The organ of corti is the actual site of the sense of hearing inside the cochlea of the inner ear. It is a system of sensory and supporting cells and nerve fibres.
The organ of corti sits on top of a basilar membrane (Membrana spiralis) and is covered in the acellular tectorial membrane (German: Tektorialmembran, purple) that divides it from the lumen of the cochlear duct (German: Schneckengang).
The cells that are responsible for the perception of sound in the organ of corti are called hair cells. They are mechanical receptors that possess stereocilia bundles on their apical cell end. The bundles tips are connected by fine cell-membrane-bridges, the so called tip-links. Two typed of hair cells are differenciated by location:
Outer hair cells (OHC): (German: äußere Haarzellen) They stand next to each other in three rows of cells on the outside of the organ of corti. On their apical cell end there are three rows of stereocilia arranged in a W-pattern. The stereocilia of the OCH are anchored inside the tectorial membrane.
Inner hair cells (IGC): (German: innere Haarzellen) They form a single row of cells on the inner side of the organ of corti, closer to the central pilalr of the cochlea (modiolus). Their stereocilia are not in contact with the tectorial membrane.
In the organ of corti there are about 15,000 hair cells and also many supporting cells (German: Stützzellen), like phalangeal (Deiters) cells and inner and outer pillar cells.
Incoming sound signals move the basilar membrane and the tectorial membrane against each other. This movement stimulates the outer hair cells to actively change their own length which increases the wave movement of the basilar membrane to be strong enough to stimulate the inner hair cells which are the actual site of sensory perception. The vestibulocochlear nerve transmits the resulting nerve impulses to the CNS.