Middle ear


Middle ear
Middle ear
Bones and muscles in the tympanic cavity in the middle ear
Latin auris media
Gray's subject #230 1037
Nerve glossopharyngeal nerve
MeSH Middle+ear
Dorlands/Elsevier Middle ear

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 middle 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.

Contents

Sound transfer

Ordinarily, waves travel 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".[1] 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 merged 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 that shows that the lever arm ratio is actually variable, depending on frequency. Between 0.1 and 1 kHz it is approximately 2, it then rises to around 5 at 2 kHz and then falls off steadily above this frequency.[2] 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 1 cm distance. In addition the eardrum itself moves in a very chaotic fashion at frequencies >3 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 kHz. The combined transfer function of the outer ear and middle ear gives humans a peak sensitivity to frequencies between 1 kHz and 3 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 or Tympanic reflex.

Nerves

Of surgical importance are two branches of the facial nerve that 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

The middle ear of tetrapods is homologous with the spiracle of fishes, an opening from the pharynx to the side of the head in front of the main gill slits. In fish embryos, the spiracle forms as a pouch in the pharynx, which grows outward and breaches the skin to form an opening; in most tetrapods, this breach is never quite completed, and the final vestige of tissue separating it from the outside world becomes the eardrum. The inner part of the spiracle, still connected to the pharynx, forms the eustachian tube.[3]

In reptiles, birds, and early fossil tetrapods, there is only a single auditory ossicle, the stapes. This runs directly from the eardrum to the fenestra ovalis.[3]

The structure of the middle ear in living amphibians varies considerably, and is often degenerate. In most frogs and toads, it is similar to that of reptiles, but in other amphibians, the middle ear cavity is often absent. In these cases, the stapes either is also missing or, in the absence of an eardrum, connects to the quadrate bone in the skull, although, it is presumed, it still has some ability to transmit vibrations to the inner ear. In many amphibians, there is also a second auditory ossicle, the operculum (not to be confused with the structure of the same name in fishes). This is a flat, plate-like bone, overlying the fenestra ovalis, and connecting it either to the stapes or, via a special muscle, to the scapula. It is not found in any other vertebrates.[3]

Mammals are unique in having three ear bones, which allow for finer detection of sound. The malleus has evolved from the articular bone of the lower jaw, and the incus from the quadrate. In other vertebrates, these bones form the joint of the jaw, but the expansion of the dentary bone in mammals has allowed those animals to develop an entirely new jaw joint, freeing up the old joint to become part of the ear. In many mammals, the middle ear also becomes protected by a bony sheath, the auditory bulla, not found in other vertebrates. This is often a separate structure, but, in humans, it is part of the temporal bone.[3]

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 tubes that 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 yawning or chewing helps relieve the pressure felt in the ears when on board an aircraft.

Otitis media is an inflammation of the middle ear.

Additional images

References

  1. ^ Joseph D. Bronzino (2006). Biomedical Engineering Fundamentals. CRC Press. ISBN 0849321212. http://books.google.com/books?id=C2ladV9aI8MC&pg=PT84&dq=middle-ear+lever+hydraulic#PPA78,M1. 
  2. ^ Koike et al.: Modeling of the human middle ear J. Acoust. Soc. Am., Vol. 111, No. 3, March 2002
  3. ^ a b c d Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 480–488. ISBN 0-03-910284-X. 

External links


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Look at other dictionaries:

  • middle ear — n. the part of the ear including the tympanic membrane and the adjacent cavity containing the three small bones (hammer, anvil, and stirrup) that transmit the vibrations of the tympanic membrane to the inner ear: see EAR1 …   English World dictionary

  • middle ear — n [singular] the part just inside your ear …   Dictionary of contemporary English

  • middle ear — noun singular the part of your ear that is between the outer part that you can see and the EARDRUM …   Usage of the words and phrases in modern English

  • middle ear — ► NOUN ▪ the air filled central cavity of the ear, behind the eardrum …   English terms dictionary

  • Middle ear — There are three sections of the ear. They are the external ear, the middle ear, and the inner ear. The middle ear consists of the ear drum (the tympanum or tympanic membrane) and, beyond it, a cavity. This cavity is connected via a canal (the… …   Medical dictionary

  • middle ear — noun the main cavity of the ear; between the eardrum and the inner ear • Syn: ↑tympanic cavity, ↑tympanum • Hypernyms: ↑cavity, ↑bodily cavity, ↑cavum • Part Holonyms: ↑audi …   Useful english dictionary

  • middle ear — tympanic cavity the part of the ear that consists of an air filled space within the petrous part of the temporal bone. It is lined with mucous membrane and is connected to the pharynx by the Eustachian tube and to the outer ear by the eardrum… …   The new mediacal dictionary

  • middle ear — mid′dle ear′ n. anat. the middle portion of the ear consisting of the eardrum and an air filled chamber lined with mucous membrane that contains the malleus, incus, and stapes • Etymology: 1885–90 …   From formal English to slang

  • middle ear — noun The cavity in the temporal bone between the eardrum and the inner ear that contains the ossicles, and which conveys sound to the cochlea. Syn: auris media, tympanum See Also: inner ear, outer ear …   Wiktionary

  • middle ear — /mɪdl ˈɪə/ (say midl ear) noun the section of the ear recessed into the temporal bone, lying between the inner ear and the eardrum and containing the three ossicles which join them; tympanum …   Australian English dictionary


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