CorpusHuman Anatomy
Special SensesReceptor

Hair cells

Also known as: mechanoreceptor hair cells, stereocilia, kinocilium, tip links, mechanotransduction, type I hair cell, type II hair cell

The mechanoreceptor cells of the cochlea and vestibular organs: epithelial cells topped by a staircase bundle of stereocilia whose deflection opens mechanically gated cation channels.

Show on body map Ear — Coronal Section

Overview

Stereocilia per bundle≈30–300
Threshold deflection≈1 nm
Transduction channelTMC1/TMC2 complex

Hair cells are modified epithelial cells (not neurons) that synapse onto primary afferent neurons of CN VIII. Their apical hair bundle consists of 30–300 stereocilia — stiff, actin-filled microvilli (not true cilia) — arranged in rows of increasing height, tapering at their base to pivot. Vestibular hair cells also have a single true kinocilium (9+2 axoneme) beside the tallest row, which defines the cell's polarity; mature cochlear hair cells lose theirs.

Adjacent stereocilia are connected by fine extracellular tip links (cadherin-23 and protocadherin-15) running from the tip of each stereocilium to the side of its taller neighbour. Deflection towards the tallest stereocilia stretches the tip links and opens mechanoelectrical transduction channels (TMC1/TMC2 complex) at the tips; K⁺ (and Ca²⁺) flow in from endolymph, depolarising the cell. Deflection the other way slackens tip links and closes the ≈10% of channels open at rest, hyperpolarising the cell. Movement perpendicular to the axis has no effect. Channel opening is extraordinarily fast (microseconds) and sensitive: bundle deflections of less than ≈1 nm — atomic dimensions — are detectable at threshold.

Depolarisation opens basolateral Cav1.3 Ca²⁺ channels, triggering glutamate release at ribbon synapses, which sustain high, continuous release rates. Resting release produces spontaneous afferent firing, which is modulated up and down by stimulation. In the vestibular maculae and cristae, flask-shaped type I hair cells are enveloped by a calyx afferent ending, while cylindrical type II hair cells have bouton endings.

Location

Spiral organ of Corti (inner and outer hair cells), maculae of the utricle and saccule, and cristae ampullares of the semicircular ducts.

Parts & subdivisions

  1. Stereocilia
    Actin-core microvilli in a staircase; the transduction apparatus is at their tips.
  2. Kinocilium
    True cilium beside the tallest stereocilia (vestibular hair cells only in adults); deflection towards it excites.
  3. Tip links
    Cadherin-23/protocadherin-15 filaments gating the transduction channels; mutated in Usher syndrome type 1.
  4. Ribbon synapse
    Specialised synapse with a presynaptic dense body tethering vesicles for sustained glutamate release (otoferlin-dependent).
  5. Type I vs type II (vestibular)
    Type I: flask-shaped, calyx afferent ending. Type II: cylindrical, bouton endings and efferent synapses.

Function

  • Convert mechanical displacement into receptor potentials
  • Encode direction of deflection (bidirectional sensitivity)
  • Release glutamate onto afferent fibres of the vestibulocochlear nerve
Physiology

Driving force for K⁺ entry = endocochlear potential (+80 mV) − hair cell potential (≈−45 to −70 mV) ≈ 125–150 mV. K⁺ exits basolaterally into low-K⁺ perilymph (KCNQ4 channels) and is recycled via gap-junction networks to the stria vascularis.

Clinical significance

Usher syndrome

Autosomal recessive deafness with retinitis pigmentosa — the commonest cause of combined deaf-blindness — due to mutations in proteins shared by hair-cell stereocilia and photoreceptor connecting cilia (e.g. myosin VIIA, cadherin-23, protocadherin-15, harmonin).

Hair-cell loss is permanent

Unlike birds and fish, mammals cannot regenerate cochlear hair cells; research into gene therapy (e.g. otoferlin gene therapy for auditory neuropathy) and ATOH1-driven regeneration is ongoing.

Test yourself

Which statement about outer hair cells is correct?
  1. They change length via prestin and amplify basilar membrane motion
  2. They provide ≈95% of the afferent fibres of the cochlear nerve
  3. They form a single row on the modiolar side of the tunnel of Corti
  4. 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.

More special senses questions

Compiled from standard anatomy and physiology references — see the Special Senses sources. For education only; not medical advice.