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Encyclopedia > Cochlea
Cochlea
Cross section of the cochlea.
Parts of the inner ear, showing the cochlea
Gray's subject #232 1050
MeSH Cochlea

The cochlea is the auditory portion of the inner ear. Its core component is the Organ of Corti, the sensory organ of hearing, which is distributed along the partition separating fluid chambers in the coiled tapered tube of the cochlea. made in inkscape File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Image File history File links Bony_labyrinth. ... The posterior semicircular canal, vertical like the superior, is directed backward, nearly parallel to the posterior surface of the petrous bone; it is the longest of the three canals, measuring from 18 to 22 mm. ... The introduction to this article provides insufficient context for those unfamiliar with the subject matter. ... The utricle, larger than the saccule, is of an oblong form, compressed transversely, and occupies the upper and back part of the vestibule, lying in contact with the recessus ellipticus and the part below it. ... The lateral or horizontal canal (external semicircular canal) is the shortest of the three canals. ... This is a page about the part of the ear. ... Categories: Stub ... Medical Subject Headings (MeSH) is a huge controlled vocabulary (or metadata system) for the purpose of indexing journal articles and books in the life sciences. ... Inner ear The inner ear is the bony labyrinth, a system of passages comprising two main functional parts: the organ of hearing, or cochlea and the vestibular apparatus, the organ of balance that consists of three semicircular canals and the vestibule. ... The organ of Corti is the organ in the inner ear of mammals that contains auditory sensory cells, or hair cells. // Structure and function It has highly specialized structures that respond to fluid-borne vibrations in the cochlea with a shearing vector in the hairs of some cochlear hair cells. ...


The name is from the Latin for snail, which is from the Greek kokhlias "snail, screw," from kokhlos "spiral shell,"(etymology) in reference to its coiled shape; the cochlea is coiled in most mammals, monotremes being the exceptions. Families †Kollikodontidae Ornithorhynchidae Tachyglossidae †Steropodontidae Monotremes (monos, single + trema, hole; refers to the cloaca) are mammals that lay eggs instead of giving birth to live young like marsupials (Metatheria) and placental mammals (Eutheria). ...

Contents

Anatomy

Structural diagram of the cochlea showing how fluid pushed in at the oval window moves, deflects the cochlear partition, and bulges back out at the round window.

Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ...

Structures

The cochlea is a spiralled, hollow, conical chamber of bone. Its structures include:

  • the scala vestibuli (containing perilymph), which lies superior to the cochlear duct and abuts the oval window.
  • the scala tympani (containing perilymph), which lies inferior to the scala media and terminates at the round window.
  • the scala media (containing endolymph), which is the membranous cochlear duct containing the organ of Corti.
  • the helicotrema is the location where the scala tympani and the scala vestibuli merge
  • Reissner's membrane separates the scala vestibuli from the scala media.
  • The basilar membrane, a main structural element that determines the mechanical wave propagation properties of the cochlear partition, separates the scala media from the scala tympani.
  • The Organ of Corti is the sensory epithelium, a cellular layer on the basilar membrane, powered by the potential difference between the perilymph and the endolymph. It is lined with hair cells—sensory cells topped with hair-like structures called stereocilia.

Scala vestibuli is a perilymph filled cavity inside the cochlea of the inner ear. ... Perilymph is a fluid located within the cochlea (part of the ear) in 2 of its 3 muscles; the scala typmani and scala vestibuli. ... Scala tympani is the name of one of the perilymph filled cavities in the cochlear labyrinth. ... Perilymph is a fluid located within the cochlea (part of the ear) in 2 of its 3 muscles; the scala typmani and scala vestibuli. ... The round window is one of two membranes that separates the inner ear from the middle ear. ... Scala media is a endolymph filled cavity inside the cochlea, located in between the scala tympani and the scala vestibuli, separated by the basilar membrane and Reissners membrane(the vestibular membrane) respectively. ... Endolymph is the fluid contained in the membranous labyrinth of the inner ear. ... The organ of Corti is the organ in the inner ear of mammals that contains auditory sensory cells, or hair cells. // Structure and function It has highly specialized structures that respond to fluid-borne vibrations in the cochlea with a shearing vector in the hairs of some cochlear hair cells. ... The helicotrema is the part of the cochlear labyrinth where the scala tympani and the scala vestibuli meet. ... Reissners membrane is a membrane inside the cochlea of the inner ear, it separates scala media from scala vestbuli and together with the basilar membrane it creates a compartment in the cochlea filled with perilymph, which is important for the function of the organ of Corti inside the scala... Cross section of the cochlea. ... The organ of Corti is the organ in the inner ear of mammals that contains auditory sensory cells, or hair cells. // Structure and function It has highly specialized structures that respond to fluid-borne vibrations in the cochlea with a shearing vector in the hairs of some cochlear hair cells. ... Hair cells are the sensory cells of the auditory system that are found within the cochleas organ of Corti. ... Section through the spiral organ of Corti. ...

Function

In brief: the cochlea is filled with a watery liquid, which moves in response to the vibrations coming from the middle ear via the oval window. As the fluid moves, thousands of "hair cells" are set in motion, and convert that motion to electrical signals that are communicated via neurotransmitters to many thousands of nerve cells. These primary auditory neurons transform the signals into electrical impulses known as action potentials, which travel along the auditory nerve to structures in the brainstem for further processing.


The stapes of the middle ear transmits to the fenestra ovalis (oval window) on the outside of the cochlea, which vibrates the perilymph (fluid) in the scala vestibuli (upper chamber of the cochlea). The stapes or stirrup is the stirrup-shaped small bone or ossicle in the middle ear which attaches the incus to the fenestra ovalis, the oval window which is adjacent to the vestibule of the inner ear. ... The fenestra ovalis or oval window is a membrane-covered opening which leads from the middle ear to the vestibule of the inner ear. ...


This motion of perilymph in turn vibrates the endolymph in the scala media, the perilymph in the scala tympani, the basilar membrane, and organ of Corti, thus causing movements of the hair bundles of the hair cells, acoustic sensor cells that convert vibration into electrical potentials. The hair cells in the organ of Corti are tuned to certain sound frequencies[1], being responsive to high frequencies near the oval window and to low frequencies near the apex of the cochlea. The organ of Corti is the organ in the inner ear of mammals that contains auditory sensory cells, or hair cells. // Structure and function It has highly specialized structures that respond to fluid-borne vibrations in the cochlea with a shearing vector in the hairs of some cochlear hair cells. ... Hair cells are the sensory cells of both the auditory system and the vestibular system in all vertebrates. ...


The hair cells are arranged in four rows in the organ of Corti along the entire length of the cochlear coil. Three rows consist of outer hair cells (OHCs) and one row consists of inner hair cells (IHCs). The inner hair cells provide the main neural output of the cochlea. The outer hair cells, instead, mainly receive neural input from the brain, which influences their motility as part of the cochlea’s mechanical pre-amplifier. The input to the OHC is from the olivary body via the medial olivocochlear bundle. The organ of Corti is the organ in the inner ear of mammals that contains auditory sensory cells, or hair cells. // Structure and function It has highly specialized structures that respond to fluid-borne vibrations in the cochlea with a shearing vector in the hairs of some cochlear hair cells. ... In anatomy, the olivary bodies or simply olives (Latin oliva and olivae, singular and plural, respectively) are a pair of prominent oval structures in the medulla oblongata, the lower portion of the brainstem. ...


For very low frequencies (below 20Hz), the pressure waves propagate along the complete route of the cochlea - up scala vestibuli, around helicotrema and down scala tympani to the round window. Frequencies this low do not activate the organ of Corti and are below the threshold for hearing. Higher frequencies do not propagate to the helicotrema but are transmitted through the endolymph in the cochlear duct to the perilymph in the scala tympani.


A very strong movement of the endolymph due to very loud noise may cause hair cells to die. This is a common cause of partial hearing loss and is the reason why users of firearms or heavy machinery should wear earmuffs or earplugs. Earmuffs are objects designed to cover a persons ears for protection. ... Silicone rubber earplugs for protection against water, dust etc. ...


Detailed anatomy

The walls of the hollow cochlea are made of bone, with a thin, delicate lining of epithelial tissue. This coiled tube is divided through most of its length by a membrane partition. Two fluid-filled spaces (scalae) are formed by this dividing membrane.


The fluid in both is called perilymph: a clear solution of electrolytes and proteins. The two scalae (fluid-filled chambers) communicate with each other through an opening at the top (apex) of the cochlea called the helicotrema, a common space that is the one part of the cochlea that lacks the lengthwise dividing membrane. The helicotrema is the part of the cochlear labyrinth where the scala tympani and the scala vestibuli meet. ...


At the base of the cochlea each scala ends in a membrane that faces the middle ear cavity. The scala vestibuli ends at the oval window, where the footplate of the stapes sits. The footplate rocks when the ear drum moves the ossicular chain; sending the perilymph rippling with the motion, the waves moving away from footplate and towards helicotrema. Those fluid waves then continue in the perilymph of the scala tympani. The scala tympani ends at the round window, which bulges out when the waves reach it -providing pressure relief. This one-way movement of waves from oval window to round window occurs because the middle ear directs sound to the oval window, but shields the round window from being struck by sound waves from the external ear. It is important, because waves coming from both directions, from the round and oval window would cancel each other out. In fact, when the middle ear is damaged such that there is no tympanic membrane or ossicular chain, and the round window is oriented outward rather than set under a bit of a ledge in the round window niche, there is a maximal conductive hearing loss of about 60 dB. Scala vestibuli is a perilymph filled cavity inside the cochlea of the inner ear. ... The stapes or stirrup is the stirrup-shaped small bone or ossicle in the middle ear which attaches the incus to the fenestra ovalis, the oval window which is adjacent to the vestibule of the inner ear. ...


The lengthwise partition that divides most of the cochlea is itself a fluid-filled tube, the third scalae. This central column is called the scala media or cochlear duct. Its fluid, endolymph, also contains electrolytes and proteins, but is chemically quite different from perilymph. Whereas the perilymph is rich in sodium salts, the endolymph is rich in potassium salts. Scala media is a endolymph filled cavity inside the cochlea, located in between the scala tympani and the scala vestibuli, separated by the basilar membrane and Reissners membrane(the vestibular membrane) respectively. ... The cochlear duct (or scala media) is an endolymph filled cavity inside the cochlea, located in between the scala tympani and the scala vestibuli, separated by the basilar membrane and Reissners membrane (the vestibular membrane) respectively. ... Endolymph is the fluid contained in the membranous labyrinth of the inner ear. ...


The cochlear duct is supported on three sides by a rich bed of capillaries and secretory cells (the stria vascularis), a layer of simple squamous epithelial cells (Reissner's membrane), and the basilar membrane, on which rests the receptor organ for hearing - the organ of Corti. The cochlear duct is almost as complex on its own as the ear itself. The upper portion of the spiral ligament contains numerous capillary loops and small blood vessels, and is termed the stria vascularis. ... Reissners membrane is a membrane inside the cochlea of the inner ear, it separates scala media from scala vestbuli and together with the basilar membrane it creates a compartment in the cochlea filled with perilymph, which is important for the function of the organ of Corti inside the scala... Cross section of the cochlea. ... The organ of Corti is the organ in the inner ear of mammals that contains auditory sensory cells, or hair cells. // Structure and function It has highly specialized structures that respond to fluid-borne vibrations in the cochlea with a shearing vector in the hairs of some cochlear hair cells. ...


The ear is a very active organ. Not only does the cochlea "receive" sound, it generates it! Some of the hair cells of the cochlear duct can change their shape enough to move the basilar membrane and produce sound. This process is important in fine tuning the ability of the cochlea to accurately detect differences in incoming acoustic information. The sound produced by the inner ear is called an otoacoustic emission (OAE), and can be recorded by a microphone in the ear canal. Otoacoustic emissions are important is some types of tests for hearing impairment. An otoacoustic emission (OAE) is a sound which is generated from within the inner ear. ... A hearing impairment or hearing loss is a full or partial decrease in the ability to detect or understand sounds. ...


Comparative physiology

The coiled form of cochlea is unique to mammals. In birds and in other non-mammalian vertebrates the compartment containing the sensory cells for hearing is occasionally also called “cochlea”, although it is not coiled up. Instead it forms a blind-ended tube, also called the cochlear duct. This difference apparently evolved in parallel with the differences in frequency range of hearing and in frequency resolution between mammals and non-mammalian vertebrates. Most bird species do not hear above 4–5 kHz, the currently known maximum being ~ 11 kHz in the barn owl. Some marine mammals hear up to 200 kHz. The superior frequency resolution in mammals is due to their unique mechanism of pre-amplification of sound by active cell-body vibrations of outer hair cells. A long coiled compartment, rather than a short and straight one, provides more space for frequency dispersion and is therefore better adapted to the highly derived functions in mammalian hearing.[2] Subclasses & Infraclasses Subclass †Allotheria* Subclass Prototheria Subclass Theria Infraclass †Trituberculata Infraclass Metatheria Infraclass Eutheria Mammals (class Mammalia) are warm-blooded, vertebrate animals characterized by the presence of sweat glands, including those that produce milk, and by the presence of: hair, three middle ear bones used in hearing, and a neocortex... This article does not cite any references or sources. ... Hair cells are the sensory cells of the auditory system that are found within the cochleas organ of Corti. ... Dispersion can mean any of several things: A phenomenon that causes the separation of a wave into components of varying frequency. ...


As the study of the cochlea should fundamentally be focused upon the level of hair cells, it is important to note the anatomical and physiological differences between the hair cells of various species. In birds, for instance, instead of outer and inner hair cells, there are tall and short hair cells. There are several similarities of note in regard to this comparative data. For one, the tall hair cell is very similar in function to that of the inner hair cell and the short hair cell is very similar in function to that of the outer hair cell. One unavoidable difference, however, is that while all hair cells are attached to a tectorial membrane in birds, only the outer hair cells are attached to the tectorial membrane in mammals.


References

  1. ^ Tasaki I. Nerve impulses in individual auditory nerve fibers of guinea pig. J Neurophysiol. 17(2): 97-122, 1954
  2. ^ Vater M, Meng J, Fox RC. Hearing organ evolution and specialization: Early and later mammals. In: GA Manley, AN Popper, RR Fay (Eds). Evolution of the Vertebrate Auditory System, Springer-Verlag, New York 2004, pp 256–288.

See also

For more uses of the word labyrinth, see Labyrinth (disambiguation) The labyrinth is a system of fluid passages in the inner ear, including both the cochlea which is part of the auditory system, and the vestibular system which provides the sense of balance. ... Illustration of the internal parts of a cochlear implant. ... The Cochlear nerve (n. ... The cochlear nuclei consist of: (a) the dorsal cochlear nucleus, corresponding to the tuberculum acusticum on the dorso-lateral surface of the inferior peduncle; and (b) the ventral or accessory cochlear nucleus, placed between the two divisions of the nerve, on the ventral aspect of the inferior peduncle. ... Roadway noise is the main source of exposure Noise health effects, the collection of health consequences of elevated sound levels, constitute one of the most widespread public health threats in industrialized countries. ...

Additional images

External links

  • MeSH Cochlea
  • "Promenade 'Round the Cochlea" by R. Pujol, S. Blatrix S. et al. at University of Montpellier
  • "The Cochlea Homepage", F. Mammano and R. Nobili of the Venetian Institute of Molecular Medicine
  • MYSTERIES OF THE COCHLEA Dr. Janez Faganel Memorial Lecture at Klinični center Ljubljana

  Results from FactBites:
 
Cochlea - Wikipedia, the free encyclopedia (445 words)
Named after the Latin word for snail shell, the cochlea is a coiled, tapered tube containing the auditory branch of the mammalian inner ear.
The stapes transmits vibrations to the fenestra ovalis (oval window) on the outside of the cochlea, which vibrates the perilymph in the scala vestibuli.
This in turn vibrates the endolymph in the scala media, thus causing movements of the hair bundles of the hair cells, which are acoustic sensor cells that convert vibration into electrical potentials.
U of M News Service (627 words)
In the biological cochlea, the basilar membrane, which winds along the cochlear spiral, is stiffer at the base and becomes softer as it approaches the center.
In the biological cochlea, sensory hair cells in the spiral detect the sound waves traveling through the fluid, and translate the sound waves into electrical signals, which the auditory nerve carries to the brain.
The goal is to use the mechanical cochlea as a sensitive microphone, perhaps in tandem with a cochlear implant, Grosh said, the same way an external microphone, a microprocessor and an antenna work together in present implants.
  More results at FactBites »

 
 

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