Voiceover: You probably notice that whenever
you have a cold your nose gets all stuffy
and when your nose gets all stuffy
with a bunch of stuff you're not able to
taste things very well.
So let's imagine that
you're trying to eat a strawberry.
So you're eating the strawberry.
Normally it tastes really good
and it goes in your mouth
and as you're chewing the strawberry
you're breaking down different cellular components
and these cellular components
release little molecules and these little molecules
will travel through the back of your throat
and some of them will actually come into your nose.
And as you're chewing the strawberry
you're actually smelling little molecules
that are being released.
You're actually using your sense of smell
in conjunction with your sense of taste
with your ability to taste the strawberry.
And so if you knock out your sense of smell
which is what happens when you have a cold.
If your sense of smell is knocked out
you're relying only on your sense of taste.
And when you only rely on your sense of taste
you're not able to taste things as well.
Things don't taste as rich.
So you can actually try this.
Next time you're eating something
just go ahead and close your nose
and see if whatever it is you're eating
tastes differently after
you're no longer smelling anything.
So hopefully this goes to show a little bit about
why our sense of smell is important.
Now we can definitely live without our sense of smell,
but it does greatly enrich our everyday lives.
Smell is also known as olfaction.
Sometimes our sense of smell can be called
our olfactory sense and we'll talk a little bit
about how this word comes into play
and a little bit of the anatomy later on.
Let's go ahead and go into the anatomy
of our olfactory system really quickly.
If I again draw a profile of someone.
Here's the nose, here's the mouth,
and here's the chin.
There is a nostril and this nostril basically
is an opening to allow air
and various odor molecules to come into the nose.
There is actually a region that you can't see.
This region is known as your olfactory epithelium.
This region is the olfactory epithelium.
Olfactory epithelium. So again, olfactory olfaction.
Separating this olfactory epithelium from the brain
because the brain actually sits up here.
So we've got a little bit of the brain
and it sits up here.
So separating the brain from the olfactory epithelium
in this nasal passage over here
is a piece of bone known as the cribriform plate.
So this little piece of bone is known
as the cribriform plate.
And sitting right above the cribriform plate
is actually a little extension from the brain.
So there's actually an extension
that comes from the brain
and it sits right above the cribriform plate.
I'm drawing it here in purple
and it looks like a little bulb
and this is actually known as the olfactory bulb.
So olfactory bulb.
So again, olfaction, olfactory bulb,
and the olfactory bulb is basically
a bundle of nerves so it's actually
one of the cranial nerves that exits the brain
and so this bundle of nerves sends
little projections through the cribriform plate
into the olfactory epithelium.
So this projection of nerves
breaks off and branches off
and there are thousands and thousands of cells
sending little connections
into the olfactory epithelium.
So I'm only drawing a few of them over here,
but there are thousands of these cells
sending little connections into
the olfactory epithelium and at the very end
of each one of these little cells are receptors
and each receptor is sensitive
to one particular molecule.
It's actually sensitive to
one particular type of molecule.
So for example let's imagine that we have
a benzene ring and most things
that have benzene rings are known as
aromatic compounds and normally
you can actually smell aromatic compounds.
So a molecule has a benzene ring
it usually has some sort of scent
and that scent is picked up via the olfactory bulb
and these little extensions
that come into the olfactory epithelium.
So we have a little benzene ring,
A little odor molecule and it travels into the nose.
It will actually bind to a small receptor
on one of these nerve endings.
So what I'm going to do next is zoom in
into this region over here.
I just want to look at the olfactory bulb
and in particular I want to be able to look at
each one of these little olfactory sensory neurons
and their little projections that bind
to various molecules in the environment.
So what I'm going to do now is zoom into
that little part that I highlighted earlier.
And so we're just going to look at the olfactory bulb
so let me just label this olfactory bulb.
And as I mentioned before the olfactory bulb
was separated from the nasal passage
by something known as a cribriform plate.
The cribriform plate is basically just bone
and there are a bunch of little holes
in the cribriform plate that basically allow
the cells, these olfactory sensory cells
to send projections to the olfactory bulb.
So let's imagine that there's an olfactory sensory cell
and it sends a little projection into the olfactory bulb.
And so it sends a little projection
and this cell is situated inside
the olfactory epithelium and there are
all kinds of other epithelial cells over here
so I could just kind of draw those in.
So there are all kinds of epithelial cells
that basically secrete mucus and all kinds of things
and these olfactory epithelial cells
are situated in between the rest
of the epithelial cells.
And there are thousands of different types
of epithelial cells and each type
has a particular receptor.
So let's say that this particular olfactory sensory cell
is responsible, has a little receptor and this
little receptor is sensitive to benzene rings.
So as we have a benzene molecule
enter the nose through the nostrils
it enters the nasal passage and eventually will
bounce into this receptor
and when it binds to this receptor
it will actually trigger a cascade of events
that causes this cell to fire.
This cell will fire an action potential
and the action potential will actually end up here
in the olfactory bulb and what we have is
we actually have a whole bunch
of olfactory sensory cells that are sensitive
to the same molecule.
So we have another one over here
and it will actually synapse to the same place.
So we got another olfactory sensory cell
and this also is sensitive to this benzene molecule.
So the benzene molecule comes in
and activates a whole bunch of cells
that have benzene receptors and all the cells
that are sensitive to benzene will all
fire an action potential to one particular location
in the olfactory bulb and this particular location
is known as a glomerulus. So glomerulus.
And the glomerulus is basically
we can think of it as a designation point
for various sensory olfactory cells
that are sensitive to the same molecule.
So we could think of this glomerulus
as a benzene glomerulus because all the cells
that are sensitive to benzene will synapse over here.
And what actually happens is they will synapse
onto another cell type.
So there is another cell and this is known
as a mitral, so mitral or as a tufted cell.
Tufted cell and this mitral/tufted cell
will actually project to the brain.
So the reason we have this type of organization
is because it's a lot easier for one cell
to send a projection to the brain
then it would be for thousands of these cells
to send a projection to the brain.
So just to kind of go into this in a little bit more detail
so what we can also have are other cell types.
Let's imagine that there's this blue cell over here.
And let's imagine that this blue cell
is sensitive to 2 benzene rings
that are connected to one another.
So if we have 2 benzene rings that are connected
to one another they will bind onto a receptor
on this blue olfactory sensory cell
and this blue cell will go through the
cribriform plate and synapse in the olfactory bulb,
but it will synapse at a different location
as will other blue cells sensitive
to double benzene rings and they will synapse
onto a double benzene glomerulus and so on
and so this double benzene glomerulus
has a different mitral/tufted cell
that will send a projection to the brain.
So this is the general organization
of various cells in the olfactory epithelium
and the general idea here is that
we have hundreds of various cells that are
hypersensitive to one particular molecule
and these hundreds of cells will all send
all their projections to one glomerulus
and that one glomerulus will have several
mitral/tufted cells that will then go to the brain.
So now what I want to go into is
I just want to look at this part right here.
So I want to look at how a molecule will bind
to a receptor and how it triggers an action potential.
So let's imagine that we have
this molecule of benzene over here.
So it will enter the nasal passage
and it will bounce around until it binds
to a particular receptor.
So what we have is a particular receptor
that is in the membrane of an olfactory sensory cell.
So this benzene molecule will come in
and it will bind right here.
It will bind and when it binds it will actually cause
this receptor which is known as
a G protein-coupled receptor
and it's called a G protein-coupled receptor
because there's actually a protein inside
that is known as a G protein which it is coupled to.
It's coupled, sorry ie. The G-protein is coupled to the
receptor and when the molecule of benzene binds
it will actually cause disassociation here.
The G protein will break away
and it will cause a cascade of events inside the cell.
So this inside one of the sensory olfactory cells
that we were talking about earlier.
And so one of the effects that this G protein will have
is it will actually bind to an ion channel.
So it will bind to an ion channel
and the ion channel will basically allow
positive ions from outside the cell to flow inside
and this will cause the cell to depolarize
and fire an action potential that eventually
will go through the cribriform plate
and then synapse to a mitral/tufted cell
and that mitral/tufted cell will send
a synapse to the brain.
So let's just go over everything that we talked about
really quickly.
We have an odor molecule which is a benzene ring,
in this example, will bind to a GPCR.
So this GPCR is a particular odor receptor
and the G protein.
So the G protein-coupled receptor,
the G protein will actually break away.
The G protein breaks away
and activates various things inside the cell
and one of those things is
an ion channel gets activated.
That causes the cell to depolarize
and fire an action potential
and the action potential will go through
the cribriform plate and into the glomerulus
and it will activate a mitral/tufted cell
which will then enter the brain
and then your brain is able to perceive that odor
through this pathway over here.