Ear

The ear is the sense organ that detects sound. The vertebrate ear shows a common biology from fish to humans, with variations in structure according to order and species. It is not only a receiver for sound, but also plays a major role in the sense of balance and body position.

The word 'ear' can be used for the whole vertebrate ear, or just the visible portion. In most animals, the visible ear is a flap of tissue called the pinna. The pinna may be all that shows of the ear, but it has only a tiny role in hearing and none at all in the sense of balance. In people, the pinna is more often called the auricle. All vertebrates have two ears, placed symmetrically on each side of the head; this arrangement is important for their ability to balance and their ability to localize sound.

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, birdsongs and spoken language. Although the ear is the sense organ that recognizes sound, it is the brain and central nervous system that 'hears'. Sound waves are perceived by the brain through electrical signals generated in nerve cells in the auditory portion of the central nervous system. The ear changes the pressure of sound waves from the outside world into a signal of nerve impulses (action potentials) that are sent to the brain.

The outer part of the ear collects sound; the sound pressure is amplified through the middle portion of the ear and, in land animals, passes from a medium of air into a medium of fluid. The change from air to fluid 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, heavy bone- the heaviest bone of the body. The hollow channels of the inner ear are filled with fluid, and lined by a sensory epithelium that is studded with hair cells. The part of the inner ear that is responsible for hearing is the cochlea. In mammals the cochlea is a hollow, coiled fluid-filled tube. the tube is divided lengthways by the basilar membrane, on which is perched the Organ of Corti, the structure which contains the actual sensory cells responsible for our sense of hearing.

The microscopic 'hairs' of these sensory cells are structural protein filaments that project out into the fluid. The hair cells are mechanoreceptors, and they release a chemical neurotransmitter when stimulated. Sound waves moving through fluid push the filaments, if the filament moves enough it activates the hair cell. In this way sound waves are transformed into chemical signals from the hair cells that then activate the nerve endings of fibres of the auditory nerve. The primary auditory neurons thus fire action potentials whenever the hair cells to which they are connected are stimulated.

In vision, the rods and cones of the retina play a similar role with light as hair cells do with sound. The nerve impulses travel in the auditory nerve fibres from the left and right ear to the cochlear nucleus in each side of the brainstem and eventually, through several other 'relay stations', up to the part of the cerebral cortex that is 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.

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) is usually a result of abnormalities of the inner ear, rather than the nerves or tracts of the central auditory system.

Parts of the Ear

Outer Ear (pinna, ear canal, surface of eardrum)

The outer ear is the external part of the ear, and includes the pinna] (also called auricle), the ear canal, and the most superficial layer of the ear drum (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 part is not vital for hearing. The complicated design of the human outer ear helps to capture sound, but the most important functional part is the ear canal. Unless the canal is open, hearing will be dampened. Ear wax (medical name - cerumen) is produced by glands in the skin of the outer part 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, commonly called the eardrum.

The pinna helps to direct sound through the ear canal to the eardrum. In animals with mobile pinnae (like the horse), each pinna can be aimed independently to better receive the sound. For these animals, the pinnae help to localize exactly where the sound is coming from. Human beings localize sound within the central nervous system, by comparing the loudness from each ear in brain circuits that are connected to both ears.

The outer ear and culture

Although the function of the human auricle is rudimentary in terms of hearing, the ears have an obvious effect on facial appearance. In Western societies, protruding ears (present in about 5% of the Europeans & descendants) are 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 jewellery 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 very heavy earrings, or traumatic pull of an earring, 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 auriculoplasty. In the rare cases when no pinna is formed (atresia), or is extremely small (microtia), it is possible to reconstruct the auricle. 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 for 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 often 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 (air-filled cavity behind the eardrum)

The middle ear includes most of the eardrum (tympanic membrane) and the 3 ear bones ossicles: 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 handle) that is attached to the mobile portion of the ear drum. The incus is the bridge between the malleus and stapes. The stapes is the smallest named bone in the human body. The arrangement of these three bones is a sort of Rube Goldberg device: movement of the tympanic membrane moves the first bone, which moves the second, which moves the third. When this third bone pushes down, it moves fluid within the cochlea (a portion of the inner ear). This fluid only moves when the stapes footplate is depressed into 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 the chamber of the middle ear to the back of the pharynx. The middle ear in humans is 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 temporal bone, which can be felt as a bump in the skull behind the pinna, also contains air, which ventilates 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 in the opposite direction; active opening of the Eustachian tube is required as the diver goes deeper into higher pressure.

The tympanic membrane and ossicles amplify the sound from the opening of the ear canal to the cochlea. Several mechanisms that combine to amplify the sound. The first is the "hydraulic principle". The surface area of the tympanic membrane is many times that of the stapes footplate, so sound energy that strikes the tympanic membrane is concentrated to the smaller footplate. A second mechanism is the "lever principle"; the angles between the articulating ear ossicles increase the force applied to the stapes footplate from that of the movement of the malleus (hammer). 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".

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.

The receptor cells involved in balance are also hair cells, 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. Depolarisation of vestibular hair cells stimulates the Vestibular portion of the eighth cranial nerve.

Damage to the Human Ear

Outer Ear Trauma

The auricle can easily be damaged; because it is skin-covered cartilage, with only a thin padding of connective tissue, rough handling can cause enough swelling that jeopardizes the blood-supply to its framework, the auricular cartilage. That entire cartilage framework is fed by a thin covering membrane called the perichondrium (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: 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 usually can be repaired with good results. As there is no cartilage, there is no risk of deformity from a blood clot or pressure injury to the ear lobe.

Other injuries to the outer ear occur fairly frequently, but rarely have such permanent consequences. Some of the more common ones include blast injuries 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 usually 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 can not 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 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 main damage mechanisms to the inner ear in industrialized society, and both injure hair cells. The first is exposure to very loud sounds (noise trauma), and the second is exposure to drugs and other substances (ototoxicity). In 1972, the U.S. Environmental Protection Agency told the U.S. 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.

Non-vertebrate hearing organs

Only vertebrate animals have ears, although many invertebrates can detect sounds 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 sound.

See also

• Georg von Békésy, winner of the 1961 Nobel Prize for his research on the cochlea

External links

• Protein behind hearing
• Effect of sound waves (Flash animation)
• 3D Ear page