Organ of Corti

The organ of Corti is located in the scala media of the cochlea of the inner ear between the vestibular duct and the tympanic duct and is composed of mechanosensory cells, known as hair cells. 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. 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==

, middle ear, and inner ear, and how sound is conducted through the outer ear, to the ossicles of the middle ear, through to the inner ear and the cochlea, where the organ of Corti sits. 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. 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 perilymphatic pressure waves pass. The stereocilia atop the IHCs move with this fluid displacement and in response their cation, 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, lies on the basilar membrane at the base of the scala media. 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. 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 scala media, develops after the formation and growth of the cochlear duct. 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, 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, Massachusetts Eye and Ear, and the 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==