An ear for music
The cochlea - a pea-sized snail-shaped structure in the middle ear - is the key intermediary between music and the brain.
Sound waves are collected by the outer ear and funnelled onto the eardrums where they are amplified by the ossicles - tiny bones in the middle ear. The sounds are now ready to be translated in the cochlea into the language of the brain: nerve impulses.
This happens in the organ of Corti. A crucial structure is the basilar membrane lining the cochlea. At one end the membrane is narrow and stiff and vibrates in response to high pitches. The other end is wider and more flexible and vibrates most in response to deeper sounds.
Vibrations of the basilar membrane are detected by tiny hair cells embedded in the membrane. There are two types of hair cell: outer hair cells and inner hair cells.
Outer hair cells act to amplify the incoming sound signals. Inner hair cells convert mechanical stimulation to an electrical signal: the vibrations open ion channels, allowing positively charged ions to enter the cell. This influx depolarises the hair cell, leading to the opening of voltage-sensitive calcium channels. The resulting flux of calcium ions triggers the release of neurotransmitters from the hair cells, generating an action potential in the auditory nerve.
But the information doesn't flow one way. The brain can send signals that sharpen up responses of hair cells, so we can concentrate on specific sound sources in complex environments.
Images:
Illustration of outer and inner ear; Medical Art Service, Munich
Illustration of organ of Corti; Medical Art Service, Munich
Hair cell of inner ear; Dr David Furness