Jump to content

Ear

Edit links
From Wikipedia, the free encyclopedia

This is an old revision of this page, as edited by 199.195.109.138 (talk) at 16:35, 27 April 2012 (→Mammalian ear). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

For other uses, see Ear (disambiguation).
Ear
Human (external) ear
Details
Identifiers
LatinCras
MeSHD004423
NeuroLex IDbirnlex_1062
TA98A01.1.00.005
A15.3.00.001
TA26861
FMA52780
Anatomical terminology

The ear is the organ that detects sound. It not only receives sound, but also aids in 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 mammals, the visible ear is a flap of tissue that is also called the pinna and is the first of many steps in hearing. In humans, the pinna is often called the auricle. 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.

Introduction to ears and hearing

Audition is the scientific name for the sense 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. [[File:Anatomy of the Human Ear.svg|thumb|300px|Anatomy of the human ear. The length of the auditory canal is exaggerated for viewing purposes 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. The human ear 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]


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.[2]

Damage to the human ear

Outer ear trauma

Auricle

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

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.[3] 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%).[4] Furthermore, according to the National Health and Nutrition Examination Survey (NHANES), approximately twenty-two million (17%) US workers reported exposure to hazardous workplace noise.[5] Workers exposed to hazardous noise further exacerbate the potential for developing noise induced hearing loss when they do not wear (hearing protection).

Vestigial structures

[[File:Darwin-s-tubercle.jpg|Right|thumb|Comparative anatomy of primate ears: Human (left) and Barbary Macaque (right).]]

Human ear (from Descent of Man)

It has long been known that humans, and indeed 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.[6] These undeveloped muscles are 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.[6] 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.[7]

The outer structure of the ear also shows some vestigial features, such as the node or point on the helix of the ear known as Darwin's tubercle which is found in around 10% of the population, this feature is labelled (a) in the accompanying figure.

Invertebrate hearing organs

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.[8]

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.[9]

Simpler structures allow 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[10] and allow them to respond to the sound.

See also

3

References

  1. ^ Greinwald, John H. Jr MD; Hartnick, Christopher J. MD The Evaluation of Children With Hearing Loss. Archives of Otolaryngology — Head & Neck Surgery. 128(1):84-87, January 2002
  2. ^ Anson and Donaldson, Surgical Anatomy of the Temporal Bone, 4th Edition, Raven Press, 1992
  3. ^ Senate Public Works Committee, Noise Pollution and Abatement Act of 1972, S. Rep. No. 1160, 92nd Cong. 2nd session.
  4. ^ Tak SW, Calvert GM, "Hearing Difficulty Attributable to Employment by Industry and Occupation: An Analysis of the National Health Interview Survey - United States, 1997 to 2003," J. Occup. Env. Med. 2008, 50:46-56
  5. ^ Tak SW, Davis RR, Calvert GM "Exposure to Hazardous Workplace Noise and Use of Hearing Protection Devices Among US WOrkers, 1999-2004," Am. J. Ind. Med. 2009, 52:358-371
  6. ^ a b Darwin, Charles (1871). The Descent of Man, and Selection in Relation to Sex. John Murray: London.
  7. ^ Mr. St. George Mivart, Elementary Anatomy, 1873, p. 396.
  8. ^ Yack, JE, and JH Fullard, 1993. What is an insect ear? Ann. Entomol. Soc. Am. 86(6): 677-682.
  9. ^ Piper, Ross (2007), Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, Greenwood Press.
  10. ^ Scoble, M. J. 1992. The Lepidoptera: Form, function, and diversity. Oxford University Press
Look up ear in Wiktionary, the free dictionary.
Wikimedia Commons has media related to Ears.


g:Ausis]]

Retrieved from "https://en.wikipedia.org/w/index.php?title=Ear&oldid=489493035"