Cochlear hair cell

The hair cells of the cochlea are the cells that hear. They live in the organ of Corti, which sits inside the cochlea, the snail shaped spiral of the inner ear, and is the receptor organ for hearing. They are called hair cells because of the stiff little hairs they carry on top, the stereocilia.

The trick is in those hairs. When sound makes the structure they rest on vibrate, the stereocilia tilt toward the tallest of them. As they tilt, they pull on the filaments that join them to one another, the tip links, and that tension opens channels, which are doors in the cell membrane. Sound reaches the nerve as a hair that bends to one side and opens a door.

Potassium flows in through those doors from the endolymph, the fluid that surrounds the cell on its upper side. That inflow changes the cell's electrical charge, which is called depolarizing, and the cell then releases neurotransmitter, mainly glutamate, onto the nerve fibers of the spiral ganglion. At that point sound has stopped being movement and has become a nerve message.