Cochlea

Cochlea

Also known as: scala vestibuli, scala tympani, modiolus, helicotrema, spiral ganglion, tonotopy, travelling wave

The snail-shaped part of the inner ear (2½–2¾ turns) in which sound-induced fluid waves displace the basilar membrane tonotopically and stimulate the hair cells of the spiral organ.

Show on body map Ear — Coronal Section

Overview

Turns2½–2¾

Length of cochlear duct≈32–35 mm

Spiral ganglion neurons≈30 000–35 000

Frequency range≈20 Hz (apex) – 20 kHz (base)

The bony cochlea is a tapering canal (≈32–35 mm long) wound 2½–2¾ times around a conical bony core, the modiolus, which contains the spiral ganglion (cell bodies of ≈30 000 bipolar cochlear neurons) and the cochlear nerve fibres. A thin shelf of bone, the osseous spiral lamina, projects from the modiolus. The cochlea is ≈9 mm across at its base and ≈5 mm high; its base forms the promontory of the middle ear and its apex (cupula) points anterolaterally.

In cross-section the canal is divided into three scalae: the scala vestibuli (upper; perilymph; begins at the oval window), the scala media (cochlear duct) (middle; endolymph; contains the spiral organ) and the scala tympani (lower; perilymph; ends at the round window). The scala vestibuli is separated from the cochlear duct by the thin vestibular (Reissner) membrane, and the cochlear duct from the scala tympani by the basilar membrane. The scala vestibuli and tympani communicate at the apex through the helicotrema.

Mechanics: stapes vibration produces a pressure difference across the cochlear partition and a travelling wave that moves from base to apex, growing until it peaks at a place determined by frequency, then dies away. Because the basilar membrane is narrow and stiff at the base and wide and floppy at the apex, high frequencies peak near the base and low frequencies near the apex — a tonotopic map (place coding) preserved through the whole auditory pathway. Low frequencies (<≈4 kHz) are also coded by phase-locking of nerve firing (temporal coding). Loudness is coded by firing rate and the number of fibres recruited.

Location: Anteromedial part of the bony labyrinth in the petrous temporal bone; base towards the internal acoustic meatus, apex directed anterolaterally.

Parts & subdivisions

Modiolus
Central bony axis housing the spiral ganglion and cochlear nerve.
Osseous spiral lamina
Bony shelf from the modiolus carrying nerve fibres to the organ of Corti.
Scala vestibuli
Perilymph; from the vestibule/oval window to the helicotrema.
Cochlear duct (scala media)
Endolymph; contains the spiral organ.
Scala tympani
Perilymph; from the helicotrema to the round window; site of cochlear implant electrodes.
Basilar membrane
Tonotopically tuned partition supporting the organ of Corti.
Helicotrema
Apical communication between scala vestibuli and scala tympani.
Spiral ganglion
≈30 000–35 000 bipolar neurons; ≈90–95% (type I, myelinated) contact inner hair cells; ≈5–10% (type II, unmyelinated) contact outer hair cells.

Anatomy

Innervation: Cochlear division of VIII (afferent, spiral ganglion); olivocochlear efferents from the superior olivary complex (medial — to outer hair cells; lateral — to type I afferent dendrites).
Arterial supply: Common cochlear artery from the labyrinthine artery (AICA), forming spiral modiolar arteries.

Function

Frequency analysis of sound (tonotopy) — acting as a mechanical spectrum analyser
Transduce sound into neural signals via the spiral organ
Amplify faint sounds (outer hair-cell cochlear amplifier)

Physiology: Frequency map (Greenwood): ≈20 kHz at the base → ≈20 Hz at the apex; ≈3.5 mm of basilar membrane per octave in the mid-range. Frequency discrimination ≈0.2–0.3% (≈2–3 Hz at 1 kHz). Otoacoustic emissions (sounds generated by outer hair cells) can be recorded in the canal — the basis of newborn hearing screening.

Clinical significance

Noise-induced hearing loss

Prolonged exposure above ≈85 dB(A) or impulse noise damages outer hair cells, producing a characteristic audiometric notch at ≈4 kHz (3–6 kHz). Tinnitus is common. Prevented by hearing protection. Even "temporary" threshold shifts can cause permanent synaptic loss ("hidden hearing loss").

Presbycusis

Age-related, bilateral, symmetrical high-frequency sensorineural hearing loss, beginning at the cochlear base (hair-cell loss, strial atrophy, neural loss). Patients struggle to follow speech in background noise (consonants are high-frequency). Managed with hearing aids; untreated hearing loss is a major modifiable risk factor for dementia.

Congenital infection

Congenital CMV is the commonest non-genetic cause of congenital sensorineural hearing loss; congenital rubella causes the triad of deafness, cataract and patent ductus arteriosus.

Test yourself

What is the main mechanism by which the middle ear increases sound pressure delivered to the cochlea?

The ≈17:1 area ratio of tympanic membrane to stapes footplate
Resonance of the external acoustic meatus
Contraction of the stapedius muscle
Air in the mastoid air cells amplifying vibration

Answer: The ≈17:1 area ratio of tympanic membrane to stapes footplate

Concentrating force from the ≈55 mm² effective drum area onto the ≈3.2 mm² footplate gives ≈17×; the ossicular lever adds ≈1.3×, for a total gain of ≈22× (≈25–30 dB). Stapedius contraction reduces transmission.

Where along the cochlea are high-frequency sounds detected, and why?

At the apex, where the basilar membrane is narrow and stiff
At the base, where the basilar membrane is narrow and stiff
At the apex, where the basilar membrane is wide and floppy
Uniformly along the cochlea; frequency is coded only by firing rate

Answer: At the base, where the basilar membrane is narrow and stiff

The basilar membrane is ≈0.1 mm wide and stiff at the base (≈20 kHz) and ≈0.5 mm wide and compliant at the apex (≈20 Hz). This place (tonotopic) code is preserved throughout the auditory pathway; basal hair cells are lost first in presbycusis and noise damage.

A unilateral lesion at which level would cause complete deafness in one ear?

Lateral lemniscus
Inferior colliculus
Primary auditory cortex
Cochlear nuclei

Answer: Cochlear nuclei

All cochlear nerve fibres synapse ipsilaterally in the cochlear nuclei; above this level the pathway is bilateral from the superior olivary complex onwards, so central lesions do not cause unilateral deafness.