Spiral organ (organ of Corti)

Organum spirale

Also known as: organ of Corti, inner hair cells, outer hair cells, tectorial membrane, pillar cells, Deiters cells, tunnel of Corti, prestin

The sensory epithelium of hearing on the basilar membrane, with one row of ≈3 500 inner hair cells (the true sensory cells) and three rows of ≈12 000 outer hair cells (the cochlear amplifier), covered by the tectorial membrane.

Show on body map Ear — Coronal Section

Overview

Inner hair cells≈3 500 (1 row)

Outer hair cells≈12 000 (3 rows)

Afferents to IHCs≈90–95% of cochlear nerve fibres

Cochlear amplifier gain≈40–50 dB

The organ of Corti runs the length of the cochlear duct on the basilar membrane. Its framework is formed by supporting cells: inner and outer pillar (rod) cells enclosing the triangular tunnel of Corti, phalangeal (Deiters) cells cradling the outer hair cells, and the border, Hensen and Claudius cells. The heads of hair cells and supporting cells are joined by tight junctions to form the reticular lamina, which separates endolymph (above, bathing the stereocilia) from cortilymph/perilymph (below, bathing the cell bodies). Above it lies the gelatinous tectorial membrane, attached medially to the spiral limbus.

Inner hair cells (IHCs) — ≈3 500 flask-shaped cells in a single row — are the true sensory receptors: each is contacted by ≈10–20 type I afferent fibres, and together they supply ≈90–95% of cochlear nerve fibres. Their stereocilia are not firmly embedded in the tectorial membrane and are moved by fluid flow. Outer hair cells (OHCs) — ≈12 000 cylindrical cells in three rows (sometimes four or five at the apex) — have their tallest stereocilia embedded in the tectorial membrane and receive mainly efferent (medial olivocochlear) innervation and only ≈5–10% of afferents (type II). Their lateral membrane contains the motor protein prestin: depolarisation shortens and hyperpolarisation lengthens the cell at acoustic frequencies (electromotility), pumping energy into the basilar membrane — the cochlear amplifier that provides ≈40–50 dB of gain and sharp tuning.

Transduction: upward deflection of the basilar membrane causes shear between the reticular and tectorial membranes, bending stereocilia towards the tallest row → tip links open mechanotransduction channels → K⁺ entry from endolymph → depolarisation → voltage-gated Ca²⁺ influx → glutamate release at ribbon synapses onto spiral ganglion afferents.

Location: On the basilar membrane within the cochlear duct, along its full length from base to apex.

Parts & subdivisions

Inner hair cells
One row, ≈3 500; flask-shaped; ≈90–95% of afferent innervation (type I); the primary sensory cells.
Outer hair cells
Three rows, ≈12 000; cylindrical; prestin-driven electromotility; cochlear amplifier; source of otoacoustic emissions; efferent control by the medial olivocochlear bundle.
Tectorial membrane
Acellular gel (collagen II, tectorins) attached to the spiral limbus; OHC stereocilia embedded in it.
Pillar cells and tunnel of Corti
Rigid supporting cells forming a triangular tunnel; separate inner from outer hair cells.
Deiters (phalangeal) cells
Support outer hair cells, with phalangeal processes forming part of the reticular lamina.
Reticular lamina
Tight-junctional apical surface sealing endolymph from perilymph.

Anatomy

Innervation: Afferent: type I spiral ganglion neurons (IHCs) and type II (OHCs) → cochlear nerve. Efferent: medial olivocochlear (to OHCs — reduce amplification, protect from noise, improve hearing in noise) and lateral olivocochlear (to type I dendrites beneath IHCs).
Histology: Hair cells with apical stereocilia arranged in a staircase (V/W-shaped in OHCs, linear in IHCs); mature cochlear hair cells lack a kinocilium. Supporting cells rich in microtubules.
Development: From the ventral otocyst; hair-cell fate requires the transcription factor ATOH1 (Math1). Mammalian cochlear hair cells are not regenerated after birth — the basis of permanent sensorineural loss.

Function

Transduce sound-induced vibration into neural signals (inner hair cells)
Amplify and sharpen basilar membrane vibration (outer hair cells)
Generate otoacoustic emissions

Clinical significance

Otoacoustic emissions and newborn screening

Healthy OHCs emit faint sounds recordable in the ear canal; their presence in a newborn excludes significant cochlear hearing loss. Automated auditory brainstem response (ABR) is used additionally to detect auditory neuropathy (normal OAEs but absent ABR — e.g. otoferlin mutation, hyperbilirubinaemia).

Selective hair-cell loss

OHCs are more vulnerable than IHCs to noise, ageing and ototoxins. Loss of OHCs causes reduced sensitivity, poor frequency selectivity and loudness recruitment (abnormally rapid growth of loudness).

Test yourself

Which statement about outer hair cells is correct?

They change length via prestin and amplify basilar membrane motion
They provide ≈95% of the afferent fibres of the cochlear nerve
They form a single row on the modiolar side of the tunnel of Corti
They lack stereocilia

Answer: They change length via prestin and amplify basilar membrane motion

The ≈12 000 outer hair cells (3 rows) are electromotile (prestin), forming the cochlear amplifier and generating otoacoustic emissions. The ≈3 500 inner hair cells (1 row) receive ≈90–95% of afferent fibres.