Hearing begins with the transduction of sound energy into electrical signals in the inner ear, through a sequence of events so fast and sensitive as to be limited only by the thermal noise of molecules moving in inner ear liquids. This entry reviews that sequence with particular focus on the remarkable properties of auditory receptor cells.

Sound originates in the motion of vibrating objects, such as the vocal cords during speech. The vibration energy—a form of mechanical energy—is transmitted to air molecules and thence to the eardrum. Vibrations of the eardrum in turn move the bones of the middle ear, which push in and out, piston-like, on the liquid-filled chambers of the hearing organ in the inner ear. The mammalian hearing organ, called the cochlea, is ...