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The organ of Corti, or spiral organ, is the receptor organ for hearing and is located in the mammalian
cochlea The cochlea is the part of the inner ear Inner Ear (established in 2007 in Bodø, Norway) is a Norwegian record label initiated and led by the brothers and jazz musicians Tore Johansen and Roger Johansen (musician), Roger Johansen. They opene ...

cochlea
. This highly varied strip of
epithelial cells Epithelium () is one of the four basic types of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume ...
allows for transduction of auditory signals into nerve impulses'
action potential In physiology Physiology (; ) is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence ...

action potential
. Transduction occurs through vibrations of structures in the inner ear causing displacement of cochlear fluid and movement of
hair cell Hair cells are the sensory receptor Sensory neurons, also known as afferent neurons, are neuron A neuron or nerve cell is an membrane potential#Cell excitability, electrically excitable cell (biology), cell that communicates with other cel ...
s at the organ of Corti to produce electrochemical signals.The Ear
Pujol, R., Irving, S., 2013
Italian anatomist
Alfonso Giacomo Gaspare Corti Alfonso Giacomo Gaspare Corti (22 June 1822 – 2 October 1876) was born at Gambarana, near Pavia Pavia (, , , ; la, Ticinum; Medieval Latin: ) is a town and comune of south-western Lombardy in northern Italy, south of Milan on the lower Ti ...

Alfonso Giacomo Gaspare Corti
(1822–1876) discovered the organ of Corti in 1851. The structure evolved from the basilar papilla and is crucial for
mechanotransduction Mechanotransduction ('' mechano'' + ''transduction'') is any of various mechanism Mechanism may refer to: *Mechanism (engineering) In engineering, a mechanism is a Machine, device that transforms input forces and movement into a desired set of ou ...
in mammals.


Structure

The organ of Corti is located in the
scala media The cochlear duct (bounded by the scala media) is an endolymph Endolymph is the fluid contained in the membranous labyrinth The membranous labyrinth is a collection of fluid filled tubes and chambers which contain the receptors for the senses o ...
of the
cochlea The cochlea is the part of the inner ear Inner Ear (established in 2007 in Bodø, Norway) is a Norwegian record label initiated and led by the brothers and jazz musicians Tore Johansen and Roger Johansen (musician), Roger Johansen. They opene ...

cochlea
of the
inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear The ear is the organ of hearing and, in mammals, balance. In mammals, the ear is usually described as having three parts—the outer ear T ...

inner ear
between the
vestibular duct The vestibular duct or scala vestibuli is a perilymph-filled cavity inside the cochlea The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, ...
and the
tympanic duct The tympanic duct or scala tympani is one of the perilymph Perilymph is an extracellular fluid located within the inner ear. It is found within the scala tympani and scala vestibuli of the cochlea. The ionic composition of perilymph is comparable ...
and is composed of mechanosensory cells, known as
hair cell Hair cells are the sensory receptor Sensory neurons, also known as afferent neurons, are neuron A neuron or nerve cell is an membrane potential#Cell excitability, electrically excitable cell (biology), cell that communicates with other cel ...
s. Strategically positioned on the
basilar membrane The basilar membrane is a stiff structural element within the cochlea The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus (c ...
of the organ of Corti are three rows of
outer hair cells Hair cells are the sensory receptor Sensory neurons, also known as afferent neurons, are neuron A neuron or nerve cell is an electrically excitable cell Cell most often refers to: * Cell (biology), the functional basic unit of life C ...
(OHCs) and one row of
inner hair cells Hair cells are the sensory receptor Sensory neurons, also known as afferent neurons, are neuron A neuron or nerve cell is an electrically excitable cell Cell most often refers to: * Cell (biology), the functional basic unit of life C ...
(IHCs). Surrounding these hair cells are supporting cells: Deiters cells, also called phalangeal cells, which have a close relation with the OHCs, and pillar cells, which separate and support both the OHCs and the IHCs. Projecting from the tops of the hair cells are tiny finger-like projections called
stereocilia Stereocilia (or stereovilli) are non-motile apical cell modifications. They are distinct from cilia The cilium (; the plural is cilia) is an organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biolog ...

stereocilia
, which are arranged in a graduated fashion with the shortest stereocilia on the outer rows and the longest in the center. This gradation is thought to be the most important anatomic feature of the organ of Corti because this allows the sensory cells superior tuning capability. If the cochlea were uncoiled, it would roll out to be about 33 mm long in women and 34 mm in men, with about 2.28 mm of standard deviation for the population. The cochlea is also tonotopically organized, meaning that different frequencies of sound waves interact with different locations on the structure. The base of the cochlea, closest to the outer ear, is the most stiff and narrow and is where the high-frequency sounds are transduced. The apex, or top, of the cochlea is wider and much more flexible and loose and functions as the transduction site for low-frequency sounds.


Function

The function of the organ of Corti is to convert ( transduce) sounds into electrical signals that can be transmitted to the brainstem through the auditory nerve. It is the auricle and
middle ear The middle ear is the portion of the ear The ear is the organ of hearing and, in mammals, balance. In mammals, the ear is usually described as having three parts—the outer ear The outer ear, external ear, or auris externa is t ...

middle ear
that act as mechanical transformers and amplifiers so that the sound waves end up with amplitudes 22 times greater than when they entered the ear.


Auditory transduction

In normal hearing, the majority of the auditory signals that reach the organ of Corti in the first place come from the outer ear.
Sound waves In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular ...
enter through the
auditory canal The ear canal (external acoustic meatus, external auditory meatus, EAM) is a pathway running from the outer ear The outer ear, external ear, or auris externa is the external part of the ear The ear is the organ of hearing and, in ...
and vibrate the
tympanic membrane In the anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science which deals with the structural organization ...

tympanic membrane
, also known as the eardrum, which vibrates three small bones called the
ossicles The ossicles (also called auditory ossicles) are three bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a spe ...
. As a result, the attached
oval window The oval window (or ''fenestra vestibuli'' or ''fenestra ovalis'') is a membrane-covered opening from the middle ear to the cochlea The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrint ...
moves and causes movement of the
round window The round window is one of the two openings from the middle ear into the inner ear. It is sealed by the secondary tympanic membrane (round window membrane), which vibrates with opposite phase to vibrations entering the inner ear through the oval w ...
, which leads to displacement of the cochlear fluid. However, the stimulation can happen also via direct vibration of the cochlea from the skull. The latter is referred to as Bone Conduction (or BC) hearing, as complementary to the first one described, which is instead called Air Conduction (or AC) hearing. Both AC and BC stimulate the basilar membrane in the same way (Békésy, G.v., Experiments in Hearing. 1960). The basilar membrane on the tympanic duct presses against the hair cells of the organ as
perilymph Perilymph is an extracellular fluid In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structu ...
atic pressure waves pass. The stereocilia atop the IHCs move with this fluid displacement and in response their
cation An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are u ...
, or positive ion selective, channels are pulled open by
cadherin Cadherins (named for "calcium-dependent adhesion") are a type of cell adhesion molecule Cell adhesion molecules (CAMs) are a subset of cell surface proteins that are involved in the molecular binding, binding of cells with other cells or with th ...
structures called that connect adjacent stereocilia. The organ of Corti, surrounded in potassium-rich fluid
endolymph Endolymph is the fluid contained in the membranous labyrinth The membranous labyrinth is a collection of fluid filled tubes and chambers which contain the receptors for the senses of equilibrium and hearing. It is lodged within the bony labyrinth ...
, lies on the
basilar membrane The basilar membrane is a stiff structural element within the cochlea The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus (c ...
at the base of the
scala media The cochlear duct (bounded by the scala media) is an endolymph Endolymph is the fluid contained in the membranous labyrinth The membranous labyrinth is a collection of fluid filled tubes and chambers which contain the receptors for the senses o ...
. Under the organ of Corti is the
scala tympani The tympanic duct or scala tympani is one of the perilymph-filled cavities in the inner ear of the human. It is separated from the cochlear duct by the basilar membrane, and it extends from the round window to the helicotrema, where it continu ...
and above it, the
scala vestibuli The vestibular duct or scala vestibuli is a perilymph Perilymph is an extracellular fluid located within the inner ear. It is found within the scala tympani and scala vestibuli of the cochlea. The ionic composition of perilymph is comparable to t ...
. Both structures exist in a low potassium fluid called
perilymph Perilymph is an extracellular fluid In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structu ...
. Because those stereocilia are in the midst of a high concentration of potassium, once their cation channels are pulled open, potassium ions as well as calcium ions flow into the top of the hair cell. With this influx of positive ions the IHC becomes depolarized, opening voltage-gated calcium channels at the basolateral region of the hair cells and triggering the release of the neurotransmitter
glutamate Glutamic acid (symbol Glu or E; the ionic form is known as glutamate) is an α-amino acid Amino acids are organic compounds that contain amino (–NH2) and Carboxylic acid, carboxyl (–COOH) functional groups, along with a Substituent, sid ...
. An electrical signal is then sent through the
auditory nerve The cochlear nerve (also auditory nerve or acoustic nerve) is one of two parts of the vestibulocochlear nerve The vestibulocochlear nerve (auditory vestibular nerve), known as the eighth cranial nerve, transmits sound and equilibrium (balance) ...
and into the
auditory cortex The auditory cortex is the part of the temporal lobe that processes auditory information in humans and many other vertebrates. It is a part of the auditory system, performing basic and higher functions in hearing, such as possible relations to ...

auditory cortex
of the brain as a neural message.


Cochlear amplification

The organ of Corti is also capable of modulating the auditory signal. The outer hair cells (OHCs) can amplify the signal through a process called electromotility where they increase movement of the basilar and tectorial membranes and therefore increase deflection of stereocilia in the IHCs. A crucial piece to this cochlear amplification is the motor protein
prestin Prestin is a protein that is critical to sensitive hearing in mammals. It is encoded by the ''SLC26A5'' (solute carrier anion transporter family 26, member 5) gene. Prestin is the motor protein of the Hair cell#Outer hair cells .E2.80.93 acoustic ...

prestin
, which changes shape based on the voltage potential inside of the hair cell. When the cell is depolarized, prestin shortens, and because it is located on the membrane of OHCs it then pulls on the basilar membrane and increasing how much the membrane is deflected, creating a more intense effect on the inner hair cells (IHCs). When the cell hyperpolarizes prestin lengthens and eases tension on the IHCs, which decreases the neural impulses to the brain. In this way, the hair cell itself is able to modify the auditory signal before it even reaches the brain.


Development

The organ of Corti, in between the
scala tympani The tympanic duct or scala tympani is one of the perilymph-filled cavities in the inner ear of the human. It is separated from the cochlear duct by the basilar membrane, and it extends from the round window to the helicotrema, where it continu ...
and the
scala media The cochlear duct (bounded by the scala media) is an endolymph Endolymph is the fluid contained in the membranous labyrinth The membranous labyrinth is a collection of fluid filled tubes and chambers which contain the receptors for the senses o ...
, develops after the formation and growth of the
cochlear duct The cochlear duct (bounded by the scala media) is an endolymph Endolymph is the fluid contained in the membranous labyrinth The membranous labyrinth is a collection of fluid filled tubes and chambers which contain the receptors for the senses o ...
. The inner and outer hair cells then differentiate into their appropriate positions and are followed by the organization of the supporting cells. The topology of the supporting cells lends itself to the actual mechanical properties that are needed for the highly specialized sound-induced movements within the organ of Corti. Development and growth of the organ of Corti relies on specific genes, many of which have been identified in previous research ( SOX2,
GATA3 GATA3 is a transcription factor In molecular biology Molecular biology is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemica ...
,
EYA1 Eyes absent homolog 1 is a protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew i ...
,
FOXG1 Forkhead box protein G1 is a protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew ...
,
BMP4 Bone morphogenetic protein 4 is a protein that in humans is encoded by ''BMP4'' gene. BMP4 is found on chromosome 14q22-q23 BMP4 is a member of the bone morphogenetic protein family which is part of the transforming growth factor-beta superfamily. ...
,
RAC1 Rac1, also known as Ras-related C3 botulinum toxin substrate 1, is a protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by ...
, and more), to undergo such differentiation. Specifically, the cochlear duct growth and the formation of hair cells within the organ of Corti. Mutations in the genes expressed in or near the organ of Corti before the differentiation of hair cells will result in a disruption in the differentiation, and potential malfunction of, the organ of Corti.


Clinical significance


Hearing loss

The organ of Corti can be damaged by excessive sound levels, leading to noise-induced impairment. The most common kind of hearing impairment,
sensorineural hearing loss Sensorineural hearing loss (SNHL) is a type of hearing loss Hearing loss is a partial or total inability to hear Hearing, or auditory perception, is the ability to perceive Sound, sounds by detecting Vibration, vibrations, changes in ...
, includes as one major cause the reduction of function in the organ of Corti. Specifically, the active amplification function of the outer hair cells is very sensitive to damage from exposure to trauma from overly-loud sounds or to certain
ototoxic Ototoxicity is the property of being toxicity, toxic to the ear (''oto-''), specifically the cochlea or vestibulocochlear nerve, auditory nerve and sometimes the vestibular system, for example, as a side effect of a drug. The effects of ototoxicit ...
drugs. Once outer hair cells are damaged, they do not regenerate, and the result is a loss of sensitivity and an abnormally large growth of loudness (known as ''recruitment'') in the part of the spectrum that the damaged cells serve. While hearing loss has always been considered irreversible in mammals, fish and birds routinely repair such damage. A 2013 study has shown that the use of particular drugs may reactivate genes normally expressed only during hair cell development. The research was carried out at
Harvard Medical School Harvard Medical School (HMS) is the graduate medical school A medical school is a tertiary education Tertiary education, also referred to as third-level, third-stage or post-secondary education, is the education Education is the ...
,
Massachusetts Eye and Ear Massachusetts Eye and Ear (Mass. Eye and Ear, or MEE) is a specialty hospital located in Boston, Massachusetts, United States, which focuses on ophthalmology (eye), otolaryngology (ear/nose/throat), and related medicine and medical research, resear ...
, and the
Keio University , abbreviated as or , is a private research university located in Minato, Tokyo, Japan. It is the oldest institute of western higher education in Japan. Its founder, Fukuzawa Yukichi, originally established it as a school for Rangaku, Western st ...

Keio University
School of Medicine in Japan.


Additional images

File:Gray903.png, Transverse section of the cochlear duct of a fetal cat. File:Gray928.png, Diagrammatic longitudinal section of the cochlea File:Gray929.png, Floor of ductus cochlearis File:Gray930.png, Limbus laminæ spiralis and membrana basilaris File:Gray931.png, Section through the spiral organ of Corti (magnified)


Notes


References

* * History. (n.d.). * * * *Nicholls, J. G., Martin, A. R., Fuchs, P. A., Brown, D. A., Diamond, M. E., & Weisblat, D. A. (2012). From Neuron to Brain (5th ed., pp. 456–459). Sunderland, MA: Sinauer Associates, Inc. *Pritchard U. "On the organ of Corti in mammals". 2 March 1876, ''
Proceedings of the Royal Society of London ''Proceedings of the Royal Society'' is the parent title of two scientific journal In academic publishing Academic publishing is the subfield of publishing which distributes academic research and scholarship. Most academic work is publis ...
, volume 24, pp. 346–52 *Pujol, R., & Irving, S. (2013). The Ear.


External links

* Dissecting the molecular basis of organ of Corti development
Organ of Corti 3D animation
* http://lobe.ibme.utoronto.ca/presentations/OHC_Electromotility/sld005.htm Diagram at University of Toronto * http://mayoresearch.mayo.edu/mayo/research/ent_research/images/image02.gif Diagram at Mayo * http://www.iurc.montp.inserm.fr/cric51/audition/english/corti/fcorti.htm at University of Montpellier 1 {{Authority control Auditory system Ear