2-Minute Neuroscience: The Cochlea

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.


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