Welcome to 2 minute neuroscience, where I simplistically explain neuroscience topics
in 2 minutes or less. In this installment I will discuss the cochlea
When sound waves travel through the canal of our ear, they hit the tympanic membrane,
or eardrum, and cause it to vibrate. This vibration prompts movement in the ossicles,
a trio of tiny bones that transmit the vibration to a structure called the oval window, which
sits in the wall of the cochlea. The cochlea is a tiny coiled structure in the inner ear
that resembles a snail shell.
The interior of the cochlea consists of three fluid-filled canals that run parallel to one
another: the scala vestibuli, the scala media, and the scala tympani. The scala vestibuli
and scala tympani contain a fluid called perilymph and the scala media contains a fluid called
endolymph. When the oval window is depressed by the ossicles it creates waves that travel
through the fluid of the cochlea. These waves cause a structure called the basilar membrane
to move as well.
To visualize the function of the basilar membrane it can be helpful to imagine the cochlea uncoiled.
When waves flow through the fluid in the cochlea, they create small waves within the basilar
membrane itself that travel down the membrane. Different sections of the basilar membrane
respond to different frequencies of sound and as the waves progress down the membrane,
they reach their peak at the part of the membrane that responds to the frequency of the sound
wave created by the original stimulus. In this way, the basilar membrane accurately
translates the frequency of sounds picked up by the ear into representative neural activity
that can be sent to the brain.
The translation of the movement of the basilar membrane into electrical impulses occurs in
the organ of Corti, which is the receptor organ of the ear. It sits atop the basilar
membrane and contains receptor cells known as hair cells. Hair cells are so named because
protruding from the top of each cell is a collection of small "hairs" called stereocilia.
When the basilar membrane vibrates, this causes movement of the hair cells and their stereocilia;
movement of the stereocilia opens ion channels and causes the release of neurotransmitters
to propagate the auditory signal to the vestibulocochlear nerve, which will carry the information regarding
the auditory stimulus to the brain to be analyzed and perceived.