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For other uses, see Ear (disambiguation).

Ear
Human (external) ear
Details
SystemAuditory system
Identifiers
LatinAuris
MeSHD004423
NeuroLex IDbirnlex_1062
TA98A01.1.00.005
A15.3.00.001
TA26861
FMA52780
Anatomical terminology
This article is one of a series documenting the anatomy of the
Human ear
Outer ear
Middle ear
Inner ear

The ear is the organ of the sense of hearing, and in mammals is also an organ of balance. In mammals, the ear is usually described as having three parts—the outer ear, middle ear and the inner ear. The outer ear consists of the auricle or pinna, and the ear canal. The middle ear includes the tympanic cavity and the three ossicles. The inner ear consists of the bony labyrinth which contains the semicircular canals, and the utricle and saccule of the vestibular system, to do with balance, and the cochlea a part of the auditory system.

The ear develops from the first pharyngeal pouch and six small swellings that develop in the early embryo called otic placodes, which are derived from ectoderm.

A number of conditions may relate to the ear, including hearing loss, tinnitus and balance disorders including vertigo, however these may also relate to diseases affecting the pathways in the brain relating to hearing and balance.

Although the entire organ is considered as the ear, it is often just referred to as the visible outer part. 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 stage in hearing. The ears of vertebrates are placed somewhat symmetrically on either side of the head, an arrangement that aids sound localization.

Structure

Ear anatomy showing an exaggerated ear canal
An eardrum

The human ear consists of three parts—the outer ear, middle ear and inner ear.[1]

The ear canal of the outer ear meets the start of the middle ear at the eardrum (also known as the tympanic membrane).

The eardrum separates the outer ear from the air-filled tympanic cavity of the middle ear. The middle ear contains the three small bones (ossicles) involved in the transmission of sound, and is connected to the throat at the nasopharynx, via the pharyngeal opening of the Eustachian tube.

The inner ear contains the otolith organs—the utricle and saccule and the semicircular canals belonging to the vestibular system and the cochlea of the auditory system.[1]Hair cells responsible for changing mechanical changes into electrical stimuli are present in the organ of Corti in the cochlea.

Outer ear

Main article: Outer ear

The outer ear is the external portion of the ear and includes the fleshy visible auricle or pinna, the ear canal, and the outer layer of the eardrum (tympanic membrane).[1][2] The outer ear is the only visible portion of the ear in most animals; consequently, the word "ear" is often used to refer to the external part alone.[3]

Human ear (from Descent of Man). Darwin's tubercle shown at a.

The auricle consists of the curving outer rim (the helix), the inner curved rim (the antihelix), and opens into the ear canal (also known as the external acoustic meatus). The tragus protrudes and partially obscures the ear canal, as does the facing antitragus. The hollow region in front of the ear canal is called the concha. The ear canal stretches for a distance of about 1 inch (2.5 cm), and consists of an initial part surrounded by cartilage which changes to being surrounded by bone as it meets the eardrum. This bony part is known as the auditory bulla and is formed by the tympanic part of the temporal bone. The skin surrounding the ear canal contains ceruminous and sebaceous glands that produce the protective ear wax. The ear canal ends at the external surface of the eardrum.[2]

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.[2] In humans these muscles have no action or very little. [4] The ear muscles are supplied by the facial nerve, which also supplies sensation to the skin of the ear itself, as well as to the external ear cavity. The great auricular nerve, auricular nerve, auriculotemporal nerve, lesser and greater occipital nerves of the cervical plexus all supply sensation to parts of the outer ear and the surrounding skin.[2]

The auricle consists of a single piece of elastic cartilage with a complicated relief on its inner surface and a fairly smooth configuration on its posterior surface . A tubercle known as Darwin's tubercle, is sometimes present and this lies in the descending part of the helix and corresponds to the ear-tip of mammals. The earlobe consists of areola and adipose tissue.[5] The symmetrical arrangement of the ears allows for the localisation of sound. This is carried out by comparing arrival-times and intensities from each ear, in brain circuits in the superior olivary complex and the trapezoid bodies that are connected via pathways to both ears.[6]

Middle ear

Main article: Middle ear
The middle ear

The middle ear lies between the outer ear and the inner ear. It consists of an air-filled cavity called the tympanic cavity and includes the auditory ossicles and their attaching ligaments, the Eustachian or auditory tube and the round and oval windows. The ossicles are three small bones that function together to receive, amplify and transmit the sound from the eardrum to the inner ear. The ossicles are the malleus (hammer), incus (anvil), and the stapes ( stirrup) the smallest named bone in the body. The middle ear also connects to the upper throat at the nasopharynx via the pharyngeal opening of the Eustachian tube.[2][7]

The three ossicles transmit sound from the tympanic membrane to the secondary tympanic membrane which is situated within the round window to the inner ear. The malleus is connected at its longest part (the manubrium or handle) by a ligament to the tympanic membrane where it rests, and receives the vibrations of the eardrum produced by sound waves. The incus bone is the bridge between the malleus and stapes. The footplate of the stapes bone connects to the oval window and is moved by the movements of the malleus and incus. The three bones are arranged so that movement of the eardrum 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 (the auditory portion of the inner ear).[2] The round window allows for the fluid within the inner ear to move. As the stapes pushes the secondary tympanic membrane, fluid in the inner ear moves and pushes the membrane of the round window out by a corresponding amount (into the middle ear). The ossicles help in amplification of sound waves by nearly 15-20 times.[1]

Inner ear

Inner ear
Main article: Inner ear

The inner ear is made up of a bony labyrinth and a membranous labyrinth. A central area known as the vestibule contains the vestibular sensory organs for balance and motion, the utricle and saccule, and also the semicircular canals. The bony labyrinth contains the auditory portion of the inner ear, the cochlea.[2][8]

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 vestibular duct, the cochlear duct, and the tympanic duct.[2]

Blood supply

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.[2][9] 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.[2] 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.[2]

Function

The outer ear receives sound, transmitted through the ossicles of the middle ear to the inner ear, where it is converted to a nervous signal in the cochlear and transmitted along the vestibulocochlear nerve

Hearing

Main article: Hearing

Sound waves travel through the outer ear, are modulated by the middle ear, and are transmitted to the vestibulocochlear nerve in the inner ear. 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 eardrum, and causes it to vibrate. The three ossicles bones 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 eardrum. 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 vestibule and the cochlea.[10]

The inner ear houses the apparatus necessary to change the vibrations transmitted from the outside world via the middle ear into signals passed along the vestibulocochlear nerve to the brain. 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.[10]

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.[11]

Balance

Main articles: Balance (ability) and Equilibrioception

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.[10][12]

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.[10]

Development

During embryogenesis the ear develops as three distinct structures: the inner ear, the middle ear and the outer ear.[13] Each structure originates from a different germ layer: the ectoderm, endoderm and mesenchyme.[14][15]

Inner ear

The first part of the ear to develop is the inner ear,[15] which begins to form from the ectoderm around the 22nd day of the embryo’s development.[14] Specifically the inner ear derives from a set of neurogenic placodes called otic placodes. Each otic placode forms the otic vesicle. This epithelial mass will immerse itself and eventually be surrounded by mesenchyme to form the bony labyrinth.[16]

Before any of the vital components of the inner ear can be formed, a group of sensory cells called a saccule forms the inner ear’s epithelium. Part of the saccule will eventually give rise and connect to the cochlear duct. This duct appears approximately during the sixth week and connects to the saccule through the ductus reuniens.[14] As the cochlear duct’s mesenchyme begins to differentiate, three cavities are formed: the scala vestibule, the scala tympani and the scala media.[14][16] Both the scala vestibule and the scala tympani contain an extracellular fluid called perilymph. The scala media contains endolymph.[16] A set of membranes called the vestibular membrane and the basilar membrane separate the cochlear duct from the vestibular duct and the tympanic duct. A spiral ligament and a cartilaginous process called the modiolus connect and support the cochlear duct to the rest of the cartilaginous structures that surround it.[14] The organ of Corti is made up of sensory cells and a tectorial membrane. The utricule and saccule generate sensory areas called the maculae acusticae. The otic vesicle in turn forms the vestibulocochlear nerve. The structures of the inner part work together in the adult ear to convert the signals that they receive from the middle and external ears and transfer them to the brain where they can be processed.[17]

Molecular regulation

Most of the genes responsible for the regulation of inner ear formation and its morphogenesis are members of the homeobox gene family such as Pax, Msx and Otx homeobox genes. The development of inner ear structures such as the cochlea is regulated by Dlx5/Dlx6, Otx1/Otx2 and Pax2, which in turn are controlled by the master gene Shh. Shh is secreted by the notochord.[18]

Middle ear

The middle ear, which includes the tympanic cavity and the Eustacian tube, originates from the first pharyngeal pouch.[15] More specifically, the tubotympanic recess originates from the distal part of the pouch while the eustachian tube from the part next to it. This last structure will establish the final connection between the tympanic cavity and the nasopharynx.[14] The auditory ossicles (malleus, incus and stapes), originate to form the second pharyngeal pouch are embedded in the tympanic cavity and normally appear during the first half of fetal life. The first two (malleus and incus) derive from the first pharyngeal arch and the stapes from the second.[14] Eventually cells from the tissue surrounding the ossicles will experience apoptosis and a new layer of endodermal epithelial will constitute the formation of the tympanic cavity wall.[14][15] The mastoid process will appear as the tympanic cavity continues to grow.[17]

Outer ear

Unlike structures of the inner and middle ear, which develop from pharyngeal pouches, the ear canal originates from the dorsal portion of the first pharyngeal cleft.[14][15] It is fully expanded by the end of the 18th week of development.[16] The tympanic membrane or eardrum is made up of three layers (ectoderm, endoderm and connective tissue) all of which form the fibrous membrane of the joint capsule. The auricle originates as a fusion of six hillocks, prominences that are derived from the first pharyngeal arch. The first three form the tragus, crus of the helix, and helix, respectively. The final three hillocks are derived from the second pharyngeal arch and form the antihelix, antitragus, and earlobe.[14][15][16] The outer ears are firstly situated in the lower neck region. As the mandible forms they move towards their final position level with the eyes. Once the outer ear is fully developed, it functions both to capture sound from the outside and to conduct it through the ear canal to the tympanic membrane.[13][17]

Clinical significance

Deafness

Main article: Hearing loss

Deafness is either partial or total hearing loss. This may be a result of injury or damage, congenital disease, or physiological causes. When deafness is a result of injury or damage to the outer ear or middle ear, it is known as conductive hearing loss. When deafness is a result of injury or damage to the inner ear, vestibulochoclear nerve, or brain, it is known as sensorineural hearing loss.

Abnormalities of the middle ear 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 eardrum. Sometimes artificial ear bones are placed to substitute for damaged ones, or a disrupted ossicular chain is rebuilt in order to conduct sound effectively.

Congenital abnormalities

Approximately one out of one thousand children suffer some type of congenital deafness related to the development of the inner ear.[19] Inner ear congenital anomalies are related to sensorineural hearing loss and are generally diagnosed with a computed tomography (CT) scan or a magnetic resonance imaging (MRI) scan.[20] Hearing loss problems also derive from inner ear anomalies because its development is separate from that of the middle and external ear.[15] Middle ear anomalies can occur because of errors during head and neck development. The first pharyngeal pouch syndrome associates middle ear anomalies to the malleus and incus structures as well as to the non-differentiation of the annular stapedial ligament. Temporal bone and ear canal anomalies are also related to this structure of the ear and are known to be associated with sensorineural hearing loss and conductive hearing loss.[20] Auricular anomalies and minor malformations are common. These types of anomalies include chromosome syndromes such as ring 18. Children may also present cases of abnormal ear canals and low ear implantation.[15] Small auricles can develop when the auricular hillocks do not develop properly. Atresia of the ear canal can occur if the ear canal does not channelize properly or if there is an obstruction.[15] Reconstructive surgery to treat hearing loss is considered as an option for children older than five.[20]

Vertigo

Main article: Vertigo

Vertigo refers to the inappropriate perception of motion. This is due to dysfunction of the vestibular system. One common type 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. Ménière's disease, labyrinthitis, strokes, and other infective and congenital diseases may also result in the perception of vertigo.[21]

Injury

Outer ear

Injuries to the external ear occur fairly frequently, and can leave minor to major deformity. Injuries include, laceration, avulsion injuries, burn and repeated twisting or pulling of an ear, for discipline or even torture.[22] Chronic damage to the ears can cause a phenomenon called cauliflower ear a common condition of boxers and wrestlers, in which the cartilage around the ears becomes lumpy and distorted owing to persistence of a haematoma around the perichondrium, which can impair blood supply and healing.[23] The external ear is frequently affected in frostbite,[24] and owing to its exposed position can be affected by skin cancers including Squamous-cell carcinoma and Basal-cell carcinomas.[25]

Middle ear

The ear drum may become perforated in the event of a large sound or explosion, when diving or flying (called [[barotrauma]]), or because of attempts at cleaning or objects inserted into the ear. Another common cause is following an infection such as [[otitis media]].[26] These may cause a discharge from the ear called Otorrhea,[27] and are often investigated by otoscopy and audiometry. Treatment may include watchful waiting, antibiotics and possibly surgery, if the injury is prolonged or the position of the ossicles is affected.[28] 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.[29] A cholesteatoma is a cyst of squamous skin cells that may develop from birth or secondary to other causes such as chronic ear infections. It may impair hearing or cause dizziness or vertigo, and is usually investigated by otoscopy and may require a CT scan. The treatment for cholesteatoma is surgical.[30]

Inner ear

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). A large number of people are exposed to sound levels on a daily basis that are likely to lead to significant hearing loss.[31] 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%).[32] Furthermore, according to the National Health and Nutrition Examination Survey (NHANES), approximately twenty-two million (17%) US workers reported exposure to hazardous workplace noise.[33] Workers exposed to hazardous noise further exacerbate the potential for developing noise-induced hearing loss when they do not wear (hearing protection).

Society and culture

Stretching of the earlobe and various cartilage piercings

The ears have been ornamented with jewelry for thousands of years, traditionally by piercing of the earlobe. In ancient and modern cultures, ornaments have been placed to stretch and enlarge the earlobes, allowing for larger plugs 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.[34]

Injury to the ears has been present since Roman times as a method of reprimand or punishment - "In Roman times, when a dispute arose that could not be settled amicably, the injured party cited the name of the person thought to be responsible before the Praetor; if the offender did not appear within the specified time limit, the complainant summoned witnesses to make statements. If they refused, as often happened, the injured party was allowed to drag them by the ear and to pinch them hard if they resisted. Hence the French expression "se faire tirer l’oreille", of which the literal meaning is "to have one's ear pulled" and the figurative meaning "to take a lot of persuading". We use the expression "to tweak (or pull) someone's ears" to mean "inflict a punishment"." [22]

The auricles 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.[35] The first surgery to reduce the projection of prominent ears was published in the medical literature by Ernst Dieffenbach in 1845 and the first case report in 1881.[36]

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. Based on technology developed in 1997 and demonstrated by the "earmouse", an ear shape can be grown in the patient's body using their own cartilage to seed a frame temporarily implanted under the skin.

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.[37][38][39]

Pointy ears have been a characteristic of some creatures in folklore such as French croquemitaine, Brazilian curupira[40] or Japanese earth spider.[41] It has been a feature of characters on art as old as that of Ancient Greece[42] and medieval Europe.[43] Pointy ears are common characteristic of many creatures in the fantasy genre.[44] It's a common characteristics of races such as, among others, elves,[45][46][47] faeries,[48][49] pixies,[50] hobbits,[51] or orcs.[52] They are also a characteristic of creatures from the horror genre, such as vampires.[53][54] Pointy ears are also found in the science fiction genre; for example among the Vulcan and Romulan races of the Star Trek universe[55] or the Nightcrawler character from the X-Men universe.[56]

Other animals

Primate ears: human and Barbary macaque with Darwin's tubercle highlighted
Various bat pinnae

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 wide range of animals including the bat, aye-aye, lesser galago, bat-eared fox, mouse lemur and others.[57][58][59]

Some large primates such as gorillas and orang-utans (and also humans) have undeveloped ear muscles that are non-functional vestigial structures, yet are still large enough to be easily identified.[60] An ear muscle that cannot move the ear, for whatever reason, has lost that 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.[60] In such primates the inability to move the ear is compensated for, mainly by the ability to easily 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.[61]

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.

Invertebrates

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.[62] 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.[63]

Simpler structures allow other arthropods 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[64] and allow them to respond to sound.

See also

3

References

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