Editing Cochlea (section)

===Hearing===
{{Main|Hearing}}

The ''[[stapes]]'' (stirrup) ossicle bone of the middle ear transmits vibrations to the ''[[fenestra ovalis]]'' (oval window) on the outside of the cochlea, which vibrates the perilymph in the ''vestibular duct'' (upper chamber of the cochlea). The ossicles are essential for efficient coupling of sound waves into the cochlea, since the cochlea environment is a fluid–membrane system, and it takes more pressure to move sound through fluid–membrane waves than it does through air. A pressure increase is achieved by reducing the area ratio from the tympanic membrane (drum) to the oval window (''stapes'' bone) by 20. As pressure = force/area, results in a pressure gain of about 20 times from the original sound wave pressure in air. This gain is a form of [[impedance matching]] – to match the soundwave travelling through air to that travelling in the fluid–membrane system.

At the base of the cochlea, each 'duct' ends in a membranous portal that faces the middle ear cavity: The [[vestibular duct]] ends at the [[oval window]], where the footplate of the ''stapes'' sits. The footplate vibrates when the pressure is transmitted via the ossicular chain. The wave in the perilymph moves away from the footplate and towards the [[helicotrema]]. Since those fluid waves move the cochlear partition that separates the ducts up and down, the waves have a corresponding symmetric part in perilymph of the tympanic duct, which ends at the round window, bulging out when the oval window bulges in.

The perilymph in the vestibular duct and the [[endolymph]] in the cochlear duct act mechanically as a single duct, being kept apart only by the very thin [[Reissner's membrane]].
The vibrations of the endolymph in the cochlear duct displace the basilar membrane in a pattern that peaks a distance from the oval window depending upon the soundwave frequency. The [[organ of Corti]] vibrates due to [[outer hair cell]]s further amplifying these vibrations. [[Inner hair cell]]s are then displaced by the vibrations in the fluid, and depolarise by an influx of K+ via their [[tip-link]]-connected channels, and send their signals via neurotransmitter to the primary auditory neurons of the [[spiral ganglion]].<ref>{{cite journal |last1=Nin |first1=Fumiaki |last2=Hibino |first2=Hiroshi |last3=Doi |first3=Katsumi |last4=Suzuki |first4=Toshihiro |last5=Hisa |first5=Yasuo |last6=Kurachi |first6=Yoshihisa |title=The endocochlear potential depends on two K + diffusion potentials and an electrical barrier in the stria vascularis of the inner ear |journal=Proceedings of the National Academy of Sciences |date=5 February 2008 |volume=105 |issue=5 |pages=1751–1756 |doi=10.1073/pnas.0711463105|doi-access=free |pmid=18218777 |pmc=2234216 }}</ref>

The hair cells in the organ of Corti are tuned to certain sound frequencies by way of their location in the cochlea, due to the degree of stiffness in the basilar membrane.<ref>{{cite journal |author = Guenter Ehret |date = Dec 1978 |title = Stiffness gradient along the basilar membrane as a way for spatial frequency analysis within the cochlea |url = http://vts.uni-ulm.de/docs/2009/6797/vts_6797_9398.pdf |journal = J Acoust Soc Am | volume = 64 |issue = 6|pages = 1723–6 |pmid = 739099 | doi=10.1121/1.382153}}</ref> This stiffness is due to, among other things, the thickness and width of the basilar membrane,<ref>{{Cite book |last=Camhi |first=Jeffrey M. |url=https://archive.org/details/neuroethologyner0000camh |title=Neuroethology : nerve cells and the natural behavior of animals |date=1984 |publisher=Sunderland, Mass. : Sinauer Associates |isbn=978-0-87893-075-3}}</ref> which along the length of the cochlea is stiffest nearest its beginning at the oval window, where the stapes introduces the vibrations coming from the eardrum. Since its stiffness is high there, it allows only high-frequency vibrations to move the basilar membrane, and thus the hair cells.  The farther a wave travels towards the cochlea's apex (the ''helicotrema''), the less stiff the basilar membrane is; thus lower frequencies travel down the tube, and the less-stiff membrane is moved most easily by them where the reduced stiffness allows: that is, as the basilar membrane gets less and less stiff, waves slow down and it responds better to lower frequencies. In addition, in mammals, the cochlea is coiled, which has been shown to enhance low-frequency vibrations as they travel through the fluid-filled coil.<ref>{{cite journal |vauthors=Manoussaki D, Chadwick RS, Ketten DR, Arruda J, Dimitriadis EK, O'Malley JT |year = 2008 | title = The influence of cochlear shape on low-frequency hearing |journal = Proc Natl Acad Sci U S A |volume = 105 |issue = 16| pages = 6162–6166 |doi = 10.1073/pnas.0710037105 |pmid=18413615 |pmc=2299218 |bibcode = 2008PNAS..105.6162M |doi-access = free }}</ref>  This spatial arrangement of sound reception is referred to as [[tonotopy]].

For very low frequencies (below 20&nbsp;Hz), the waves propagate along the complete route of the cochlea – differentially up vestibular duct and tympanic duct all the way to the ''helicotrema''. Frequencies this low still activate the organ of Corti to some extent but are too low to elicit the perception of a [[pitch (psychophysics)|pitch]].  Higher frequencies do not propagate to the ''helicotrema'', due to the stiffness-mediated tonotopy.

A very strong movement of the basilar membrane due to very loud noise may cause hair cells to die. This is a common cause of partial hearing loss and is the reason why users of firearms or heavy machinery often wear [[earmuff]]s or [[earplug]]s.