Expansion
Organ of Corti for hearing, maculae and cristae for balance
Expansion
Cochlea: organ of Corti
- Sits on the basilar membrane within the scala media
- Inner hair cells (about 3500, single row): the true sensory cells, carrying about 95 per cent of afferent fibres
- Outer hair cells (about 12000, three rows): amplify and sharpen tuning by electromotility. Their activity generates otoacoustic emissions, exploited in newborn hearing screening
- Stereocilia are deflected against the tectorial membrane
Vestibular apparatus
- Maculae of utricle and saccule: hair cells with an otolithic membrane bearing calcium carbonate otoconia, sensing linear acceleration and gravity
- Cristae ampullares in the semicircular canals: hair cells with a gelatinous cupula, sensing angular acceleration
Hair cell transduction is common to both: deflection of stereocilia towards the tallest kinocilium opens mechanically gated channels, depolarising the cell.
Displaced otoconia entering a semicircular canal cause benign paroxysmal positional vertigo, which is why repositioning manoeuvres work.
Hair cells do not regenerate in mammals, which is why noise-induced and age-related hearing loss is permanent.