The ear is the sense organ that detects sounds. The
vertebrate ear shows a common biology from fish to
humans, with variations in structure according to order and species. It not only acts as a
receiver for sound, but plays a major role in the sense of balance and body position.
The ear is part of the auditory system.
The word "ear" may be used correctly to describe rajdeep upal, or just the visible portion. In most animals, the visible ear
is a flap of tissue that is also called the pinna. The pinna may be all that shows of the
ear, but it serves only the first of many steps in hearing and plays no role in the sense of balance. In people, the pinna is
often called the auricle. Vertebrates have a pair of ears, placed symmetrically on opposite sides of the head. This
arrangement aids in the ability to localize sound sources.
Introduction to ears and hearing
Anatomy of the human ear. (The length of the auditory canal is greatly exaggerated in this image)
Audition is the scientific name for the perception of sound. Sound is a form of energy that moves through air, water, and other matter, in waves of pressure. Sound is the
means of auditory communication, including frog calls, bird songs and spoken language. Although the ear is the vertebrate sense
organ that recognizes sound, it is the brain and central nervous system that "hears". Sound waves are perceived by the brain
through the firing of nerve cells in the auditory portion of the central nervous system. The ear
changes sound pressure waves from the outside world into a signal of nerve impulses sent to the brain.
The outer part of the ear collects sound. That sound pressure is amplified through the
middle portion of the ear and, in land animals, passed from the medium of air into a liquid medium. The change from air to liquid
occurs because air surrounds the head and is contained in the ear canal and middle ear, but not in the inner ear. The
inner ear is hollow, embedded in the temporal bone, the densest bone of the body. 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 push the filaments; if the filaments bend over enough it causes the hair cells to fire. In this way sound waves are
transformed into nerve impulses. In vision, the rods and cones of the retina play a similar role with light as the hair cells do with sound. The nerve impulses travel from the left
and right ears through the eighth cranial nerve to both sides of the brain stem and up to
the portion of the cerebral cortex dedicated to sound. This auditory part of the
cerebral cortex is in the temporal lobe.
The part of the ear that is dedicated to sensing balance and position also sends impulses through the eighth cranial nerve,
the VIIIth nerve's Vestibular Portion. Those impulses are sent to the vestibular portion of the central nervous system.
Humans can generally hear sounds with frequencies between 20 Hz and 20 kHz (the audio range). Although the sensation of 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 the nerves or tracts of the central
auditory system.[1]
Mammalian ear
Bat
pinnae come in different sizes and shapes
The shape of outer ear of mammals varies widely across species. However the inner workings of mammalian ears (including
humans') are very similar.
Parts of the ear
Outer ear (pinna, ear canal, surface of ear drum)
The outer ear is the most external portion of the ear. The outer ear includes the
pinnae (also called auricle), the ear canal, and the very most superficial layer of the ear drum
(also called the tympanic membrane). In humans, and almost all vertebrates, the only visible portion of the ear is the outer ear.
Although the word "ear" may properly refer to the pinna (the flesh covered cartilage appendage on either side of the head), this
portion of the ear is not vital for hearing. The complicated design of the human outer ear does help capture sound (and
imposes filtering that helps distinguish the direction of the sound source), but the most important functional aspect of the
human outer ear is the ear canal itself. Unless the canal is open, hearing will be dampened. Ear wax (medical name -
cerumen) is produced by glands in the skin of the outer portion of the ear canal. This outer ear
canal skin is applied to cartilage; the thinner skin of the deep canal lies on the bone of the skull. Only the thicker
cerumen-producing ear canal skin has hairs. The outer ear ends at the most superficial layer of the tympanic membrane. The
tympanic membrane is commonly called the ear drum.
The pinna helps direct sound through the ear canal to the tympanic membrane (eardrum). 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.
Human outer ear and culture
Extensive ear modification.
Although the function of the human auricle is rudimentary in terms of hearing, the ears do have an effect on facial
appearance. In Western societies, protruding ears (present in about 5% of the Europeans & descendants) 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 jewelery for thousands of years, traditionally by piercing of the earlobe. In some cultures, ornaments are placed to stretch and
enlarge the earlobes to make them very large. Tearing of the earlobe from the weight of heavy earrings, or from traumatic pull of
an earring (for example by snagging on a sweater being removed), is fairly common.[2] 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 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. However, when babies are born without an auricle on one or both sides, or when the auricle is very tiny, the 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.[3][4][5]
Middle ear
The middle ear, an air-filled cavity behind the ear drum (tympanic membrane),
includes the three ear bones or ossicles: the malleus (or hammer), incus (or anvil), and stapes
(or stirrup). The opening of the Eustachian tube is also within the middle ear. 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 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).
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 pharynx. 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.
The arrangement of the tympanic membrane and ossicles works to efficiently couple the sound from the opening of the ear canal
to the cochlea. There are several simple mechanisms that combine to increase the sound pressure. The first is the "hydraulic
principle". The surface area of the tympanic membrane is many times that of the stapes footplate. Sound energy strikes the
tympanic membrane and is concentrated to the smaller footplate. A second mechanism is the "lever principal". The dimensions of
the articulating ear ossicles lead to an increase in the force applied to the stapes footplate compared with that applied to the
malleus. A third mechanism channels the sound pressure to one end of the cochlea, and protects the other end from being struck by
sound waves. In humans, this is called "round window protection", and will be more fully discussed in the next section.
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.
Inner ear: cochlea, vestibule, and semi-circular canals
The inner ear includes both the organ of hearing (the cochlea) and a sense organ that is attuned to the effects of both gravity and motion (labyrinth or vestibular apparatus). The balance portion of the inner ear consists of three
semi-circular canals and the vestibule.
The inner ear is encased in the hardest bone of the body. Within this ivory hard bone, there are fluid-filled hollows. Within the
cochlea are three fluid filled spaces: the tympanic canal, the vestibular canal, and the middle canal. The eighth cranial nerve
comes from the brain stem to enter the inner ear. When sound strikes the ear drum, the movement is transferred to the footplate
of the stapes, which presses into one of the fluid-filled ducts of the cochlea. The fluid inside this duct is moved, flowing
against the receptor cells of the Organ of Corti, which fire. These stimulate the
spiral ganglion, which sends information through the auditory portion of the eighth
cranial nerve to the brain.
Hair cells are also the receptor cells involved in balance, although the hair cells of the auditory and vestibular systems of
the ear are not identical. Vestibular hair cells are stimulated by movement of fluid in the semicircular canals and the utricle
and saccule. Firing of vestibular hair cells stimulates the Vestibular portion of the eighth cranial nerve.[6]
Damage to the human ear
Outer ear trauma
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.
Wrestler's Ear is one term used to describe the result, because wrestling is one of the most common ways such an injury
occurs. Cauliflower ear is another name for the same condition, because the
thickened auricle can resemble that vegetable.
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 a major deformity. Some of the more common ones
include, laceration from glass, knives, and bite injuries, 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.
Middle ear trauma
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.
Inner ear trauma
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.[7] The worldwide implication for industrialized countries would place this exposed
population in the hundreds of millions.
Vestigial structures
It has long been known that humans, and indeed other 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.[8] These undeveloped muscles are known as vestigial
structures. A 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.[8]
Non-vertebrate hearing organs
Only vertebrate animals have ears, although many invertebrates are able to detect sound using other kinds of sense organs. In
insects, tympanal organs are used to hear distant sounds. They are not confined to the
head, but can occur in different locations depending on the group of insects.[9]
Simpler structures allow arthropods to detect near-at-hand 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[10] and allow them to respond to the
sound.
References
- ^ Greinwald, John H. Jr MD; Hartnick, Christopher J. MD The Evaluation of
Children With Sensorineural Hearing Loss. Archives of Otolaryngology — Head & Neck Surgery. 128(1):84-87, January 2002
- ^ Deborah S. Sarnoff, Robert H.
Gotkin, and Joan Swirsky (2002). Instant Beauty: Getting Gorgeous on Your Lunch Break. St. Martin's Press. ISBN
031228697X.
- ^ Lam SM. Edward Talbot Ely: father of aesthetic otoplasty. [Biography.
Historical Article. Journal Article] Archives of Facial Plastic Surgery. 6(1):64, 2004 Jan-Feb.
- ^ Siegert R. Combined reconstruction of congenital auricular atresia and
severe microtia. [Evaluation Studies. Journal Article] Laryngoscope. 113(11):2021-7; discussion 2028-9, 2003 Nov.
- ^ Trigg DJ. Applebaum EL. Indications for the surgical repair of unilateral
aural atresia in children. [Review] [33 refs] [Journal Article. Review] American Journal of Otology. 19(5):679-84; discussion
684-6, 1998 Sep.
- ^ Anson and Donaldson, Surgical Anatomy of the Temporal Bone, 4th Edition,
Raven Press, 1992
- ^ Senate Public Works Committee, Noise Pollution and Abatement Act of 1972,
S. Rep. No. 1160, 92nd Cong. 2nd session.
- ^ a b Darwin, Charles (1871).
The Descent of Man, and Selection in Relation to
Sex. John Murray: London.
- ^ Yack, JE, and JH Fullard, 1993. What is an insect ear? Ann. Entomol. Soc.
Am. 86(6): 677-682.
- ^ Scoble, MJ. 1992. The Lepidoptera: Form, function, and diversity. Oxford
Univ. Press.
See also
External links
