Ear`s Cochlea and Corti

Shaped like a snail's shell, the cochlea is 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. The second, larger, part of the inner ear is an apparatus that has nothing to do with hearing. It is a series of fluid-filled chambers and ducts, which contain more clusters of hair cells. When these are activated, signals from these sensors pass to the brain along a branch of the eighth cranial nerve. When the head is turned, the signals from these cells are processed in the brain, which then activates the eye and body muscles so that they are ready for the turning action. Other receptor cells sense the direction of gravity and straight line movements. The main purpose of these cells is to control balance and, because humans have only two legs and balance is unstable, continuous corrections must be applied so that we do not fall over. These signals also coordinate eye and head movements. Of all the fancies about the ear, the notion which has the most charm is that the ear can pick up the sounds of the ocean in a seashell. The fact is, what is heard is an echo of the pulse in one's head.