The cochlea and Organ of Corti are both parts of the inner ear. The cochlea is the auditory part and one of its main components is the Organ of Corti, which is the sensory organ of hearing. The cochlea is shaped like a snail’s shell and formed from three ducts that run in parallel: the scala media, which contains sound-sensing hairs; the scala vestibuli, which runs from the oval window; and the scala tympani. The ducts are coiled into two and one-half turns. Vibrations of the stirrup at the oval window cause pressure waves, which change as they pass along the duct. The duct ends at the round window, a membrane that faces into the middle ear cavity.
At the bottom of the cochlea duct is the basilar membrane with its Organ of Corti and the sound-sensitive hair cells. There are 12,000 outer hair cells and 3,500 inner ones. The tips of the outer cells are embedded in a flap, the tectorial membrane, sticking out into the duct. Pressure changes in the cochlear duct make the membrane vibrate, transmitting bending and shearing movement to the hair cells. This stimulates them to produce a nerve signal, which is carried to the brain by the cochlear nerve. Sound at any particular frequency makes some parts of the membrane vibrate more than others, stimulating a specific group of hair cells so that the sound can be recognized; but the extent of the basilar movement depends on the loudness of the sound.