– When you think about your ear
you definitely think about, “Oh, hearing.”
Okay, we’re done. See you later.
Just kidding. There’s definitely more
to the ear than just hearing. The ear is involved in
receiving perceptions of sound but it’s also involved in
receiving perceptions of balance or position in space.
And so let’s look at the anatomy of the ear so that we can
identify how exactly it works. First of all, your ear
has three part: outer, that’s what
you can see, middle, that’s filled
with air, and inner, that’s filled
with fluid. Let’s make a note of that, because that will help us
understand how it functions. The middle ear is
filled with air. Really?
There we go. And the outer ear
is filled with air, unless you jump in the water, in which case then it might
get filled with water. This big leaf-like thing
is called the pinna. Here’s your ear canal. What is our technical
term for that? The external auditory meatus. The external auditory meatus,
the EAM. The external auditory
meatus travels all the way to what is this
structure right here? That’s your tympanic membrane. That’s your eardrum.
Eardrum is not good enough. Tympanic membrane. Your tympanic membrane separates the outer ear from
the middle ear. Your tympanic membrane vibrates
when soundwaves hit it. Now, tongue was set up to
receive chemical stimuli. Nose, set up to receive
chemical stimuli. Ear, set up to receive
mechanical stimuli. Soundwaves.
The soundwaves we’re going to make
them pink for fun. Because we have soundwaves
for fun, man. And they come in and
they literally apply a force to your tympanic membrane.
Your soundwaves, most of the time you can’t
actually feel soundwaves, you can feel the air
but feel the mechanical vibrations that are making
your sound when you talk. You can totally feel
the vibrations here. And those vibrations come out
and the only reason why you hear anything is because my
soundwaves are going in and wiggling your tympanic membrane. Wow.
That’s really like intimate. A little disturbing.
Well, we’ll just stop right there. Your tympanic membrane
starts wiggling. It’s going to literally transfer
the mechanical waves from the sound that’s coming
out into the middle ear. Now, if the waves just stopped, if we had just air between
the tympanic membrane and our ultimate sensory structures
which are in the inner ear, then game over. You’re not going to
hear anything. Remove the structures
in the middle ear done. We got nothing going on.
If we wiggle the tympanic membrane and we throw a bone, like the malleus,
into the mix, once we wiggle the tympanic
membrane what’s going to happen to the malleolus
that’s touching it? It’s going to wiggle too. And then, if the
malleus is touching, oh, you know, the incus.
What’s going to happen to the incus when the malleus
starts wiggling? It’s going to
start wiggling too. And if they both know the
incus is now touching who? My friend, stapes.
What’s stapes going to do? We got the wiggles going on. Now this is crazy.
Are you ready for this? Soundwave comes
in at a magnitude of 1. The mechanical
movement has like a, I don’t know,
just I’m making up– just think of it as a size 1. A size 1 earthquake,
just tiny. After it wiggles,
the malleus, the incus, and the stapes, your inner ear bones,
I mean your middle ear bones, the size of the wave is going
to be amplified 22 times. What?
Okay. So that’s like me coming in
and going knock, knock, knock.
That’s a 1. And what you hear when I
go knock, knock, knock is 22 times louder
than that. True story. I mean is that like– wow,
what a great setup. That’s perfect.
So what. Well, stapes. Who’s stapes touching you guys?
Very important. Look, I’ll make it green
so you can see it. Stapes has its hands on another membrane called
the oval window. This is the oval window. The oval window and the tympanic
membrane are the two structures that separate
or that create the boundary of the middle ear
that is filled with air. If a window, a membrane,
can vibrate the bones, which then vibrate a membrane, what is going to happen inside
these fluid-filled structures? Dude. Have you guys been
to the Discovery Museum? That’s like my kids,
well back in the day when I was staying home
with them all day long, we’d ride the bus
to the Discovery Museum and we’d play
with the water thing down there. And they had this like
water wave maker and, oh, hours my children could stay and make waves
at the water wave maker. And you push the,
I don’t know, block in through the water and it would make
this great big wave. When I’m yelling
at you like this, ’cause it’s so exciting. The oval window is
going to vibrate. And what did I tell you is
true about the inner ear. It’s filled with fluid.
It’s like a wave maker. We just transferred the
anatomy of your ear. Dude this is so freakin’ cool. Transferred this mechanical
sound wave into water waves inside your inner ear structures
which include your cochlea. Okay we can’t have it be that
color ’cause you can’t see. Let’s try black.
What. Novel idea.
Cochlea. The cochlea looks like
a little snail. And these guys have fluid
filled in them also. These are the
semicircular canals. Here comes the wave.
A wave maker. Because I’m yelling at
you and you hear me. So the waves come in, depending on how far the
waves go up the cochlea that will determine the pitch
of the sound that you hear. So as I’m talking to you
in all these different pitches and you can hear
like all these different accents and excitements
and whatever is happening, it’s happening because the waves are like rushing in at different
distances into the cochlea. All right.
It’s a mechanical wave. Guess what? You’ve got cells lining
the cochlea. And I’ll draw you one.
It looks a little– like a, like a, this. Does this look kind of familiar? They’re called hair cells. Hair cell.
And the hair cells synapse, send messages to–
who do you think? Sensory neurons.
Okay. Hair cell has its little
twingers in the fluid. So when I’m yelling at you
and the fluid wave comes in and the hair cell appendages, twingers, move,
if they move this way, then the hair cell says, “Dude send that message
to the brain.” And fires a message
to the brain. When they move back it says,
“Oh, there’s nothing going on. Everything is normal.”
Really? And that’s true.
And that’s how you hear. And that’s what’s
happening right now. What about these semicircular
canals. The cochlea is where sound
happens. The semicircular
canals are involved in three-dimensional
perception of position. So, if you are on your side, now this is also
just shockingly cool. Also fluid-filled. However, the sound waves don’t
travel in this structure. Instead, true story, dogs, there are rocks
in this thing. What?
There are. And you know what, those semicircular
canals are set up now. I have no idea because I’m
spatially challenged. But I can visualize two axes,
like an X and a Y. And then I know
that somewhere there is a Z, and there’s another axis
that makes it three-dimensional. And those semicircular
canals are set up to perceive all orientations of space. So if I knock my head
over this way they’re set up so that the rocks in the tubes, right, are going to fall
because of gravity in a specific orientation
in those three tubes because of how they’re set up
in three-dimensional space. You totally understood
that didn’t you? Because I’m so
skilled at explaining three-dimensional things.
But here I am with my head on my side because all the
rocks fell in my ear. And my semicircular canals
are saying, “Really? Why are you sitting there
with your head on your side?” I could probably fall
asleep in this position because well we just will
stay focused right now. The rocks just fell back and I
totally have a perception of, “Oh, yeah, I moved. I’m standing back
upright again.” Oh, the rocks just fell and they send the message for
three-dimensional space. Now they are fluid-filled. So if you get
waves going through the fluid-filled
semicircular canals, if you get– by spinning,
and then you stop. Do the waves stop? No, which is why you feel
like you’re spinning still when you are not spinning. No, you are not.
Dude, the ear is amazing. Now we have some information
coming in regarding balance, perception three-dimensional
location and space. We’ve got some information
coming in regarding sound. Let’s talk about how we’re
going to send that information to the brain and how we’re
actually going to process. Like, what’s the pathway
once we get into the brain? But first of all pat
your ears and say, “Dude, you guys are amazing.