Middle ear 55972 213372358 2008-05-19T01:12:12Z Razorflame 5792322 Reverted edits by [[Special:Contributions/75.37.214.97|75.37.214.97]] ([[User talk:75.37.214.97|talk]]) to last version by XLinkBot {{Infobox Anatomy | Name = {{PAGENAME}} | Latin = auris media | GraySubject = 230 | GrayPage = 1037 | Image = | Caption = | Image2 = | Caption2 = | Map = Middle ear map | MapPos = None| MapCaption = Bones and muscles in the tympanic cavity in the middle ear | Precursor = | System = | Artery = | Vein = | Nerve = [[glossopharyngeal nerve]] | Lymph = | MeshName = Middle+ear | MeshNumber = A09.246.397 | DorlandsPre = a_73 | DorlandsSuf = 12169777 | }} The '''middle ear''' is the portion of the ear internal to the [[eardrum]], and external to the [[oval window]] of the [[cochlea]]. The mammalian middle ear contains three [[ossicles]], which couple vibration of the eardrum into waves in the fluid and membranes of the [[inner ear]]. The hollow space of the middle ear has also been called the [[tympanic cavity]], or ''cavum tympani''. The [[eustachian tube]] joins the tympanic cavity with the nasal cavity ([[nasopharynx]]), allowing pressure to equalize between the inner ear and throat. The primary function of the middle ear is to efficiently transfer acoustic energy from compression waves in air to fluid–membrane waves within the [[cochlea]]. ==Sound transfer== Ordinarily, when sound waves in air strike liquid, most of the energy is reflected off the surface of the liquid. The middle ear allows the impedance matching of sound traveling in air to acoustic waves traveling in a system of fluids and membranes in the inner ear. This system should not be confused, however, with the propagation of sound as compression waves in a liquid. The middle ear couples sound from air to the fluid via the oval window, using the principle of "mechanical advantage" in the form of the "hydraulic principle" and the "lever principle".<ref>{{cite book | title = Biomedical Engineering Fundamentals | author = Joseph D. Bronzino | publisher = CRC Press | isbn = 0849321212 | year = 2006 | url = http://books.google.com/books?id=C2ladV9aI8MC&pg=PT84&ots=ZJCVHNRI2O&dq=middle-ear+lever+hydraulic&as_brr=3&sig=Z7AqP33OzBZH3rD7kP9HnQQJllw#PPA78,M1 }}</ref> The vibratory portion of the tympanic membrane is many times the surface area of the footplate of the stapes; furthermore, the shape of the articulated ossicular chain is like a lever, the long arm being the long process of the malleus, and the body of the incus being the fulcrum and the short arm being the lenticular process of the incus. The collected pressure of sound vibration that strikes the tympanic membrane is therefore concentrated down to this much smaller area of the footplate, increasing the force but reducing the velocity and displacement, and thereby coupling the acoustic energy. The middle ear is able to dampen sound conduction substantially when faced with very loud sound, by noise-induced reflex contraction of the middle-ear muscles. ==Ossicles== The middle ear contains three tiny bones known as the [[ossicles]]: ''[[malleus]]'', ''[[incus]]'', and ''[[stapes]]''. The ossicles were given their Latin names for their distinctive shapes; they are also referred to as the ''hammer'', ''anvil'', and ''stirrup'', respectively. The ossicles directly couple sound energy from the ear drum to the [[oval window]] of the cochlea. While the [[stapes]] is present in all tetrapods, the [[malleus]] and [[incus]] evolved from lower and upper jaw bones present in reptiles. See [[Evolution of mammalian auditory ossicles]]. The ossicles are classically supposed to mechanically convert the vibrations of the [[eardrum]], into amplified pressure waves in the fluid of the [[cochlea]] (or [[inner ear]]) with a lever arm factor of 1.3. Since the area of the eardrum is about 17 fold larger than that of the [[oval window]], the sound pressure is concentrated, leading to a pressure gain of at least 22. The eardrum is fused to the malleus, which connects to the incus, which in turn connects to the stapes. Vibrations of the stapes footplate introduce pressure waves in the [[inner ear]]. There is a steadily increasing body of evidence which shows that the lever arm ratio is actually variable, depending on frequency. Between 0.1 and 1&nbsp;kHz it is approximately 2, it then rises to around 5 at 2&nbsp;kHz and then falls off steadily above this frequency.<ref>Koike et al.: Modeling of the human middle ear J. Acoust. Soc. Am., Vol. 111, No. 3, March 2002</ref> The measurement of this lever arm ratio is also somewhat complicated by the fact that the ratio is generally given in relation to the tip of the [[malleus]] (also known as the [[umbo]]) and the level of the middle of the [[stapes]]. The [[eardrum]] is actually attached to the [[malleus]] handle over about a 1cm distance. In addition the [[eardrum]] itself moves in a very chaotic fashion at frequencies >3&nbsp;kHz. The linear attachment of the [[eardrum]] to the [[malleus]] actually smooths out this chaotic motion and allows the ear to respond linearly over a wider frequency range than a point attachment. The auditory ossicles can also reduce sound pressure (the inner ear is very sensitive to overstimulation), by uncoupling each other through particular muscles. The middle ear efficiency peaks at a frequency of around 1&nbsp;kHz. The combined transfer function of the outer ear and middle ear gives humans a peak sensitivity to frequencies between 1&nbsp;kHz and 3&nbsp;kHz. ==Muscles== The movement of the ossicles may be stiffened by two muscles, the stapedius and tensor tympani, which are under the control of the [[facial nerve]] and [[trigeminal nerve]], respectively. These muscles contract in response to loud sounds, thereby reducing the transmission of sound to the inner ear. This is called the [[acoustic reflex]]. ==Nerves== Of surgical importance are two branches of the [[facial nerve]] which also pass through the middle ear space. These are the horizontal and ''chorda tympani'' branches of the facial nerve. Damage to the horizontal branch during surgery can lead to partial, mastoid process paralysis. ==Comparative anatomy== Mammals are unique in having three ear bones. The incus and stapes have evolved from bones of the jaw, and allow finer detection of sound. Some mammals, such as the [[cat]], have an enlarged middle ear encased in a thin, bulbous bone; this structure is known as a bulla. ==Disorders of the middle ear== The middle ear is hollow. If the animal moves to a high-altitude environment, or dives into the water, there will be a pressure difference between the middle ear and the outside environment. This pressure will pose a risk of bursting or otherwise damaging the tympanum if it is not relieved. This is one of the functions of the [[Eustachian tube]]s which connect the middle ear to the [[nasopharynx]]. The Eustachian tubes are normally pinched off at the nose end, to prevent being clogged with [[mucus]], but they may be opened by lowering and protruding the jaw; this is why [[yawn]]ing helps relieve the pressure felt in the ears when on board an aircraft. [[Otitis media]] is an inflammation of the middle ear. ==Additional images== <gallery> Image:Gray919.png </gallery> ==External links== *[http://www.iurc.montp.inserm.fr/cric/audition/english/ear/fear.htm Promenade Around the Cochlea - Middle ear at iurc.montp.inserm.fr] [[Category:Auditory system]] [[ar:أذن وسطى]] [[da:Mellemøre]] [[de:Mittelohr]] [[es:Oído medio]] [[et:Keskkõrv]] [[it:Orecchio medio]] [[nl:Middenoor]] [[ja:中耳]] [[no:Mellomøre]] [[pl:Ucho środkowe]] [[pt:Ouvido médio]] [[sk:Stredné ucho]] [[fi:Välikorva]] [[sv:Mellanöra]] [[zh:中耳]] ==References== <references/> {{Auditory system}}