Middle ear

From Wikipedia, the free encyclopedia

Middle ear
Malleus Tensor Tympani Incus Stapedius Labyrinth Stapes Auditory Canal Tympanic Membrane (Ear Drum) Eustachian Tube Tympanic cavity
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	a_73/12169777

The middle ear is the portion of the ear internal to the eardrum, and external to the oval window of the cochlea. The middle ear contains three ossicles, which amplify vibration of the eardrum into pressure waves in the fluid in 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 function of the middle ear is to efficiently transfer sound energy from air to the liquid contained within the cochlea. It also produces a gain of around 10-20 times (20-26dB).

1 Sound transfer
2 Ossicles
3 Muscles
4 Nerves
5 Comparative anatomy
6 Disorders of the middle ear
7 External links
8 References

[edit] Sound transfer

Ordinarily, when sound waves in air strike liquid, more than 99% of the energy is reflected off the surface of the liquid. Most people have an intuitive understanding of this, having experienced hearing underwater. The sounds of the shore or poolside are almost inaudible, remaining in the air. The middle ear allows the impedance matching of sound traveling in air and sound traveling in liquid, overcoming the interface between them.

There are several specific ways in which the middle ear amplifies sound from air to the fluid in the oval window. The first of these is the "hydraulic principle". The vibratory portion of the tympanic membrane is many times the surface area of the footplate of the stapes. The collected pressure of sound vibration that strikes the tympanic membrane is concentrated down to this much smaller area of the footplate, increasing the force and thereby amplifying the sound.

The second way in which the middle ear amplifies air-conducted sound to the fluid of the cochlea is called the "lever principle". 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 third way in which the middle ear amplifies sound is by "round window protection". The cochlea is a fluid filled tube, wound about on itself. One end of this tube is at the oval window, the other end is at the round window. Both of these windows, opening in the bone, lay on the deep wall of the middle ear space. Whereas the tympanic membrane and ossicular chain preferentially direct sound to the oval window, the round window is protected by middle ear structures from having sound waves impinge on its surface. The middle ear structures that protect the round window are the intact tympanic membrane and the round window niche. If there were no tympanic membrane or ossicular chain, sound waves would strike the round window and oval window at the same time - and much of the energy would be lost. Fluid movement inside the cochlea, in regards to both the oval and round window and the stimulation of hair cells, is more fully explained in the next section on the cochlea.

The middle-ear is able to dampen sound conduction a bit when faced with very loud sound by noise induced reflex contraction of the middle ear muscles.

[edit] 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.

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 total amplification 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 0.1-4 kHz it changes from 2 to and then falls off towards 1 at >5kHz.^[1] The measurement of this lever arm ratio is also somewhat complicated byy 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 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 ratio in area between the tympanic membrane and the oval window results in an effective amplification of approximately 14 dB, peaking 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.

[edit] 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.

[edit] 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, unilateral facial paralysis.

[edit] 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.

[edit] 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 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 yawning helps relieve the pressure felt in the ears when on board an aircraft.