The visible part of the ear is the pinna, which collects sound waves and passes them along the auditory canal to a membrane called the eardrum. When sound waves hit the eardrum, it vibrates. The eardrum transmits the vibration to three bones, or ossicles, in the middle ear, which are called the hammer, the anvil, and the stirrup. The diagram of the ear shows how they got these names: they actually look like a hammer, an anvil, and a stirrup. In response to the vibration, these ossicles move one after another. Their function is to amplify the sound vibrations.
From the ossicles, vibrations move through a membrane called the oval window to the cochlea of the inner ear. The cochlea is a coiled, fluid-filled tunnel.
Inside the cochlea are receptors called cilia or hair cells that are embedded in the basilar membrane. The basilar membrane runs along the whole length of the coiled cochlea. Vibrations that reach the inner ear cause the fluid in the cochlea to move in waves. These waves in turn make the hair cells move.
The movement triggers impulses in neurons that connect with the hair cells. The axons of these neurons come together to form the auditory nerve, which sends impulses from the ear to the brain. In the brain, the thalamus and the auditory cortex, which is in the temporal lobe of the cerebrum, receive auditory information.
Two theories explain how people distinguish the pitch of different sounds: place theory and frequency theory.
Place theory explains how people discriminate high-pitched sounds that have a frequency greater than 5000 Hz. Place theory states that sound waves of different frequencies trigger receptors at different places on the basilar membrane. The brain figures out the pitch of the sound by detecting the position of the hair cells that sent the neural signal.