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| Middle ear | |
|---|---|
 A diagram of the anatomy of the middle ear | |
| Details | |
| Nerve | Glossopharyngeal nerve |
| Identifiers | |
| Latin | auris media |
| MeSH | D004432 |
| TA98 | A15.3.02.001 |
| TA2 | 6877 |
| FMA | 56513 |
| Anatomical terminology | |
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| This article is one of a series documenting the anatomy of the |
| Human ear |
|---|
Themiddle ear is the portion of theear medial to theeardrum, and distal to theoval window of the cochlea (of theinner ear).
The mammalian middle ear contains threeossicles (malleus, incus, and stapes), which transfer the vibrations of the eardrum into waves in the fluid and membranes of theinner ear. The hollow space of the middle ear is also known as thetympanic cavity and is surrounded by thetympanic part of the temporal bone. Theauditory tube (also known as the Eustachian tube or the pharyngotympanic 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 fromcompression waves in air to fluid–membrane waves within thecochlea.
The middle ear contains three tiny bones known as theossicles:malleus,incus, andstapes. The ossicles were given their Latin names for their distinctive shapes; they are also referred to as thehammer,anvil, andstirrup, respectively. The ossicles directly couple sound energy from the eardrum to theoval window of the cochlea. While the stapes is present in alltetrapods, the malleus and incusevolved from lower and upper jaw bones present in reptiles.
The ossicles are classically supposed to mechanically convert the vibrations of theeardrum into amplified pressure waves in the fluid of thecochlea (orinner ear), with a lever arm factor of 1.3. Since the effective vibratory area of the eardrum is about 14 fold larger than that of the oval window, the sound pressure is concentrated, leading to a pressure gain of at least 18.1. 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 theinner 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.[1] 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 theumbo) and the level of the middle of the stapes. The eardrum is actually attached to the malleus handle over about a 0.5 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 combinedtransfer function of the outer ear and middle ear gives humans a peak sensitivity to frequencies between 1 kHz and 3 kHz.
The movement of the ossicles may be stiffened by two muscles. Thestapedius muscle, the smallest skeletal muscle in the body, connects to the stapes and is controlled by thefacial nerve; thetensor tympani muscle is attached to the upper end of the medial surface of the handle of malleus[2] and is under the control of themedial pterygoid nerve which is a branch of themandibular nerve of thetrigeminal nerve. These muscles contract in response to loud sounds, thereby reducing the transmission of sound to the inner ear. This is called theacoustic reflex.
Of surgical importance are two branches of thefacial nerve that also pass through the middle ear space. These are the horizontal portion of the facial nerve and thechorda tympani. Damage to the horizontal branch during ear surgery can lead to paralysis of the face (same side of the face as the ear). The chorda tympani is the branch of the facial nerve that carries taste from theipsilateral half (same side) of the tongue.
Ordinarily, when sound waves in air strike liquid, most of the energy is reflected off the surface of the liquid. The middle ear allows theimpedance 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 liquid.
Theacoustic impedance of air is about, while the impedance of cochlear fluids () is approximately equal to that of sea water. Because of this high impedance, only of incident energy could be directly transmitted from the air to cochlear fluids.
The middle ear's impedance matching mechanism increases the efficiency of sound transmission. Two processes are involved:
Together, they amplify pressure by 26 times, or about 30 dB. The actual value is around 20 dB across 200 to 10000 Hz.[3][4]
The middle ear couples sound from air to the fluid via theoval window, using the principle of "mechanical advantage" in the form of the "hydraulic principle" and the "lever principle".[5] The vibratory portion of the tympanic membrane (eardrum) is many times the surface area of the footplate of thestapes (the third ossicular bone which attaches to the oval window); furthermore, the shape of the articulated ossicular chain is a complexlever, the long arm being the long process of themalleus, the fulcrum being the body of theincus, and the short arm being the lenticular process of theincus. 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.
The middle ear is hollow in the tympanic cavity and Eustachian tube. In a high-altitude environment or on diving into 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 (eardrum) if it is not relieved. If middle ear pressure remains low, theeardrum (tympanic membrane) may becomeretracted into the middle ear.[citation needed] One of the functions of theEustachian tubes that connect the middle ear to thenasopharynx is to help keep middle ear pressure the same as air pressure. The Eustachian tubes are normally pinched off at the nose end, to prevent being clogged withmucus, but they may be opened by lowering and protruding the jaw; this is whyyawning or chewing helps relieve the pressure felt in the ears when on board an aircraft.Eustachian tube obstruction may result in fluid build up in the middle ear, which causes aconductive hearing loss.Otitis media is an inflammation of the middle ear.
The middle ear is well protected from most minor external injuries by its internal location, but is vulnerable to pressure injury (barotrauma).
Recent findings indicate that the middle ear mucosa could be subjected tohuman papillomavirus infection.[6] Indeed, DNAs belonging to oncogenic HPVs, i.e., HPV16 and HPV18, have been detected in normal middle ear specimens, thereby indicating that the normal middle ear mucosa could potentially be a target tissue for HPV infection.[6]
The middle ear oftetrapods isanalogous with thespiracle of fishes, an opening from thepharynx 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.[7]
Inreptiles,birds, and early fossil tetrapods, there is a single auditory ossicle, thecolumella which is homologous with the stapes, or "stirrup" of mammals. This is connected indirectly with the eardrum via a mostly cartilaginous extracolumella and medially to the inner-ear spaces via a widened footplate in the fenestra ovalis.[7] The columella is an evolutionary derivative of the bone known as the hyomandibula in fish ancestors, a bone that supported the skull and braincase.
The structure of the middle ear in livingamphibians varies considerably and is often degenerate. In mostfrogs 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 thequadrate 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, theoperculum (not to be confused with thestructure 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 thescapula. It is not found in any other vertebrates.[7]
Mammals are unique in havingevolved a three-ossicle middle-ear independently of the various single-ossicle middle ears of other land vertebrates, all during the Triassic period of geological history. Functionally, the mammalian middle ear is very similar to the single-ossicle ear of non-mammals, except that it responds to sounds of higher frequency, because these are better taken up by the inner ear (which also responds to higher frequencies than those of non-mammals). The malleus, or "hammer", evolved from thearticular bone of the lower jaw, and the incus, or "anvil", from the quadrate. In other vertebrates, these bones form the primary jaw joint, but the expansion of thedentary bone in mammals led to the evolution of an entirely new jaw joint, freeing up the old joint to become part of the ear. For a period of time, both jaw joints existed together, one medially and one laterally. The evolutionary process leading to a three-ossicle middle ear was thus an "accidental" byproduct of the simultaneous evolution of the new, secondary jaw joint. In many mammals, the middle ear also becomes protected within a cavity, theauditory bulla, not found in other vertebrates. A bulla evolved late in time and independently numerous times in different mammalian clades, and it can be surrounded by membranes, cartilage or bone. The bulla in humans is part of thetemporal bone.[7]
Recently found fossils such asMorganucodon show intermediary steps of middle ear evolution. A new morganucodontan-like species,Dianoconodon youngi, shows parts of themandible (= dentary) that permit an auditory function, although these bones are still attached to the mandible.[8]
