View source for Inner ear

{{Short description|Innermost part of the vertebrate ear}}
{{Infobox anatomy
| Name        = '''Inner ear'''
| Latin       = auris interna
| Image       = Blausen 0329 EarAnatomy InternalEar.png
| Caption     =
| Width       = 330
| Image2      =
| Caption2    =
| Precursor   =
| System      =
| Artery      = [[Labyrinthine artery]]
| Vein        =
| Nerve       =
| Lymph       =
}}
{{Ear series|expanded=Inner}}
[[Image:Gray920.png|thumb|right|Inner ear]]

The '''inner ear''' ('''internal ear''', '''auris interna''') is the innermost part of the vertebrate [[ear]]. In [[vertebrate]]s, the inner ear is mainly responsible for sound detection and balance.<ref>Torres, M., Giráldez, F. (1998) The development of the vertebrate inner ear. ''[[Mechanisms of Development]]'' 71 (1–2) pp. 5–21</ref> In [[mammal]]s, it consists of the [[bony labyrinth]], a hollow cavity in the [[temporal bone]] of the skull with a system of passages comprising two main functional parts:<ref>J.M. Wolfe et al. (2009). ''Sensation & Perception''. 2nd ed. Sunderland: [[Sinauer Associates|Sinauer Associated]] Inc</ref>
* The [[cochlea]], dedicated to hearing; converting sound pressure patterns from the outer ear into electrochemical impulses which are passed on to the brain via the [[auditory nerve]].
* The [[vestibular system]], dedicated to [[balance (ability)|balance]].

The inner ear is found in all vertebrates, with substantial variations in form and function. The inner ear is innervated by the [[eighth cranial nerve]] in all vertebrates.

==Structure==
[[File:Gray923.png|thumb|The [[cochlea]] and [[Vestibule of the ear|vestibule]], viewed from above.]]
The labyrinth can be divided by layer or by region.

===Bony and membranous labyrinths===
The [[bony labyrinth]], or osseous labyrinth, is the network of passages with bony walls lined with [[periosteum]]. The three major parts of the bony labyrinth are the [[vestibule of the ear]], the [[semicircular canals]], and the [[cochlea]]. The [[membranous labyrinth]] runs inside of the bony labyrinth, and creates three parallel fluid filled spaces. The two outer are filled with [[perilymph]] and the inner with endolymph.<ref>{{Cite journal |last1=Rask-Andersen |first1=Helge |last2=Liu |first2=Wei |last3=Erixon |first3=Elsa |last4=Kinnefors |first4=Anders |last5=Pfaller |first5=Kristian |last6=Schrott-Fischer |first6=Annelies |last7=Glueckert |first7=Rudolf |date=November 2012 |title=Human Cochlea: Anatomical Characteristics and their Relevance for Cochlear Implantation |journal=[[The Anatomical Record: Advances in Integrative Anatomy and Evolutionary Biology]] |volume=295 |issue=11 |pages=1791–1811 |doi=10.1002/ar.22599 |pmid=23044521 |s2cid=25472441}}</ref>

===Vestibular and cochlear systems===
In the [[middle ear]], the energy of [[sound pressure|pressure waves]] is translated into mechanical vibrations by the three auditory ossicles. Pressure waves move the tympanic membrane which in turns moves the malleus, the first bone of the middle ear. The malleus articulates to incus which connects to the stapes. The footplate of the stapes connects to the oval window, the beginning of the inner ear. When the stapes presses on the oval window, it causes the perilymph, the liquid of the inner ear, to move. The middle ear thus serves to convert the energy from sound pressure waves to a force upon the perilymph of the inner ear. The oval window has only approximately 1/18 the area of the tympanic membrane and thus produces a higher [[pressure]]. The cochlea propagates these mechanical signals as waves in the fluid and membranes and then converts them to nerve impulses which are transmitted to the brain.<ref>{{cite book
 | title = Auditory Neuroscience
 | author = Jan Schnupp, Israel Nelken and Andrew King
 | publisher = MIT Press
 | year = 2011
 | isbn = 978-0262113182
 | url = https://mustelid.physiol.ox.ac.uk/drupal/?q=ear
 | access-date = 2011-04-13
 | archive-url = https://web.archive.org/web/20120307161941/https://mustelid.physiol.ox.ac.uk/drupal/?q=ear
 | archive-date = 2012-03-07
 | url-status = dead
 }}</ref>

The vestibular system is the region of the inner ear where the semicircular canals converge, close to the cochlea. The vestibular system works with the visual system to keep objects in view when the head is moved. Joint and muscle receptors are also important in maintaining balance. The brain receives, interprets, and processes the information from all these systems to create the sensation of balance.

The vestibular system of the inner ear is responsible for the sensations of balance and motion.  It uses the same kinds of fluids and detection cells ([[hair cells]]) as the cochlea uses, and sends information to the brain about the attitude, rotation, and linear motion of the head.  The type of motion or attitude detected by a hair cell depends on its associated mechanical structures, such as the curved tube of a semicircular canal or the calcium carbonate crystals ([[otolith]]) of the [[saccule]] and [[utricle (ear)|utricle]].

===Development===
The human inner ear develops during week 4 of [[embryonic development]] from the [[auditory placode]], a thickening of the [[ectoderm]] which gives rise to the [[bipolar neuron]]s of the [[Scarpa's ganglion|cochlear]] and [[Spiral ganglion|vestibular ganglions]].<ref>{{cite book|last=Hyman|first=Libbie Henrietta |title=Hyman's comparative vertebrate anatomy |url=https://books.google.com/books?id=VKlWjdOkiMwC|access-date=2011-05-14|edition=3|year=1992|publisher=[[University of Chicago]] Press|isbn=0226870138|page=634}}</ref> As the auditory placode invaginates towards the embryonic [[mesoderm]], it forms the auditory vesicle or ''otocyst''.

The [[auditory vesicle]] will give rise to the utricular and saccular components of the [[membranous labyrinth]]. They contain the sensory hair cells and [[otolith]]s of the [[macula of utricle]] and [[macula of saccule|of the saccule]], respectively, which respond to [[linear acceleration]] and the force of [[gravity]]. The utricular division of the auditory vesicle also responds to [[angular acceleration]], as well as the [[endolymphatic sac]] and [[Endolymphatic duct|duct]] that connect the saccule and utricle.

Beginning in the fifth week of development, the auditory vesicle also gives rise to the [[cochlear duct]], which contains the spiral [[organ of Corti]] and the [[endolymph]] that accumulates in the membranous labyrinth.<ref name=USMLE>{{cite book|last=Brauer|first=Philip R. |title=Human embryology: the ultimate USMLE step 1 review |url=https://books.google.com/books?id=_Cb_XXR5HCQC|access-date=2011-05-14|year=2003|publisher=[[Elsevier]] Health Sciences|isbn=156053561X|page=61}}</ref> The [[Reissner's membrane|vestibular wall]] will separate the cochlear duct from the perilymphatic [[scala vestibuli]], a cavity inside the cochlea. The [[basilar membrane]] separates the cochlear duct from the [[scala tympani]], a cavity within the cochlear labyrinth. The lateral wall of the cochlear duct is formed by the [[spiral ligament]] and the [[stria vascularis]], which produces the [[endolymph]]. The [[hair cells]] develop from the lateral and medial ridges of the cochlear duct, which together with the [[tectorial membrane]] make up the organ of Corti.<ref name=USMLE />

===Microanatomy===
[[File:Cochlea-crosssection.svg|thumb|A cross-section of the [[cochlea]] showing the [[organ of Corti]].]]
[[File:Organ of corti.svg|thumb|Cross-section through the spiral organ of Corti at greater magnification.]]

Rosenthal's canal or the spiral canal of the cochlea is a section of the bony labyrinth of the inner ear that is approximately 30&nbsp;mm long and makes 2¾ turns about the [[modiolus (cochlea)|modiolus]], the central axis of the cochlea that contains the [[spiral ganglion]].

Specialized inner ear cell include: hair cells, pillar cells, Boettcher's cells, Claudius' cells, spiral ganglion neurons, and Deiters' cells (phalangeal cells).

The hair cells are the primary auditory receptor cells and they are also known as auditory sensory cells, acoustic hair cells, auditory cells or cells of Corti. The [[organ of Corti]] is lined with a single row of inner hair cells and three rows of outer hair cells. The hair cells have a hair bundle at the apical surface of the cell, which consists of an array of actin-based stereocilia. Each stereocilium inserts as a rootlet into a dense filamentous actin mesh known as the cuticular plate. Disruption of these bundles results in hearing impairments and balance defects.

Inner and outer pillar cells in the organ of Corti support hair cells. Outer pillar cells are unique because they are free standing cells which only contact adjacent cells at the bases and apices. Both types of pillar cell have thousands of cross linked [[microtubule]]s and [[actin]] filaments in parallel orientation. They provide mechanical coupling between the [[basement membrane]] and the [[mechanoreceptor]]s on the hair cells.

[[Boettcher cell|Boettcher's cells]] are found in the organ of Corti where they are present only in the lower turn of the cochlea. They lie on the basilar membrane beneath Claudius' cells and are organized in rows, the number of which varies between species. The cells interdigitate with each other, and project [[microvillus|microvilli]] into the intercellular space. They are supporting cells for the auditory hair cells in the organ of Corti. They are named after German pathologist [[Arthur Böttcher]] (1831–1889).

[[Claudius cell|Claudius' cells]] are found in the organ of Corti located above rows of Boettcher's cells. Like Boettcher's cells, they are considered supporting cells for the auditory hair cells in the organ of Corti. They contain a variety of [[aquaporin]] water channels and appear to be involved in ion transport. They also play a role in sealing off endolymphatic spaces. They are named after the German anatomist [[Friedrich Matthias Claudius]] (1822–1869).

[[Deiters cell|Deiters' cells]] (phalangeal cells) are a type of [[neuroglia]]l cell found in the organ of Corti and organised in one row of inner phalangeal cells and three rows of outer phalangeal cells. They are the supporting cells of the hair cell area within the cochlea. They are named after the German pathologist Otto Deiters (1834–1863) who described them.

[[Hensen cell|Hensen's cells]] are high columnar cells that are directly adjacent to the third row of Deiters' cells.

[[Hensen's stripe]] is the section of the tectorial membrane above the inner hair cell.

[[Nuel's spaces]] refer to the fluid-filled spaces between the outer pillar cells and adjacent hair cells and also the spaces between the outer hair cells.

[[Hardesty's membrane]] is the layer of the tectoria closest to the reticular lamina and overlying the outer hair cell region.

[[Reissner's membrane]] is composed of two cell layers and separates the scala media from the scala vestibuli.

[[Huschke's teeth]] are the tooth-shaped ridges on the spiral limbus that are in contact with the tectoria and separated by interdental cells.

=== Blood supply ===
The bony labyrinth receives its blood supply from three arteries:
1 – Anterior tympanic branch (from maxillary artery).
2 – Petrosal branch (from middle meningeal artery).
3 – Stylomastoid branch (from posterior auricular artery).
The [[membranous labyrinth]] is supplied by the [[labyrinthine artery]].
Venous drainage of the inner ear is through the labyrinthine vein, which empties into the [[sigmoid sinus]] or [[inferior petrosal sinus]].

==Function==
Neurons within the ear respond to simple tones, and the brain serves to process other increasingly complex sounds. An average adult is typically able to detect sounds ranging between 20 and 20,000&nbsp;Hz. The ability to detect higher pitch sounds decreases in older humans.

The human ear has evolved with two basic tools to encode sound waves; each is separate in detecting high and low-frequency sounds. [[Georg von Békésy]] (1899–1972) employed the use of a microscope in order to examine the basilar membrane located within the inner-ear of cadavers. He found that movement of the basilar membrane resembles that of a traveling wave; the shape of which varies based on the frequency of the pitch. In low-frequency sounds, the tip (apex) of the membrane moves the most, while in high-frequency sounds, the base of the membrane moves most.<ref>{{cite book |last=Schacter |first=Daniel |title=Psychology |publisher=[[Worth Publishers]] |year=2012 |isbn=978-1464135606 |location=New York}}</ref>

==Disorders==
{{main|Vestibulopathy}}
Interference with or infection of the labyrinth can result in a syndrome of ailments called [[labyrinthitis]].  The symptoms of labyrinthitis include temporary nausea, disorientation, vertigo, and dizziness. Labyrinthitis can be caused by viral infections, bacterial infections, or physical blockage of the inner ear.<ref>{{cite conference |title=Labyrinthine dysfunction during diving |conference=1st [[Undersea and Hyperbaric Medical Society]] Workshop. |volume=UHMS Publication Number WS6-15-74. |publisher=Undersea and Hyperbaric Medical Society |year=1973 |pages=11 |url=http://archive.rubicon-foundation.org/4291 |access-date=2009-03-11 |archive-date=2009-07-03 |archive-url=https://web.archive.org/web/20090703203756/http://archive.rubicon-foundation.org/4291 |url-status=usurped }}</ref><ref name="pmid4619861">{{cite journal |author=Kennedy RS |title=General history of vestibular disorders in diving |journal=Undersea Biomedical Research |volume=1 |issue=1 |pages=73–81 |date=March 1974 |pmid=4619861 |url=http://archive.rubicon-foundation.org/2663 |access-date=2009-03-11 |archive-date=2010-06-15 |archive-url=https://web.archive.org/web/20100615051951/http://archive.rubicon-foundation.org/2663 |url-status=usurped }}</ref>

Another condition has come to be known as autoimmune inner ear disease (AIED). It is characterized by idiopathic, rapidly progressive, bilateral sensorineural hearing loss. It is a fairly rare disorder while at the same time, a lack of proper diagnostic testing has meant that its precise incidence cannot be determined.<ref>Ruckenstein, M. J. (2004). "Autoimmune Inner Ear Disease".'' [[Current Opinion in Otolaryngology & Head and Neck Surgery]], 12''(5), pp. 426-430.</ref>

==Other animals==
Birds have an auditory system similar to that of mammals, including a cochlea. Reptiles, amphibians, and fish do not have cochleas but hear with simpler auditory organs or vestibular organs, which generally detect lower-frequency sounds than the cochlea.  The cochlea of birds is also similar to that of crocodiles, consisting of a short, slightly curved bony tube within which lies the basilar membrane with its sensory structures.<ref>{{Cite web|url=https://www.britannica.com/science/sound-reception/Hearing-in-birds|title=Bird cochlea}}</ref>

===Cochlear system===
{{See also|Evolution of the cochlea}}
In [[reptile]]s, sound is transmitted to the inner ear by the [[stapes]] (stirrup) bone of the middle ear. This is pressed against the [[oval window]], a membrane-covered opening on the surface of the vestibule. From here, sound waves are conducted through a short '''perilymphatic duct''' to a second opening, the [[round window]], which equalizes pressure, allowing the incompressible fluid to move freely. Running parallel with the perilymphatic duct is a separate blind-ending duct, the '''lagena''', filled with [[endolymph]]. The lagena is separated from the perilymphatic duct by a [[basilar membrane]], and contains the sensory hair cells that finally translate the vibrations in the fluid into nerve signals. It is attached at one end to the saccule.<ref name=VB>{{cite book |author=Romer, Alfred Sherwood|author2=Parsons, Thomas S.|year=1977 |title=The Vertebrate Body |publisher=Holt-Saunders International |location= Philadelphia, PA|pages= 476–489|isbn= 003910284X}}</ref>

In most reptiles the perilymphatic duct and lagena are relatively short, and the sensory cells are confined to a small '''basilar papilla''' lying between them. However, in [[mammal]]s, [[bird]]s, and [[crocodilian]]s, these structures become much larger and somewhat more complicated. In birds, crocodilians, and [[monotreme]]s, the ducts are simply extended, together forming an elongated, more or less straight, tube. The endolymphatic duct is wrapped in a simple loop around the lagena, with the basilar membrane lying along one side. The first half of the duct is now referred to as the [[scala vestibuli]], while the second half, which includes the basilar membrane, is called the [[scala tympani]]. As a result of this increase in length, the basilar membrane and papilla are both extended, with the latter developing into the [[organ of Corti]], while the lagena is now called the [[cochlear duct]]. All of these structures together constitute the cochlea.<ref name=VB/>

In [[theria]]n mammals, the lagena is extended still further, becoming a coiled structure (cochlea) in order to accommodate its length within the head. The organ of Corti also has a more complex structure in mammals than it does in other [[amniote]]s.<ref name=VB/>

The arrangement of the inner ear in living [[amphibian]]s is, in most respects, similar to that of reptiles. However, they often lack a basilar papilla, having instead an entirely separate set of sensory cells at the upper edge of the saccule, referred to as the '''papilla amphibiorum''', which appear to have the same function.<ref name=VB/>

Although many fish are capable of hearing, the lagena is, at best, a short diverticulum of the saccule, and appears to have no role in sensation of sound. Various clusters of hair cells within the inner ear may instead be responsible; for example, [[Osteichthyes|bony fish]] contain a sensory cluster called the '''macula neglecta''' in the utricle that may have this function. Although fish have neither an outer nor a middle ear, sound may still be transmitted to the inner ear through the bones of the skull, or by the [[swim bladder]], parts of which often lie close by in the body.<ref name=VB/>

===Vestibular system===
By comparison with the [[cochlea]]r system, the [[vestibular system]] varies relatively little between the various groups of [[gnathostome|jawed vertebrates]]. The central part of the system consists of two chambers, the saccule and utricle, each of which includes one or two small clusters of sensory hair cells. All jawed vertebrates also possess three semicircular canals arising from the utricle, each with an [[osseous ampullae|ampulla]] containing sensory cells at one end.<ref name=VB/>

An [[endolymphatic duct]] runs from the saccule up through the head and ending close to the brain. In [[cartilaginous fish]], this duct actually opens onto the top of the head, and in some [[teleost]]s, it is simply blind-ending. In all other species, however, it ends in an [[endolymphatic sac]]. In many reptiles, fish, and amphibians this sac may reach considerable size. In amphibians the sacs from either side may fuse into a single structure, which often extends down the length of the body, parallel with the [[spinal canal]].<ref name=VB/>

The primitive [[lamprey]]s and [[hagfish]], however, have a simpler system. The inner ear in these species consists of a single vestibular chamber, although in lampreys, this is associated with a series of sacs lined by [[cilia]]. Lampreys have only two semicircular canals, with the horizontal canal being absent, while hagfish have only a single, vertical, canal.<ref name=VB/>

===Equilibrium===
The inner ear is primarily responsible for balance, equilibrium and orientation in three-dimensional space. The inner ear can detect both static and dynamic equilibrium. Three [[semicircular ducts]] and two chambers, which contain the [[saccule]] and [[utricle (ear)|utricle]], enable the body to detect any deviation from equilibrium. The macula sacculi detects vertical acceleration while the [[macula utriculi]] is responsible for horizontal acceleration. These microscopic structures possess stereocilia and one kinocilium which are located within the gelatinous otolithic membrane. The membrane is further weighted with otoliths. Movement of the stereocilia and kinocilium enable the hair cells of the saccula and utricle to detect motion. The semicircular ducts are responsible for detecting rotational movement.<ref>Anatomy & Physiology The Unity of Form and Function. N.p.: McGraw-Hill College, 2011. Print.</ref>

==Additional images==
<gallery>
 Image:Anatomy of the Human Ear.svg|Human ear anatomy.{{Anatomy of the human ear - color legend}}
 Image:Ear labyrinth.jpg|Ear labyrinth
 Image:Oreille Interne.png|Inner ear
 Image:Temporal bone2.jpg|Temporal bone
 Image:Gray925.png|Right human membranous labyrinth, removed from its bony enclosure and viewed from the antero-lateral aspect
 Image:1408 Frequency Coding in The Cochlea.jpg|Frequency coding in the cochlea
</gallery>

==See also==
* [[Hearing]]
* [[Outer ear]]
* [[Tip link]]

==References==
{{Reflist}}
* Ruckenstein, M. J. (2004). [https://journals.lww.com/co-otolaryngology/Abstract/2004/10000/Autoimmune_inner_ear_disease.13.aspx "Autoimmune Inner Ear Disease"]. ''Current Opinion in Otolaryngology & Head and Neck Surgery'', '''12'''(5), pp.&nbsp;426–430.
* Saladin, ''Anatomy and Physiology'' 6th ed., print
* American Speech-Language-Hearing Association, [https://web.archive.org/web/20131208002201/http://www.asha.org/public/hearing/Middle-Ear/ "The Middle Ear"],

==External links==
{{Commons category}}
* {{SUNYAnatomyLabs|30|05|01|01}}
{{Auditory and vestibular anatomy}}

{{Authority control}}

[[Category:Auditory system]]
[[Category:Inner ear]]
[[Category:Otology]]
[[Category:Audiology]]
[[Category:Otorhinolaryngology]]