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."

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 function
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.

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.