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Human (external) ear
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Human (external) ear
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Often the entire organ is considered the ear, though it may also be considered just the visible portion. In most mammals, the visible ear is a flap of tissue that is also called the pinna (or auricle in humans) and is the first of many steps in hearing. Vertebrates have a pair of ears placed somewhat symmetrically on opposite sides of the head. This arrangement aids in the ability to localize sound sources.
The shape of the outer ear of mammals varies widely across species. However the inner workings of mammalian ears (including humans') are very similar.
The outer ear is the most external portion of the ear. The outer ear includes the fleshy visible outer ear, called the pinna or auricle, the ear canal, and the outer layer of the tympanic membrane, also known as the ear drum. :855 The outer ear is the only visible portion of the ear in humans and almost all vertebrates, and consequently the word "ear" may be used to refer to the pinna alone.
The auricle consists of the curving outer rim (the helix), the inner curved rim (the antihelix), and opens into the ear canal, properly called the external acoustic meatus. The tragus protrudes and partially obscures the ear canal. The ear canal stretches for a distance of about 1 inch, and consists of an inner portion surrounded by bone, and an outer portion surrounded by cartilage. The skin surrounding the external acoustic meatus contains glands that produce ear wax (cerumen). The ear canal ends at the external surface of the ear drum (tympanic membrane)  :856
Two sets of muscles are associated with the outer ear; the intrinsic and extrinsic muscles. In some mammals these muscles can adjust the direction of the pinna. :855 In humans these muscle have very little action if any at all. These muscles are supplied by the facial nerve, which also supplies sensation to the skin of the ear itself, as well as the ear cavity. The vagus nerve, auriculotemporal nerve of the mandibular nerve, lesser occipital branch of C2, and the greater occipital nerve branch of C3 all supply sensation to portions of the outer ear and surrounding skin. :855
The auricle consists of a single piece of fibrocartilage with a complicated relief on the anterior, concave side and a fairly smooth configuration on the posterior, convex side. The Darwinian tubercle, which is present in some people, lies in the descending part of the helix and corresponds to the true ear tip of the long-eared mammals. The lobule merely contains subcutaneous tissue. In some animals with mobile pinnae (like the horse), each pinna can be aimed independently to better receive the sound. For these animals, the pinnae help localize the direction of the sound source. Human beings localize sound within the central nervous system, by comparing arrival-time differences and loudness from each ear, in brain circuits that are connected to both ears. This process is commonly referred to as EPS, or Echo Positioning System.
|Components of the middle ear|
The middle ear is an air-filled cavity behind the tympanic membrane, includes three bones (ossicles): the malleus (or hammer), incus (or anvil), and stapes (or stirrup). The middle ear also connects to the upper throat via the Eustachian tube.:858
The three ossicles transmit sound from the tympanic membrane to the ventricles of the ear. The malleus is connected to the tympanic membrane, and transmits vibrations of the membrane produced by sound waves. The malleus has a long process (the manubrium, or handle) that is attached to the mobile portion of the eardrum. The incus is the bridge between the malleus and stapes. The stapes connects to the oval window, and is the smallest named bone in the human body. The three bones are arranged so that movement of the tympanic membrane causes movement of the malleus, which causes movement of the incus, which causes movement of the stapes. When the stapes footplate pushes on the oval window, it causes movement of fluid within the cochlea (a portion of the inner ear).:858 The ossicles help in amplification of sound waves by nearly thirty times.
In humans and other land animals the middle ear (like the ear canal) is normally filled with air. Unlike the open ear canal, however, the air of the middle ear is not in direct contact with the atmosphere outside the body. The Eustachian tube connects from the chamber of the middle ear to the back of the nasopharynx. The middle ear is very much like a specialized paranasal sinus, called the tympanic cavity; it, like the paranasal sinuses, is a hollow mucosa-lined cavity in the skull that is ventilated through the nose. The mastoid portion of the human temporal bone, which can be felt as a bump in the skull behind the pinna, also contains air, which is ventilated through the middle ear.
Normally, the Eustachian tube is collapsed, but it gapes open both with swallowing and with positive pressure. When taking off in an airplane, the surrounding air pressure goes from higher (on the ground) to lower (in the sky). The air in the middle ear expands as the plane gains altitude, and pushes its way into the back of the nose and mouth. On the way down, the volume of air in the middle ear shrinks, and a slight vacuum is produced. Active opening of the Eustachian tube is required to equalize the pressure between the middle ear and the surrounding atmosphere as the plane descends. The diver also experiences this change in pressure, but with greater rates of pressure change; active opening of the Eustachian tube is required more frequently as the diver goes deeper into higher pressure.
Abnormalities such as impacted ear wax (occlusion of the external ear canal), fixed or missing ossicles, or holes in the tympanic membrane generally produce conductive hearing loss. Conductive hearing loss may also result from middle ear inflammation causing fluid build-up in the normally air-filled space. Tympanoplasty is the general name of the operation to repair the middle ear's tympanic membrane and ossicles. Grafts from muscle fascia are ordinarily used to rebuild an intact ear drum. Sometimes artificial ear bones are placed to substitute for damaged ones, or a disrupted ossicular chain is rebuilt in order to conduct sound effectively.
|Components of the inner ear|
The inner ear is split anatomically into bony and membranous labyrinths. This contains the sensory organs for balance and motion, namely the vestibules of the ear (utricle and saccule), and the semicircular canals. This also contains the sensory organ for hearing, the cochlea.:865–867
The bony labyrinth refers to a bone matrix which opens externally into the oval window, which connects with the incus, which transmits vibrations into a fluid called endolymph, which fills the membranous labyrinth. The endolymph is situated in two vestibules, the utricle and saccule, and eventually transmits to the cochlea, a spiral-shaped structure. The cochlea consists of three fluid-filled spaces: the scala tympani, the scala vestibuli and the scala media, which together are responsibly for hearing.:864–865
The blood supply of the ear differs according to each part of the ear. The external ear is supplied by the anterior and posterior auricular arteries, which are branches of the superficial temporal artery and external carotid artery respectively, and branches of the occipital artery.:855 The middle ear is supplied by mastoid branch of the occipital or posterior auricular arteries, tympanic branch of the maxillary artery and some branches from different arteries, including the middle meningeal artery, ascending pharyngeal artery, internal carotid artery, and the artery of pterygoid canal.:863 The inner ear is supplied by the anterior tympanic branch of the maxillary artery, stylomastoid branch of the posterior auricular artery, petrosal branch of middle meningeal artery, and the labyrinthine artery, arising from either the anterior inferior cerebellar artery, or the basilar artery.:868
Humans, and primates such as the orangutan and chimpanzee, have ear muscles that are minimally developed and non-functional, yet still large enough to be easily identifiable. These undeveloped muscles are vestigial structures. An ear muscle that cannot move the ear, for whatever reason, can no longer be said to have any biological function. This serves as evidence of homology between related species. In humans there is variability in these muscles, such that some people are able to move their ears in various directions, and it has been said that it may be possible for others to gain such movement by repeated trials. In such primates the inability to move the ear is compensated mainly by the ability to turn the head on a horizontal plane, an ability which is not common to most monkeys—a function once provided by one structure is now replaced by another.
Sound waves travel through the outer ear, are modulated by the middle ear, and are transmitted to a nerve in the inner ear, the vestibulocochlear nerve. This nerve transmits information to the temporal lobe of the brain, where it is registered as sound.
Sound that travels through the outer ear impacts on the tympanic membrane (ear drum), and causes it to vibrate. The three ossicles transmit this sound to a second window, the oval window, which protects the fluid-filled inner ear. In detail, the pinna of the outer ear helps to focus a sound, which impacts on the tympanic membrane. The malleus rests on the membrane, and receives the vibration. This vibration is transmitted along the incus and stapes to the oval window. Two small muscles, the tensor tympani and stapedius, also help modulate noise. The tensor tympani dampens noise, and the stapedius decreases the receptivity to high-frequency noise. Vibration of the oval window causes vibration of the endolymph within the ventricles and cochlea. :651–657
The hollow channels of the inner ear are filled with liquid, and contain a sensory epithelium that is studded with hair cells. The microscopic "hairs" of these cells are structural protein filaments that project out into the fluid. The hair cells are mechanoreceptors that release a chemical neurotransmitter when stimulated. Sound waves moving through fluid flows against the receptor cells of the Organ of Corti. The fluid pushes the filaments of individual cells; movement of the filaments causes receptor cells to become open to the potassium-rich endolymph. This causes the cell to depolarise, and creates an action potential that is transmitted along the spiral ganglion, which sends information through the auditory portion of the vestibulocochlear nerve to the temporal lobe of the brain. :651–657
The human ear can generally hear sounds with frequencies between 20 Hz and 20 kHz (the audio range). Although hearing requires an intact and functioning auditory portion of the central nervous system as well as a working ear, human deafness (extreme insensitivity to sound) most commonly occurs because of abnormalities of the inner ear, rather than in the nerves or tracts of the central auditory system. Sound below 20 Hz is considered infrasound, which the ear cannot process.
Providing balance, when moving or stationary, is also a central function of the ear. The ear facilitates two types of balance: static balance, which allows a person to feel the effects of gravity, and dynamic balance, which allows a person to sense acceleration.
Static balance is provided through the two ventricles, the utricle and the saccule. Cells lining the walls of these ventricles contain fine filaments, and the cells are covered with a fine gelatinous layer. Each cell has 50-70 small filaments, and one large filament, the kinocilium. Within the gelatinous layer lie otoliths, tiny formations of calcium carbonate. When a person moves, these otoliths shift position. This shift alters the positions of the filaments, which opens ion channels within the cell membranes, creating depolarisation and an action potential that is transmitted to the brain along the vestibulocochlear nerve. :692–694
Dynamic balance is provided through the three semicircular canals. These are three canals situated perpendicularly. At the end of each canal is a slight enlargement, known as the ampulla, which contains numerous cells with filaments in a central area called the cupula. The fluid in these canals rotates according to the momentum of the head. When a person changes acceleration, the inertia of the fluid changes. This affects the pressure on the cupula, and results in the opening of ion channels. This causes depolarisation, which is passed as a signal to the brain along the vestibulocochlear nerve. :692–694
Deafness refers to a partial or total loss of the ability to hear. This may be a result of injury or damage, congenital disease, or physiological. When deafness is a result of injury or damage to the outer ear or middle ear, it is known as conductive deafness. When deafness is a result of injury or damage to the inner ear, vestibulochoclear nerve, or brain, it is known as sensorineural deafness.
Vertigo refers to the inappropriate perception of motion. This is due to dysfunction of the vestibular system. One common cause of vertigo is benign paroxysmal positional vertigo, when an otolith is displaced from the ventricles to the semicircular canal. The displaced otolith rests on the cupola, causing a sensation of movement when there is none. Meniere's disease, labyrinthitis, strokes, and other infective and congenital diseases may also result in the perception of vertigo. :1151,1171
The auricle can be easily damaged. Because it is skin-covered cartilage, with only a thin padding of connective tissue, rough handling of the ear can cause enough swelling to jeopardize the blood-supply to its framework, the auricular cartilage. That entire cartilage framework is fed by a thin covering membrane called the perichondrium (meaning literally: around the cartilage). Any fluid from swelling or blood from injury that collects between the perichondrium and the underlying cartilage puts the cartilage in danger of being separated from its supply of nutrients. If portions of the cartilage starve and die, the ear never heals back into its normal shape. Instead, the cartilage becomes lumpy and distorted, a phenomenon called wrestler's ear (because wrestling is one of the most common ways such an injury occurs) or Cauliflower ear.
The lobule of the ear (ear lobe) is the one part of the human auricle that normally contains no cartilage. Instead, it is a wedge of adipose tissue (fat) covered by skin. There are many normal variations to the shape of the ear lobe, which may be small or large. Tears of the earlobe can be generally repaired with good results. Since there is no cartilage, there is not the risk of deformity from a blood clot or pressure injury to the ear lobe.
Other injuries to the external ear occur fairly frequently, and can leave minor to major deformity. Some of the more common ones include, laceration from glass, knives, and bite injuries, repeated twisting or pulling of the ear (a form of child discipline), avulsion injuries, cancer, frostbite, and burns.
Ear canal injuries can come from firecrackers and other explosives, and mechanical trauma from placement of foreign bodies into the ear. The ear canal is most often self-traumatized from efforts at ear cleaning. The outer part of the ear canal rests on the flesh of the head; the inner part rests in the opening of the bony skull (called the external auditory meatus). The skin is very different on each part. The outer skin is thick, and contains glands as well as hair follicles. The glands make cerumen (also called ear wax). The skin of the outer part moves a bit if the pinna is pulled; it is only loosely applied to the underlying tissues. The skin of the bony canal, on the other hand, is not only among the most delicate skin in the human body, it is tightly applied to the underlying bone. A slender object used to blindly clean cerumen out of the ear often results instead with the wax being pushed in, and contact with the thin skin of the bony canal is likely to lead to laceration and bleeding.
Like outer ear trauma, middle ear trauma most often comes from blast injuries and insertion of foreign objects into the ear. Skull fractures that go through the part of the skull containing the ear structures (the temporal bone) can also cause damage to the middle ear. Small perforations of the tympanic membrane usually heal on their own, but large perforations may require grafting. Displacement of the ossicles will cause a conductive hearing loss that can only be corrected with surgery. Forcible displacement of the stapes into the inner ear can cause a sensory neural hearing loss that cannot be corrected even if the ossicles are put back into proper position. Because human skin has a top waterproof layer of dead skin cells that are constantly shedding, displacement of portions of the tympanic membrane or ear canal into the middle ear or deeper areas by trauma can be particularly traumatic. If the displaced skin lives within a closed area, the shed surface builds up over months and years and forms a cholesteatoma. The -oma ending of that word indicates a tumour in medical terminology, and although cholesteatoma is not a neoplasm (but a skin cyst), it can expand and erode the ear structures. The treatment for cholesteatoma is surgical.
There are two principal damage mechanisms to the inner ear in industrialized society, and both injure hair cells. The first is exposure to elevated sound levels (noise trauma), and the second is exposure to drugs and other substances (ototoxicity).
In 1972 the U.S. EPA told Congress that at least 34 million people were exposed to sound levels on a daily basis that are likely to lead to significant hearing loss. The worldwide implication for industrialized countries would place this exposed population in the hundreds of millions. The National Institute for Occupational Safety and Health has recently published research on the estimated numbers of persons with hearing difficulty (11%) and the percentage that can be attributed to occupational noise exposure (24%). Furthermore, according to the National Health and Nutrition Examination Survey (NHANES), approximately twenty-two million (17%) US workers reported exposure to hazardous workplace noise. Workers exposed to hazardous noise further exacerbate the potential for developing noise-induced hearing loss when they do not wear (hearing protection).
The auricles also have an effect on facial appearance. In Western societies, protruding ears (present in about 5% of ethnic Europeans) have been considered unattractive, particularly if asymmetric. The first surgery to reduce the projection of prominent ears was published in the medical literature in 1881.
The ears have also been ornamented with jewelry for thousands of years, traditionally by piercing (Ear piercing) of the earlobe. In both modern and primitive cultures, ornaments are placed to stretch and enlarge the earlobes, allowing for larger plug (jewellery) to be slid into a large fleshy gap in the lobe. Tearing of the earlobe from the weight of heavy earrings, or from traumatic pull of an earring (for example by snagging on a sweater), is fairly common. The repair of such a tear is usually not difficult.
A cosmetic surgical procedure to reduce the size or change the shape of the ear is called an otoplasty. In the rare cases when no pinna is formed (atresia), or is extremely small (microtia) reconstruction of the auricle is possible. Most often, a cartilage graft from another part of the body (generally, rib cartilage) is used to form the matrix of the ear, and skin grafts or rotation flaps are used to provide the covering skin. Recently ears have been grown on a rat's back and attached to human heads after. However, when babies are born without an auricle on one or both sides, or when the auricle is very tiny, the human ear canal is ordinarily either small or absent, and the middle ear often has deformities. The initial medical intervention is aimed at assessing the baby's hearing and the condition of the ear canal, as well as the middle and inner ear. Depending on the results of tests, reconstruction of the outer ear is done in stages, with planning for any possible repairs of the rest of the ear.
The pinna helps direct sound through the ear canal to the tympanic membrane (eardrum). The complex geometry of ridges on the inner surface of some mammalian ears helps to sharply focus echolocation signals, and any sound produced by the prey. These ridges can be regarded as the acoustic equivalent of a fresnel lens, and may be seen in a large variety of unrelated animals such as the bat, aye-aye, lesser galago, bat-eared fox, mouse lemur and others.
Bat pinnae come in different sizes and shapes
Only vertebrate animals have ears, though many invertebrates detect sound using other kinds of sense organs. In insects, tympanal organs are used to hear distant sounds. They are located either on the head or elsewhere, depending on the insect family.
The tympanal organs of some insects are extremely sensitive, offering acute hearing beyond that of most other animals. The female cricket fly Ormia ochracea has tympanal organs on each side of her abdomen. They are connected by a thin bridge of exoskeleton and they function like a tiny pair of eardrums but, because they are linked, they provide acute directional information. The fly uses her "ears" to detect the call of her host, a male cricket. Depending on where the song of the cricket is coming from the fly's hearing organs will reverberate at slightly different frequencies. This difference may be as little as 50 billionths of a second, but it is enough to allow the fly to home in directly on a singing male cricket and parasitize it.
Simpler structures allow arthropod to detect near-field sounds. Spiders and cockroaches, for example, have hairs on their legs which are used for detecting sound. Caterpillars may also have hairs on their body that perceive vibrations and allow them to respond to sound.
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