Citizendia

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. The posterior semicircular canal is a part of the Vestibular system and detects rotations of the head in the Sagittal plane The superior semicircular canal ( anterior semicircular canal) is a part of the Vestibular system and detects rotation of the head around a rostral-caudal (anterior-posterior The utricle, or utriculus along with the Saccule is one of the two Otolith organs located in the Vertebrate Inner ear. The lateral or horizontal canal (external semicircular canal is the shortest of the three canals "Vestibulum" and "vestibule" redirect here For other uses see Vestibule (disambiguation. Introduction The saccule is a bed of sensory cells situated in the inner ear the Peripheral organs of the Special senses the organs of Taste ( Peripheral gustatory or Medical Subject Headings ( MeSH) is a huge Controlled vocabulary (or metadata system for the purpose of indexing journal articles and books The inner Ear is the bony labyrinth, a system of passages comprising two main functional parts the organ of hearing or Cochlea 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. The organ of Corti (or spiral organ) is the organ in the Inner ear of Mammals that contains auditory sensory cells or " 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. Greek (el ελληνική γλώσσα or simply el ελληνικά — "Hellenic" is an Indo-European language, spoken today by 15-22 million people mainly Monotremes (from the Greek monos 'single' + trema 'hole' referring to the Cloaca) are Mammals that lay eggs ( Prototheria) instead

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.
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.

Structures

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

Function

A description of the function of the cochlea involving "some Resemblance to the System of Strings in a Harpsichord," by Benjamin Martin, 1772
A description of the function of the cochlea involving "some Resemblance to the System of Strings in a Harpsichord," by Benjamin Martin, 1772

The cochlea is filled with a watery liquid, which moves in response to the vibrations coming from the middle ear via the oval window. A harpsichord is a Musical instrument played by means of a keyboard. 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 (or vestibular 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 organ of Corti (or spiral organ) is the organ in the Inner ear of Mammals that contains auditory sensory cells or " Hair cells " Hair cells are the Sensory receptors of both the Auditory system and the Vestibular system in all Vertebrates. 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. This spatial arrangement of sound reception is referred to as tonotopy. Tonotopy (from Greek tono- and topos = place the place of tones is the spatial arrangement of where sound is perceived transmitted or received

The hair cells are arranged in four rows in the organ of Corti along the entire length of the cochlear coil. The organ of Corti (or spiral organ) is the organ in the Inner ear of Mammals that contains auditory sensory cells or " Hair cells " 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. In Anatomy, the olivary bodies or simply olives (Latin oliva and olivae, singular and plural respectively are a pair of prominent oval structures

For very low frequencies (below 20Hz), the waves propagate along the complete route of the cochlea – differentially up scala vestibuli and scala tympani all the way to the helicotrema. Frequencies this low still activate the organ of Corti to some extent, but are too low to elicit the perception of a pitch. Pitch is the property of a sound that allows the construction of melodies pitches are compared as "higher" and "lower" and are quantified as frequencies Higher frequencies do not propagate to the helicotrema.

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 person's Ears for Protection. An earplug is a device that is meant to be inserted in the Ear canal to protect the wearer's hearing from loud noises or the intrusion of water foreign bodies dust or excessive

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 (from Greek ἕλιξ meaning coil and τρη̂μα meaning hole is the part of the Cochlear labyrinth where the Scala

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. Scala vestibuli is a Perilymph filled cavity inside the Cochlea of the Inner ear. The oval window (or vestibular window) is a membrane-covered opening which leads from the Middle ear to the vestibule 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 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.

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. The cochlear duct (or scala media) is an Endolymph filled cavity inside the Cochlea, located in between the Scala tympani and the Scala The cochlear duct (or scala media) is an Endolymph filled cavity inside the Cochlea, located in between the Scala tympani and the Scala Its fluid, endolymph, also contains electrolytes and proteins, but is chemically quite different from perilymph. Endolymph is the fluid contained in the Membranous labyrinth of the Inner ear. Whereas the perilymph is rich in sodium salts, the endolymph is rich in potassium salts.

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 upper portion of the Spiral ligament contains numerous Capillary loops and small Blood vessels, and is termed the stria vascularis. Reissner's membrane ( vestibular membrane, vestibular wall) is a membrane inside the Cochlea of the Inner ear. The basilar membrane within the Cochlea of the Inner ear is a stiff structural element that separates two liquid-filled tubes that run along the coil of the cochlea The organ of Corti (or spiral organ) is the organ in the Inner ear of Mammals that contains auditory sensory cells or " Hair cells " The cochlear duct is almost as complex on its own as the ear itself.

The ear is an 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. An otoacoustic emission (OAE is a Sound which is generated from within the Inner ear. Otoacoustic emissions are important in some types of tests for hearing impairment.

Comparative physiology

The coiled form of cochlea is unique to mammals. Mammals ( class Mammalia) are a class of Vertebrate Animals characterized by the presence of Sweat glands, including sweat glands 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. Vertebrates are members of the Subphylum Vertebrata, Chordates with backbones or spinal columns The grouping sometimes includes 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. Hair cells are the Sensory receptors of both the Auditory system and the Vestibular system in all Vertebrates. 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]

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

Additional images

Vestibular system

Ear labyrinth

Right osseous labyrinth. For more uses of the word labyrinth, see Labyrinth (disambiguation The labyrinth is a System of fluid passages in the A cochlear implant (CI is a surgically implanted electronic device that provides a sense of Sound to a person who is profoundly deaf or severely hard of hearing The cochlear nerve (also auditory nerve or VIIIth nerve) is part of the Vestibulocochlear nerve, (or 8th Cranial nerve) that is found in higher The cochlear nuclei consist of (a the Dorsal cochlear nucleus, corresponding to the Tuberculum acusticum on the dorso-lateral surface of Noise health effects are the Health consequences of elevated Sound levels Elevated workplace or other Noise can cause Hearing impairment Lateral view.

Interior of right osseous labyrinth.

The cochlea and vestibule, viewed from above.

Diagrammatic longitudinal section of the cochlea.

External links

Univerzitetni klinični center Ljubljana (UKC (University Medical Centre Ljubljana is Ljubljana 's central hospital and it is also the largest hospital in Slovenia

Dictionary

cochlea

-noun

  1. (anatomy) The complex, spirally coiled, tapered cavity of the inner ear in which sound vibrations are converted into nerve impulses.
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