Organ Of Corti
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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 involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus. A core component of the cochlea is the Organ of Corti, the sensory org ...
. This highly varied strip of
epithelial cells Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercell ...
allows for transduction of auditory signals into nerve impulses'
action potential An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, ...
. 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 receptors of both the auditory system and the vestibular system in the ears of all vertebrates, and in the lateral line organ of fishes. Through mechanotransduction, hair cells detect movement in their environment. ...
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 (15 June 1822 – 2 October 1876) was an Italian anatomist. He born at Gambarana, near Pavia in 1822. Education A famous friend of Corti's father, Antonio Scarpa, may have kindled his boyhood interest in anatomy an ...
(1822–1876) discovered the organ of Corti in 1851. The structure evolved from the basilar papilla and is crucial for
mechanotransduction In cellular biology, mechanotransduction ('' mechano'' + '' transduction'') is any of various mechanisms by which cells convert mechanical stimulus into electrochemical activity. This form of sensory transduction is responsible for a number of ...
in mammals.


Structure

The organ of Corti is located in the
scala media Scala or SCALA may refer to: Automobiles * Renault Scala, multiple automobile models * Škoda Scala, a Czech compact hatchback Music * Scala (band), an English electronic music group * Escala (group), an electronic string quartet formerly known ...
of 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. A core component of the cochlea is the Organ of Corti, the sensory org ...
of the
inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the ...
between the
vestibular duct The vestibular duct or scala vestibuli is a perilymph-filled cavity inside the cochlea of the inner ear that conducts sound vibrations to the cochlear duct. It is separated from the cochlear duct by Reissner's membrane and extends from the vesti ...
and the
tympanic duct The tympanic duct or scala tympani is one of the perilymph-filled cavities in the inner ear of humans. It is separated from the cochlear duct by the basilar membrane, and it extends from the round window to the helicotrema, where it continues as ...
and is composed of mechanosensory cells, known as
hair cell Hair cells are the sensory receptors of both the auditory system and the vestibular system in the ears of all vertebrates, and in the lateral line organ of fishes. Through mechanotransduction, hair cells detect movement in their environment. ...
s. Strategically positioned on the
basilar membrane The basilar membrane is a stiff structural element within the cochlea of the inner ear which separates two liquid-filled tubes that run along the coil of the cochlea, the scala media and the scala tympani. The basilar membrane moves up and down in ...
of the organ of Corti are three rows of
outer hair cells Hair cells are the sensory receptors of both the auditory system and the vestibular system in the ears of all vertebrates, and in the lateral line organ of fishes. Through mechanotransduction, hair cells detect movement in their environment. ...
(OHCs) and one row of
inner hair cells Hair cells are the sensory receptors of both the auditory system and the vestibular system in the ears of all vertebrates, and in the lateral line organ of fishes. Through mechanotransduction, hair cells detect movement in their environment. ...
(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 or villi) are non-motile apical cell modifications. They are distinct from cilia and microvilli, but are closely related to microvilli. They form single "finger-like" projections that may be branched, with normal cel ...
, 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 medial to the eardrum, and distal to the oval window of the cochlea (of the inner ear). The mammalian middle ear contains three ossicles, which transfer the vibrations of the eardrum into waves in the ...
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, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid. In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' by the ...
enter through the
auditory canal The ear canal (external acoustic meatus, external auditory meatus, EAM) is a pathway running from the outer ear to the middle ear. The adult human ear canal extends from the pinna to the eardrum and is about in length and in diameter. Stru ...
and vibrate the
tympanic membrane In the anatomy of humans and various other tetrapods, the eardrum, also called the tympanic membrane or myringa, is a thin, cone-shaped membrane that separates the external ear from the middle ear. Its function is to transmit sound from the air ...
, also known as the eardrum, which vibrates three small bones called the ossicles. 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 of the inner ear. Vibrations that contact the tympanic membrane travel through the three ossicles and into the in ...
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 ...
, 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 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 that of plasma and cerebrospinal fluid. The major ...
atic pressure waves pass. The stereocilia atop the IHCs move with this fluid displacement and in response their
cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
, or positive ion selective, channels are pulled open by
cadherin Cadherins (named for "calcium-dependent adhesion") are a type of cell adhesion molecule (CAM) that is important in the formation of adherens junctions to allow cells to adhere to each other . Cadherins are a class of type-1 transmembrane proteins, ...
structures called tip links that connect adjacent stereocilia. The organ of Corti, surrounded in potassium-rich fluid
endolymph Endolymph is the fluid contained in the membranous labyrinth of the inner ear. The major cation in endolymph is potassium, with the values of sodium and potassium concentration in the endolymph being 0.91  mM and 154  mM, respectively. I ...
, lies on the
basilar membrane The basilar membrane is a stiff structural element within the cochlea of the inner ear which separates two liquid-filled tubes that run along the coil of the cochlea, the scala media and the scala tympani. The basilar membrane moves up and down in ...
at the base of the
scala media Scala or SCALA may refer to: Automobiles * Renault Scala, multiple automobile models * Škoda Scala, a Czech compact hatchback Music * Scala (band), an English electronic music group * Escala (group), an electronic string quartet formerly known ...
. 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 humans. It is separated from the cochlear duct by the basilar membrane, and it extends from the round window to the helicotrema, where it continues as ...
and above it, the
scala vestibuli The vestibular duct or scala vestibuli is a perilymph-filled cavity inside the cochlea of the inner ear that conducts sound vibrations to the cochlear duct. It is separated from the cochlear duct by Reissner's membrane and extends from the vesti ...
. Both structures exist in a low potassium fluid called
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 that of plasma and cerebrospinal fluid. The major ...
. 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 that is used by almost all living beings in the biosynthesis of proteins. It is a non-essential nutrient for humans, meaning that the human body can syn ...
. 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, a cranial nerve present in amniotes, the other part being the vestibular nerve. The cochlear nerve carries auditory sensory information ...
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 ...
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 outer hair cells of the inner ear of the mamm ...
, 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 humans. It is separated from the cochlear duct by the basilar membrane, and it extends from the round window to the helicotrema, where it continues as ...
and the
scala media Scala or SCALA may refer to: Automobiles * Renault Scala, multiple automobile models * Škoda Scala, a Czech compact hatchback Music * Scala (band), an English electronic music group * Escala (group), an electronic string quartet formerly known ...
, develops after the formation and growth of the
cochlear duct The cochlear duct (bounded by the scala media) is an endolymph filled cavity inside the cochlea, located between the tympanic duct and the vestibular duct, separated by the basilar membrane and the vestibular membrane (Reissner's membrane) resp ...
. 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 that in humans is encoded by the ''GATA3'' gene. Studies in animal models and humans indicate that it controls the expression of a wide range of biologically and clinically important genes. The GATA3 transcription ...
, EYA1,
FOXG1 Forkhead box protein G1 is a protein that in humans is encoded by the ''FOXG1'' gene. Function This gene belongs to the forkhead family of transcription factors that is characterized by a distinct forkhead domain. The complete function of thi ...
,
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 superfamil ...
, RAC1, 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 in which the root cause lies in the inner ear or sensory organ (cochlea and associated structures) or the vestibulocochlear nerve (cranial nerve VIII). SNHL accounts for about 90% of rep ...
, includes as one major cause the reduction of function in the organ of Corti. Specifically, the active amplification function of the
outer hair cell Hair cells are the sensory receptors of both the auditory system and the vestibular system in the ears of all vertebrates, and in the lateral line organ of fishes. Through mechanotransduction, hair cells detect movement in their environment. ...
s is very sensitive to damage from exposure to trauma from overly-loud sounds or to certain
ototoxic Ototoxicity is the property of being toxic to the ear (''oto-''), specifically the cochlea or auditory nerve and sometimes the vestibular system, for example, as a side effect of a drug. The effects of ototoxicity can be reversible and temporary, ...
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 of Harvard University and is located in the Longwood Medical Area of Boston, Massachusetts. Founded in 1782, HMS is one of the oldest medical schools in the United States and is consi ...
, Massachusetts Eye and Ear, and the
Keio University , mottoeng = The pen is mightier than the sword , type = Private research coeducational higher education institution , established = 1858 , founder = Yukichi Fukuzawa , endowmen ...
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 main research journal of the Royal Society. The journal began in 1831 and was split into two series in 1905: * Series A: for papers in physical sciences and mathematics. * Series B: for papers in life s ...
, 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