Corti, Organ of

( organum spirale), the peripheral part of the sound-perceiving apparatus (auditory-analyzer receptor) in mammals and man, discovered by the Italian histologist A. Corti (1822–76).

The organ of Corti, or spiral organ, evolved from structures of the lateral line organs. It is located in the bony semicircular canal of the inner ear (the cochlear duct), which is filled with endolymph. The upper wall of the duct, called Reissner’s membrane, is adjacent to the scala vestibuli. The lower wall, bordering on the scala tympani, is formed from the basilar membrane, which is attached to the lamina spiralis ossea. The scala vestibuli and scala tympani are filled with perilymph. The outer wall of the cochlear duct, or the stria vascularis, contains numerous blood vessels. The organ of Corti rests on the basilar membrane and consists of internal and external hair cells and inner and outer supporting cells (columnar, Deiters’, Claudius’, Hensen’s), between which is a tunnel where the processes of nerve cells in the spiral ganglion of the cochlear nerve extend toward the bases of the hair cells. The sound-perceiving hair cells are arranged in niches formed by the bodies of supporting cells; the surface of each has 30–60 short hairs facing in the direction of the tectorial membrane. The supporting cells also perform a trophic function, directing the flow of nutrients to the hair cells.

The function of the organ of Corti is to transform the energy of acoustic oscillations into nervous excitation. Acoustic oscillations are received by the tympanic membrane and transmitted through the ossicles of the middle ear to the fluids of the inner ear (perilymph and endolymph). Oscillations of the fluids affect the relative position of the organ’s hair cells and tectorial membrane. Bending the hairs gives rise to bioelectric potentials; these are picked up and transmitted to the central nervous system by the processes of the neurons of the spiral ganglion, which are found near the base of each hair cell.

According to another view, the hairs of the sound-perceiving cells are simply sensitive antennae, which become depolarized by the afferent waves because of a redistribution of acetylcholine in the endolymph. Depolarization triggers a chain of chemical transformations in the cytoplasm of the hair cells and gives rise to neural impulses in the nerve endings in contact with the cells. Acoustic oscillations of different pitches are perceived by different sections of the organ of Corti: high frequencies cause oscillations in the lower parts of the cochlea, and low frequencies cause oscillations in the upper parts. This is explained by the nature of the hydrodynamic phenomena in the cochlear duct.