell topic. The entry is reference-educational and not a guide to diagnosing or treating inner-ear disorders.

Core questions

• How are the cochlear fluid compartments and the organ of Corti arranged?
• How does the travelling wave produce a tonotopic (place) map of frequency?
• What is the cochlear amplifier and why is it needed for normal sensitivity and tuning?
• How do inner and outer hair cells differ in role within the organ of Corti?

Key concepts

• Scala vestibuli, scala media, scala tympani
• Endolymph and perilymph
• Basilar membrane
• Organ of Corti
• Inner and outer hair cells
• Travelling wave and tonotopy (place coding)
• Cochlear amplifier
• Prestin and outer-hair-cell electromotility
• Endocochlear potential

Mechanisms

Stapes motion at the oval window drives perilymph in the scala vestibuli, launching a travelling wave along the basilar membrane that grows to a peak at a frequency-dependent place and then decays, the basis of cochlear tonotopy first demonstrated by von Bekesy (1960; Robles & Ruggero, 2001). The organ of Corti rides on this membrane: inner hair cells are the principal sensory cells reporting basilar-membrane motion to the auditory nerve, while outer hair cells provide active amplification. That amplification depends on the membrane motor protein prestin, which makes outer hair cells change length in response to voltage, feeding energy back into the travelling wave to boost sensitivity and sharpen frequency tuning (Zheng et al., 2000; Fettiplace & Fuchs, 1999). A standing electrical gradient, the endocochlear potential maintained across the scala media, supplies the driving force for transduction.

Clinical relevance

Because the cochlea maps frequency onto place and depends on active amplification by outer hair cells, loss of those cells degrades sensitivity and tuning, and the tonotopic plan underlies the design of cochlear implants. This entry describes normal cochlear anatomy and physiology for reference and is not a basis for individual diagnosis or treatment.

History

Von Bekesy's experiments establishing the travelling wave and the place principle of frequency analysis earned a Nobel Prize and remain the foundation of cochlear mechanics (von Bekesy, 1960). The later recognition that the cochlea is actively amplified, and the identification of prestin as the motor of outer-hair-cell electromotility, transformed the cochlea from a passive analyser into an active, nonlinear organ (Zheng et al., 2000; Robles & Ruggero, 2001).

Key figures

• Georg von Bekesy
• Luis Robles
• Mario Ruggero
• Peter Dallos
• Jing Zheng

Seminal works

• bekesy-1960
• robles-ruggero-2001
• zheng-2000

Frequently asked questions

How does the cochlea tell apart different frequencies?

A travelling wave along the basilar membrane peaks at a location that depends on frequency, high frequencies near the base and low frequencies near the apex, so frequency is encoded as place along the cochlea.

What is the cochlear amplifier?

An active process in which outer hair cells, using the motor protein prestin, feed mechanical energy back into the travelling wave to increase sensitivity and sharpen frequency tuning.