How the human ear works

The inner ear is called the cochlea, because of its spiral shape. It is about 4 inches long, two and a half times rolled and filled with fluid (endolymph and perilymfe). This liquid is set into motion by sound vibrations from the "base plate" of the stapes.In the cochlea is the basilar membrane, which holds for about 20,000 hair cells. The basilar membrane is wider at one end than at the base of the oval window. This gives the membrane a stiffness that varies with the length: this means that the hair cells in different positions on the membrane respond to different frequencies: the higher frequencies near the oval window register (base cochleae), lower notes below (APEX cochleae). These hairs are in turn connected to the auditory nerve that carries signals to the auditory center in the brains.In the brains lead by the auditory nerve action potentials transmitted to the actual sound perception, or listening. This takes place in the hearing center of the brains, the auditory cortex.

source: Wikipedia