Harmony in my head
The cochlea – a pea-sized, snail-shaped structure in the inner 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 into an electrical signal: the vibrations open mechanically gated ion channels, allowing positively charged ions to enter the cell. This influx depolarises the hair cell, leading to the opening of voltage-gated calcium ion 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 information doesn’t just flow one way. The brain can send signals that sharpen up responses of hair cells, so that we can concentrate on specific aspects of sound in complex environments.Lead image:
Dr David Furness, Wellcome Images CC BY NC ND