Organ of Corti

The organ of Corti (or spiral organ) is the organ in the inner ear of mammals that contains auditory sensory cells, or "hair cells."

Structure and function
It 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. The Organ of Corti contains between 15,000-20,000 auditory nerve receptors. Each receptor has its own hair cell. The shear on the hairs opens ion channels, leading to neural, electrical signaling to the auditory cortex. The pinna and middle ear amplify sound levels, so that by the time these longitudinal waves reach the Organ of Corti, they are 20 times that of the levels impinging on the pinna. This amplification is partly responsible for the delicacy of the Organ of Corti with respect to excessive sound levels, and helps to understand noise induced health effects.

The discoverer: Alfonso Corti
The organ was named after the Italian anatomist Marquis Alfonso Giacomo Gaspare Corti (1822-1876), who conducted microscopic research of the mammalian auditory system from 1849 to 1851 at the Koelliker laboratory in Würzburg (Germany). He developed new coloring techniques in microscopic anatomy, which enabled him to distinguish and describe individual components inside the highly complex cochlea that had previously been unidentified. In 1851 he was the first to describe the core sensory organ in the mammalian cochlea, the organ of Corti.

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.