(Cat meowing, siren wailing)
(whispering) Did you hear that?
You perceive sound without even realizing it most of the time.
and your brain is very good at calculating the
pitch, volume, and location of those sounds...even if you’re tone-deaf
(loudly) How do we do that?
Well, lend me your ears as we talk about the auditory system.
Welcome back to Neuro Transmissions!
Last episode we talked about how your brain “sees”
using photoreceptor cells in your retinas.
This week we’re going to talk about another sense: hearing.
So how does sound go from being a wave in the air outside of your ear
to being a signal you can perceive in your brain?
The human ear is very well designed to help us know where a sound is coming from.
The folds of cartilage around the outside are called the “pinna”.
And sound waves bounce off of them as they enter your ear canal.
The ear canal acts as a sort of amplifier for many of the sounds that enter your ear.
At the end of the ear canal, sounds hit the “tympanic membrane”.
Or, as you may be more familiar with it, the ear drum.
As the name suggests, the tympanic membrane is essentially a drum.
The playing head of most drums has a thin layer which vibrates when hit.
Just like a drum, the surface of the eardrum vibrates too!
Its purpose is to convert the sound waves traveling through air into a physical vibration
so that it can be passed on to the tiny bones of your middle ear, called ossicles.
These three ossicles, called the malleus, incus, and stapes,
act as levers to amplify the sound vibrations.
They pass the signal along to another membrane called the “oval window”
on the cochlea, which is this sort of snail-shell looking structure.
Cochlea is actually Latin for “snail shell”.
And now, finally, we’re getting to the neuroscience part of this whole structure.
If you’ve ever played in an inflatable children’s pool, you probably know that when you
push on the side of the pool, it causes ripples to move through the water contained within.
This is exactly how sound signals are transmitted into the cochlea.
The stapes bone vibrates with a sound and pushes up against the oval window,
transferring that sound into liquid motion.
Inside this structure is the Organ of Corti,
which contains the neurons responsible for sensing sounds.
The fluid inside, called perilymphatic fluid, sloshes across the surface of
tiny cells contained in the Organ of Corti, called hair cells because of their appearance.
As you can see, they have these funny little bits that stick up on top, making them look
sort of like they have a flat top hair cut!
These little hairs are called “stereocilia”.
Do you remember how last week, we talked about photoreceptor cells,
and how they have special receptors that can respond to light?
Hair cells are just like photoreceptors, but for sound instead of light.
These stereocilia get pushed by the the fluid as it sloshes through the Organ of Corti,
and the motion actually physically opens ion channels.
This allows positive ions into the cells and lets them depolarize.
The way this happens is really pretty neat.
Imagine a gate with a spring on it, and when all of the hair cells are lined up, the gate is closed.
But when you push against the hair cell, it pulls the stereocilia apart,
which pulls on the spring attached to the gate, and causes the gate to fly open
so the ions can get through!
You may have had a parent or teacher tell you not to listen to music that is too loud,
or maybe you’ve seen babies wearing giant ear muffs at concerts to protect their hearing.
A big part of hearing loss from loud noises happens if you damage these stereocilia.
If a noise is too loud, it jostles the hair cells too hard,
and the stereocilia “flop over” and become non-functional.
This can lead to hearing loss and tinnitus as you get older.
Hair cells are organized on the Organ of Corti based on the frequency of sound they respond to.
Higher frequency sounds, like a bell ringing, are registered at the base of the membrane,
while lower frequency sounds, like a foghorn, are registered at the apex.
All the way inside the spiral!
When the cells, depolarize, this allows calcium into the cell,
causing neurotransmitters to be released.
Those neurotransmitters bind to the neurons of the auditory nerve.
Usually there are many auditory nerve neurons receiving signals from a single hair cell.
This signal is passed up to the brain, to a structure called the cochlear nuclear complex.
What a mouthful!
The cochlear nucleus sends axons through several other structures in the brain
where auditory sorting and unpacking occurs.
Each of these structures helps your brain break down the frequency, pitch, and localization of sound.
Sound information is then passed to the inferior colliculus
before signals are relayed through the thalamus and up to the primary auditory cortex.
Remember the primary visual cortex in vision?
Like that, this region is the first area where auditory information gets processed.
The secondary auditory cortex surrounds this region, and does more processing.
Eventually, the auditory information is passed on to other places in the brain,
including the frontal lobe, allowing you to put together all of the various components
of the sound and make sense of it.
Auditory processing is very important to humans.
Not just because it lets us hear, but also because our language is mostly an auditory ability.
Our auditory processing systems are closely connected with
speech processing areas in the brain.
We’ll go into more detail about the neuroscience of language in a different video!
Here’s another interesting fact. Have you ever seen those viral videos where
a deaf person hears for the first time?
Those people have usually just had cochlear implants placed in their skulls.
These are amazing little machines that can bypass damaged cochlea.
These implants use a microphone and a speech processor to collect and organize sounds.
Those sounds are then turned into an electrical signal and are sent down to
the implanted electrode array which then stimulates different points on the auditory nerve.
A lot like a biological cochlea.
While a cochlear implant doesn’t perfectly recreate the sounds that we’re used to hearing,
it can provide an approximation of the soundscape around an individual,
and help individuals interpret speech!
So we’ve covered the visual system and the auditory system.
That’s two of the five senses! Only three more to go.
If you liked this video, give it a thumbs up and subscribe for new videos.
Come back next time and check out our video on the olfactory system:
the way we smell!
I’m Alie Astrocyte, and until our next transmission. Over and out!