Wound around a bony core called the modiolus, with a hole at the very tip where two of its three channels join
The cochlea — the helicotrema joins scala vestibuli and scala tympani at the apex
Anatomy · Eye, ear & senses
Cochlea
The coiled organ of hearing
Inner ear · bony labyrinth
A snail-shaped, fluid-filled spiral tunnel in the petrous part of the temporal bone, in front of the vestibule. Winding around a central bony pillar, the modiolus, it houses the organ of Corti, whose hair cells turn vibration into nerve impulses. Its nerve fibres form the cochlear division of the vestibulocochlear nerve, and an electrical prosthesis can bypass it when its hair cells are lost.
How to spot them
- ▸A coiled bony tube, about two and a half turns, resembling a snail shell, from the Greek kochlias
- ▸Coils around a central bony pillar, the modiolus, that carries the nerve fibres of the spiral ganglion
- ▸Divided into three fluid-filled channels: scala vestibuli, scala media and scala tympani
- ▸Its two membranous windows (oval and round) face the middle-ear cavity
What to know
About 3,500 inner and about 12,000 outer sensory cells, arranged in one row and three rows along its length
Cochlear hair cells: one row of inner, three rows of outer
A strip on the outer wall pumps potassium into its middle channel to create a resting electrical potential of about +80 millivolts
The stria vascularis and the endocochlear potential of the cochlea
In 1978 a Melbourne man, Rod Saunders, became the first recipient of a multiple-electrode version of a 'bionic ear' implanted in it (1978)
Graeme Clark's multichannel cochlear implant
David Kemp discovered in 1978 that it emits faint sounds of its own, now used to screen the hearing of newborns (1978)
Otoacoustic emissions from the cochlea
Its nerve cells lie in a spiral ganglion inside the bony core, and their fibres join the eighth cranial nerve
Spiral ganglion cells of the cochlea, forming the cochlear nerve
Unrolled it would measure about 30–35 mm, but the whole coil is about the size of a pea
The human cochlea
Key ideas
- Hair-cell bundles bend when the basilar membrane moves, opening ion channels and depolarising the cell. The outer hair cells also change length, amplifying the vibration.
- The three channels are separate: the two outer channels hold perilymph, the middle one holds potassium-rich endolymph. A cochlear implant bypasses damaged hair cells by stimulating the nerve directly.
Quick-fire facts
- •Coils about 2.5 turns in the human ear; the base turn produces the bulge of the promontory on the wall of the middle ear
- •Scala vestibuli and scala tympani contain perilymph; scala media (the cochlear duct) contains endolymph
- •The basilar membrane is narrow and stiff at the base and wide and floppy at the apex, so high notes peak at the base and low notes at the apex
- •Reissner's membrane separates scala vestibuli from scala media; the basilar membrane separates scala media from scala tympani
- •Hair cells in humans do not regenerate, so noise and ageing damage is permanent
- •Loud noise damages outer hair cells first, typically causing a dip in hearing at around 4 kHz
- •Graeme Clark's team at the University of Melbourne implanted the first multichannel device in 1978; the Bionic Ear went into commercial production in the 1980s
- •The cochlea is already close to adult size at birth
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Sources
- TeachMeAnatomy — The Inner Ear — Bony Labyrinth, Membranous Labyrinth
- National Institute on Deafness and Other Communication Disorders (NIH) — How Do We Hear?