- [Voiceover] If I asked you to list your senses,
you would probably mention things like
sight or sound or taste.
But it turns out that we can actually detect
a lot more than that.
One sense that's really important,
but not often thought about,
is our vestibular system.
Which is a system that is very important
for our sense of balance and spatial orientation.
And while a lot of information about balance
and body position is gained through receptors
in our bodies that let us know
where our limbs are in space,
it actually turns out that a lot of our
sense of balance and spatial awareness
comes from receptors in our inner ear.
So let's take a moment to focus
on our auditory system.
And what I want to do is focus on the inner ear,
the part here that's shaded in orange.
And to get a better idea of what that part looks like
I've pre-drawn an enlarged version.
And if you're familiar with the auditory system,
part of this will probably look familiar to you.
In particular, you might be familiar
with this spiraly, snail-shaped structure here,
which is referred to as the cochlea.
And this is the portion of your inner ear
that's full of specialized auditory receptors
that process sound and then transfer information
about it to our brain.
But what I want to focus on now
are these loops on the other side,
which are referred to as semi-circular canals.
So we have this anterior canal,
a posterior canal,
and then this lateral canal,
which is also sometimes referred to
as the horizontal canal.
and it can be kind of hard to see this
3D structure on a 2D image,
but these canals are actually all orthogonal to each other,
meaning that they are all at right angles to each other.
Let me draw some faces here
to try to make this a bit clearer.
So I'm gonna draw some axes on this head
to try to orient us.
So imagine a line that's going
from our nose to the back of our head,
and we're gonna label that the X axis.
And now imagine another line that's going
from our left ear to our right ear,
and we're gonna label that one the Y axis.
And here, let me add the X axis
to this top picture,
and the Y axis to the bottom,
and I've put these as dotted lines
to signify that they're actually coming
directly out of the screen towards us.
In addition to the X and Y axes,
though, we also have the Z axis.
And that one goes straight up and down.
And so you can see it on both images here.
And each of our semi-circular canals
actually lines up with one of these axes.
Each of these canals is filled with a fluid
that's known as endolymph.
and when we rotate along a certain plane,
it causes the endolymph to shift
within that particular semi-circular canal.
And this allows us to sense
what plane our head is rotating along.
But because we're also sensitive
to how much of the fluid is moving and how quickly,
we also can get information about
the strength of the rotation.
Another part of our inner ear
that gives us information relating
to balance and spatial orientation
are the otolithic organs,
and let me write that here.
And the otolithic organs include
the utricle and the saccule.
Which I've labeled here.
And they help us to detect linear acceleration
and head positioning.
And within these structures are crystals,
actual calcium carbonate crystals,
that are attached to hair cells
within kind of a viscous gel substance.
And if we accelerate in a direction,
or say, move from standing up to lying down,
this causes the crystals to move
because they are heavier than
the surrounding gel environment.
And when they move, they physically pull
on the hair cells that they're attached to.
Which is what triggers an action potential
that carries this information to the brain.
So let me try to draw this,
so that you can get a clearer picture of what it looks like.
I'm gonna erase my two faces here.
And let me change my pen color.
Alright, so we have a person standing here,
and within their ear,
within their otolithic organs,
are hair cells, and those hair cells
are attached to crystals.
And let me draw another hair cell next to it.
And when we're just standing up,
gravity is pushing those crystals
in one direction, namely straight down.
Let's think about what would happen if we lay down.
So now we have a person and he's lying down,
he's all ready to go to sleep.
And now because he's changed his orientation,
these crystals are going to shift in their position.
Because of course, gravity is still pointing
in the same direction.
And as they do this,
they pull the hair cells along with them.
And I just find this to be really fascinating.
I mean think about this,
you have crystals in your ear
that are physically pulled in different directions
by acceleration and gravity.
That allow you to detect head positioning.
Ear crystals that respond to gravity.
And this is real, this is actually
something that is going on in your head
right as we speak.
And I think this is just one of the
coolest sense organs that we have.
So you've experienced what happens
when the vestibular system works,
but you've probably also experienced
what happens when it goes wrong.
Because this results in things like dizziness and vertigo.
So let's think about how this might work.
And let's think back to the semi-circular canals.
As we spin, the fluid within them, the endolymph,
moves along the direction that we're turning.
But this liquid doesn't always stop spinning
right when we do.
And this is especially true for spinning vigorously
or for long periods of time.
And think about what the result of this might be.
So you have stopped moving,
but the continued movement of the endolymph
is resulting in signals being sent to your brain
that indicate that you are still moving,
even after you have stopped.
And this results in the experience of dizziness.
And when the fluid eventually stops moving
is when the dizziness subsides.
And interestingly, knowing this
also helps us figure out how we can combat dizziness.
Rotating in the opposite direction
of how you were originally spinning
encourages the movement of the endolymph
in that opposite direction.
And this can help stop the continued motion
of the endolymph in the original direction
by kind of cancelling it out.
That's not the only thing that causes dizziness, though.
Remember that one of the ways
that we get information about body position
is by sensing the orientation of crystals
in our otolithic organs with respect to gravity.
If you were in a situation without gravity,
let's say if you were an astronaut,
your otolithic organs probably wouldn't work very well.
Because gravity won't pull down on them in the same way.
And because of this, concepts like up and down
kind of become meaningless.
And apparently this can be really disorienting.
And this is also one of the dangers of scuba diving.
Buoyancy can have the same effect as gravity.
And this can sometimes result in divers
becoming really disoriented when they're underwater.
And this is especially the case
when they don't have visual cues
about which way is up or which way is down.