Ear

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(Redirected from Human ear)

For alternative meanings, see ear (disambiguation).

Image:Earcov.JPG An ear is an organ used by an animal to detect sound waves. The term may refer to the entire system responsible for collection and early processing of sound (the beginning of the auditory system), or merely the externally-visible part. Not all animals have ears in the same part of the body. Audition is the scientific name for the sense of hearing. The organ of Corti is the actual organ of hearing.

1 The mammalian ear
2 Damage mechanisms
3 Non-mammalian hearing organs
4 References
5 See also
6 External links

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The mammalian ear

Image:HumanEar.jpg Mammals, including humans, have two ears, one on each side of the head.

The outer ear is the external portion of the ear,also known as the pinna. The pinna captures the sound and transfers it through the auditory canal to the tympanic membrane (eardrum), which vibrates and transfers the sound to the ossicles of the tympanic membrane.

The middle ear includes the ossicles (three tiny bones), two muscle tendons (of the stapedius and tensor tympani muscles), and two nerve bundles (the horizontal portion of the facial nerve and a branch of the facial nerve called the chorda tympani). The Eustachian tube connects from the chamber of the middle ear to the back of the pharynx to equalize the pressure. That's why you can feel your ears "pop" when descending on an airplane

The inner ear comprises both the organ of hearing (the cochlea) and the labyrinth or vestibular apparatus, the organ of balance located in the inner ear that consists of three semicircular canals and the vestibule. Within the cochlea are located three canals: the tympanic canal, the vestibular canal, and the middle canal. When sound strikes the cochlea, the fluid inside is moved. This fluid stimulates the organ of Corti, located within the middle canal, to interpret the sound and send the information through the auditory nerve to the brain

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Damage mechanisms

The principal damage mechanism to the human ear in industrialized society is exposure to elevated sound levels. Approximately ten percent of the population in industrialized societies have significant hearing loss, and millions more are steadily progressing to that outcome. Once it was thought that only extremely high sound levels create hearing loss; however, more careful investigations showed that cumulative exposure to relatively moderate levels, such as 70 dB(A), can lead to the irreversible loss of hearing. Another myth of noise effects is the overstated role of presbycusis, or loss of hearing associated with aging. It has been demonstrated that the most important factor of hearing degradation is not aging alone, but rather the cumulative long-term exposure to environmental and occupational noise that create the harm<ref>Rosenhall, Ulf; Pedersen, Kai; Svanborg, Alvar Presbycusis and Noise-Induced Hearing Loss, Ear & Hearing, 11(4):257-263, August 1990</ref>. In the Rosenhall study, age cohort populations were tracked, with the result that noise-exposed persons had greater hearing loss than their age cohorts who were relatively unexposed to noise. In fact, it has been shown that people in non-industrialized countries do not experience the same progressive hearing loss<ref>S. Rosen and P. Olin, Hearing Loss and Coronary Heart Disease, Archives of Otollaryngology, 82:236 (1965)</ref>.

The mechanism of hearing loss arises from trauma to stereocilia of the cochlea, the principal fluid filled structure of the inner ear. The pinna (visible portion of the ear) combined with the middle ear amplifies sound pressure levels by a factor of twenty, so that extremely high sound pressure levels arrive in the cochlea, even from moderate atmospheric sound stimuli. The cilial damage is known to be cumulative and can be irreversible<ref>Schneider M.E., Belyantseva I.A., Azevedo R.B., Kachar B,. Rapid renewal of auditory hair bundles Nature. 22 Aug 2002. 418(6900): 837-838.</ref>. The most recent research indicates that high noise levels create elevated levels of reactive oxygen species in the inner ear<ref>Henderson, Donald; Bielefeld, Eric C.; Harris, Kelly Carney; Hu, Bo Hua, The Role of Oxidative Stress in Noise-Induced Hearing Loss, Ear & Hearing. 27(1):1-19, February 2006</ref>, which interfere with the regenerative process for cochlear cilia repair. This research shows why high noise levels have differing effects over a given population, and lead to a possible preventative strategy of adequate antioxidant intake.

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<ref>Senate Public Works Committee, Noise Pollution and Abatement Act of 1972, S. Rep. No. 1160, 92nd Cong. 2nd session</ref>. The worldwide implication for industrialized countries would place this exposed population in the hundreds of millions.

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