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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 the entire organ 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
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.
Anatomy of the human ear. (The length of the auditory canal is exaggerated in
this image)
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.
Mammalian ear
Bat pinnae come in different sizes and shapesThe 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 pinna (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 veins
appendage on either side of the head), this portion of the ear is not vital for
hearing. The easily 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 water (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 skin 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 organic membrane is commonly called the ear drum.
The pinna helps direct sound through the ear canal to the tympanic membrane
(eardrum). The framework of the auricle consists of a single piece of yellow
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.
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. 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.
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.
Middle Ear
MalleusTensor TympaniIncusStapediusLabyrinthStapesAuditory CanalTympanic
Membrane
(Ear Drum)Eustachian TubeTympanic cavity
Components of 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
principle". 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
Inner Ear
Posterior CanalSuperior CanalUtricleHorizontal
CanalVestibuleCochleaSaccule
Components of the inner ear
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.
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. The worldwide implication for industrialized countries would place this
exposed population in the hundreds of millions.
Vestigial structures
This short section requires expansion.
Comparative anatomy of primate ears: Human (left) and Barbary Macaque (right).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. 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.
Non-vertebrate hearing organs
This short section requires expansion.
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.
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
and allow them to respond to the sound.
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