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 intercellu ...
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 (1822–1876) discovered the organ of Corti in 1851. The structure evolved from the
basilar papilla The basilar papilla is the auditory sensory organ of lizards, amphibians, and birds, which is homologous to the organ of Corti in mammals. The basilar papilla is composed of cells called "hair cells" which are actually epithelial cells rather than ...
and is crucial for mechanotransduction in mammals.


Structure

The organ of Corti is located in the
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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 t ...
between the vestibular duct and the tympanic duct 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 of the organ of Corti are three rows of outer hair cells (OHCs) and one row of inner hair cells (IHCs). Surrounding these hair cells are supporting cells:
Deiters cell Deiters' cells, also known as outer phalangeal cells or cells of Deiters (), are a cell type found within the inner ear. They contain both micro-filaments and micro-tubules which run from the basilar membrane to the reticular membrane of the in ...
s, also called
phalangeal cell Deiters' cells, also known as outer phalangeal cells or cells of Deiters (), are a cell type found within the inner ear. They contain both micro-filaments and micro-tubules which run from the basilar membrane to the reticular membrane of the inn ...
s, 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, 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 ...
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 th ...
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. Struc ...
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 ai ...
, 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 ...
moves and causes movement of the round window, 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 ca ...
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 conve ...
, or positive ion selective, channels are pulled open by cadherin 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. ...
, lies on the basilar membrane at the base of the
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. Under the organ of Corti is the scala tympani and above it, the scala vestibuli. 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 ca ...
. 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, 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 and the
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, 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) r ...
. 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,
EYA1 Eyes absent homolog 1 is a protein that in humans is encoded by the ''EYA1'' gene. This gene encodes a member of the eyes absent (EYA) subfamily of proteins. The encoded protein may play a role in the developing kidney, branchial arches, eye, an ...
, FOXG1, BMP4, 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, 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 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 and Academic Area, Longwood Medical Area of Boston, Massachusetts. Founded in 1782, HMS is one of the oldest medical schools ...
, Massachusetts Eye and Ear, and the
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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, 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