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The organ of Corti, found only in mammals, is part of the cochlea of the inner ear and is provided with hair cells or auditory sensory cells.  It evolved from the basilar papilla found in all tetrapods, except for a few derived species that have lost it. 
The organ was named after the Italian anatomist Marquis Alfonso Giacomo Gaspare Corti (1822–1876), who conducted microscopic research of the mammalian auditory system.
Structure and functions 
The organ of Corti has highly specialized structures that respond to fluid-borne vibrations in the cochlea with a shearing vector in the hairs of some cochlear hair cells. It contains between 15,000-20,000 auditory nerve receptors. Each receptor has its own hair cell. The shear on the hairs opens non-selective transduction ion channels that are permeable to potassium and calcium, leading to hair cell plasma membrane depolarization, activation of voltage-dependent calcium channels at the synaptic basolateral pole of the cells which triggers vesicle exocytosis and liberation of glutamate neurotransmitter to the synaptic cleft and electrical signaling to the auditory cortex via spiral ganglion neurons. The pinna and middle ear act as mechanical transformers and amplifiers, so that by the time sound waves reach the organ of Corti, their pressure amplitude is 22 times that of the air impinging on the pinna. The organ of Corti can be damaged by excessive sound levels, leading to noise-induced health effects. The organ of Corti is the structure that transduces pressure waves to action potentials. The organ of Corti sits inside the cochlear duct, between the scala vestibuli and the scala tympani. The basilar membrane on the scala tympani presses against the hair cells of the organ as perilymphatic pressure waves pass.
Hearing loss 
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 recent 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, the Massachusetts Eye and Ear Infirmary, and the Keio University School of Medicine in Japan.
Additional images 
Transverse section of the cochlear duct of a fetal cat.
Diagrammatic longitudinal section of the cochlea.
Floor of ductus cochlearis.
Limbus laminæ spiralis and membrana basilaris.
Section through the spiral organ of Corti. Magnified.
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