The inner ear consists of two functional systems, both encased in the temporal bone of the skull. One, called the vestibular system is responsible for our sense of balance and equilibrium. The three semi-circular canals, set at right angles to each other, respond to motion and the forces of gravity. The other system, the coiled tube called the cochlea, is the most extraordinary structure of the ear. It is here that sound waves, now propagating as waves in the fluid-filled spaces of the cochlea, get converted to the neural impulses that are sent to the brain.

In the cochlea, thousands of tiny structures called hair cells are set into motion by the moving fluid wave. The hair cells trigger the attached nerve fibers into activity. As will be seen in the discussion about hearing loss below, it is within the cochlea that the most common type of hearing loss, sensory hearing loss, occurs when exposure to loud noise or diseases cause the destruction of many hundreds of these delicate hair cells.