Editing Auditory system (section)

== Structure ==
[[File:Anatomy of the Human Ear en.svg|thumb|right|Anatomy of the human ear (The length of the auditory canal is exaggerated in this image.).{{Anatomy of the human ear - color legend}}]]

=== Outer ear ===
{{main|Outer ear}}
The folds of cartilage surrounding the ear canal are called the [[pinna (anatomy)|auricle]]. Sound waves are reflected and attenuated when they hit the auricle, and these changes provide additional information that will help the brain determine the sound direction.

The sound waves enter the [[auditory canal]], a deceptively simple tube.  The ear canal amplifies sounds that are between 3 and 12 [[kHz]].<ref name="pmid14714940" /> The [[eardrum|tympanic membrane]], at the far end of the ear canal marks the beginning of the [[middle ear]].

=== Middle ear ===
{{main|Middle ear}}

[[File:Slide1ghe.JPG|thumb|right|200px|Auditory ossicles from a deep dissection of the tympanic cavity]]

Sound waves travel through the ear canal and hit the tympanic membrane, or [[eardrum]]. This wave information travels across the air-filled middle ear cavity via a series of delicate bones: the [[malleus]] (hammer), [[incus]] (anvil) and [[stapes]] (stirrup). These [[ossicles]] act as a lever, converting the lower-pressure eardrum sound vibrations into higher-pressure sound vibrations at another, smaller membrane called the [[oval window]] or vestibular window. The [[Ear#Middle ear|manubrium]] (handle) of the malleus articulates with the tympanic membrane, while the footplate (base) of the stapes articulates with the oval window. Higher pressure is necessary at the oval window than at the tympanic membrane because the inner ear beyond the oval window contains liquid rather than air. The [[stapedius reflex]] of the middle ear muscles helps protect the inner ear from damage by reducing the transmission of sound energy when the [[stapedius muscle]] is activated in response to sound.  The middle ear still contains the sound information in wave form; it is converted to nerve impulses in the [[cochlea]].

=== Inner ear ===
{{Infobox anatomy
| Name    = Cochlea
| Image   = Gray928.png
| Caption = Diagrammatic longitudinal section of the cochlea. The [[cochlear duct]], or ''scala media'', is labeled as ''ductus cochlearis'' at right.
| Width   = 200px
}}
{{main|Inner ear}}
The inner ear consists of the [[cochlea]] and several non-auditory structures.  The cochlea has three fluid-filled sections (i.e. the ''scala media, scala tympani and scala vestibuli)'', and supports a fluid wave driven by pressure across the [[basilar membrane]] separating two of the sections. Strikingly, one section, called the cochlear duct or ''[[scala media]],'' contains [[endolymph]]. The organ of Corti is located in this duct on the basilar membrane, and transforms mechanical waves to electric signals in neurons. The other two sections are known as the ''[[scala tympani]]'' and the ''[[scala vestibuli]].'' These are located within the bony labyrinth, which is filled with fluid called [[perilymph]], similar in composition to cerebrospinal fluid. The chemical difference between the fluids [[endolymph]] and [[perilymph]] fluids is important for the function of the inner ear due to electrical potential differences between potassium and calcium ions.{{citation needed|date=June 2022}}

The plan view of the human cochlea (typical of all [[mammal]]ian and most [[vertebrate]]s) shows where specific frequencies occur along its length. The frequency is an approximately exponential function of the length of the cochlea within the [[Organ of Corti]]. In some species, such as bats and dolphins, the relationship is expanded in specific areas to support their active sonar capability.

==== Organ of Corti ====
{{Main|Organ of Corti}}
[[File:Cochlea-crosssection.svg|thumb|right|200px|The [[organ of Corti]] located at the ''[[scala media]]'']]

The organ of Corti forms a ribbon of sensory epithelium which runs lengthwise down the cochlea's entire ''scala media''. Its hair cells transform the fluid waves into nerve signals. The journey of countless nerves begins with this first step; from here, further processing leads to a panoply of auditory reactions and sensations.

==== Hair cell ====
{{main|Hair cell}}
Hair cells are columnar cells, each with a "hair bundle" of 100–200 specialized [[stereocilia]] at the top, for which they are named. There are two types of hair cells specific to the auditory system; ''inner'' and ''outer'' ''hair'' ''cells''. Inner hair cells are the mechanoreceptors for hearing: they transduce the vibration of sound into electrical activity in [[nerve fiber]]s, which is transmitted to the brain. Outer hair cells are a motor structure. Sound energy causes changes in the shape of these cells, which serves to amplify sound vibrations in a frequency specific manner. Lightly resting atop the longest [[cilia]] of the inner hair cells is the [[tectorial membrane (cochlea)|tectorial membrane]], which moves back and forth with each cycle of sound, tilting the cilia, which is what elicits the hair cells' electrical responses.

Inner hair cells, like the [[photoreceptor cell]]s of the eye, show a  [[postsynaptic potential|graded response]], instead of the [[action potential|spikes]] typical of other neurons.  These graded potentials are not bound by the "all or none" properties of an action potential.

At this point, one may ask how such a wiggle of a hair bundle triggers a difference in membrane potential. The current model is that cilia are attached to one another by "[[Stereocilia (inner ear)#Design and constellation|tip links]]", structures which link the tips of one cilium to another. Stretching and compressing, the tip links may open an ion channel and produce the receptor potential in the hair cell. Recently it has been shown that [[CDH23|cadherin-23 CDH23]] and [[PCDH15|protocadherin-15 PCDH15]] are the adhesion molecules associated with these tip links.<ref name="Lelli Kazmierczak 2010">{{Cite journal |vauthors=Lelli A, Kazmierczak P, Kawashima Y, Müller U, Holt JR |date=August 2010 |title=Development and regeneration of sensory transduction in auditory hair cells requires functional interaction between cadherin-23 and protocadherin-15 |journal=The Journal of Neuroscience |volume=30 |issue=34 |pages=11259–69 |doi=10.1523/JNEUROSCI.1949-10.2010 |pmc=2949085 |pmid=20739546}}</ref> It is thought that a [[calcium]] driven motor causes a  shortening of these links to regenerate tensions. This regeneration of tension allows for apprehension of prolonged auditory stimulation.<ref name="Peng 2011">{{Cite journal |vauthors=Peng AW, Salles FT, Pan B, Ricci AJ |date=November 2011 |title=Integrating the biophysical and molecular mechanisms of auditory hair cell mechanotransduction |journal=Nature Communications |volume=2 |pages=523 |bibcode=2011NatCo...2..523P |doi=10.1038/ncomms1533 |pmc=3418221 |pmid=22045002}}</ref>

==== Neurons ====
{{Main|Hair cell#Neural connection|l1=Hair cell neural connection}}

Afferent neurons innervate cochlear inner hair cells, at synapses where the neurotransmitter [[glutamate]] communicates signals from the hair cells to the dendrites of the primary auditory neurons.

There are far fewer inner hair cells in the cochlea than afferent nerve fibers – many auditory nerve fibers innervate each hair cell. The neural dendrites belong to neurons of the [[auditory nerve]], which in turn joins the [[vestibular nerve]] to form the [[vestibulocochlear nerve]], or [[cranial nerve]] number VIII.<ref>{{Cite web |url=http://www.meddean.luc.edu/lumen/meded/GrossAnatomy/h_n/cn/cn1/cn8.htm |title=Meddean – CN VIII. Vestibulocochlear Nerve |access-date=2007-10-27 |archive-date=2012-10-06 |archive-url=https://web.archive.org/web/20121006084020/http://www.meddean.luc.edu/lumen/MedEd/grossanatomy/h_n/cn/cn1/cn8.htm |url-status=live }}</ref>
The region of the basilar membrane supplying the inputs to a particular afferent nerve fibre can be considered to be its [[receptive field]].

Efferent projections from the brain to the cochlea also play a role in the perception of sound, although this is not well understood. Efferent synapses occur on outer hair cells and on afferent (towards the brain) dendrites under inner hair cells