- [Lecturer] Two senses that are really important
for our survival are our sense of pain
and our sense of temperature.
Of course these have more scientific terms,
so pain is known as nociception
and temperature is known as thermoception.
We're able to sense pain and temperature
through receptors that are located throughout
our entire body, so let's look at a hand for example.
This hand happens to have four fingers,
but that's okay.
So we have pain and temperature receptors
throughout our entire hand.
Interestingly enough, there are
a few different types of temperature receptors,
and one of the most common types of temperature receptors
is a receptor known as TrpVI.
And this TrpVI receptor is not only sensitive
to temperature but it's also sensitive to pain.
So what I'm drawing here are these TrpVI receptors,
and they're spread out throughout the entire hand.
I'm just going to draw a few.
These receptors send axons that eventually
become one giant nerve, so what exactly is a receptor?
Is a receptor a nerve, is it a type of cell?
So in fact a receptor is actually
a specialized type of nerve.
It's the first nerve in this entire pathway
that eventually reaches the brain
and it's specialized in that it actually
has little sites, binding sites
for various molecules that can cause it to activate.
It also undergoes a conformational change,
or a change in shape whenever
there's a change in temperature.
So it's basically a specialized nerve.
It sends axons to this big giant nerve
that eventually leaves the arm and goes to the brain.
And this nerve actually can be split up
into three main types of fibers.
There are some fibers that are really big,
so they have a large diameter.
There are other fibers that have a medium diameter,
and there are still other fibers
that have a very small diameter.
These big fibers are known as A-beta fibers.
These medium fibers are known as A-gamma fibers,
and these tiny little fibers are known as C fibers,
so apart from their size, there's also
one big difference that distinguishes the different fibers
and that is the presence of myelin.
So the large A-beta fibers have
a whole lot of myelin on them.
The A-gamma fibers have a little bit of myelin on them,
and the C fibers have no myelin on them.
So the combination of myelin and the size
of the fiber actually changes how quickly
each fiber can send an action potential.
So when this receptor is activated,
it sends a signal and depending
on which type of fiber conducts the signal to the brain,
the signal will reach the brain at different times.
So the A-beta fibers have the highest conduction velocity
so they're very fast.
The A-gamma fibers are a little bit slower,
so they're medium speed and the C fibers are very slow.
So why is it that these A-beta fibers
are faster than the C fibers?
And that's because they have a really large diameter,
and when an axon has a very large diameter,
it actually decreases the resistance within the axon.
So as a neural impulse is traveling
through the axon, if there's a lot of resistance,
the signal actually travels fairly slow.
But if there's really little resistance,
the signal can travel pretty quickly
so that's why a larger diameter is important
because it decreases the resistance within the fiber.
Myelin is also very important
because it actually increases the conductance of the fiber.
There are a whole bunch of ions sprinkled
throughout the axon.
These ions float in and out of the cell
in order to generate the action potential,
and so these ions are actually unable to go in
and out of the cell except through some areas
that are unmyelinated.
This actually causes the cell to propagate
an action potential very quickly.
C fibers are unmyelinated and so the electrical impulse
is actually very slow to travel through this type of fiber.
So how does pain arise?
So let's imagine that we get poked by some sharp object.
What happens is as this object is entering the skin,
it actually causes cells to break open,
and when the cells break open they release
a whole bunch of little chemicals and these little chemicals
will diffuse to the nearest pain receptor,
so the nearest TrpVI receptor and it'll
actually activate the receptor.
So if we were to blow this receptor up,
if we kind of draw it a little bit bigger
it looks a little something like this,
and there's a little area for its chemicals to bind.
So when a cell breaks open, it releases chemicals
and this chemical will come in, and it will bind.
And when this molecule binds it will cause
the receptor to fire an action potential,
and this action potential will eventually
reach the brain through one of these fibers.
Interestingly enough, nature has actually
evolved its own chemical that can
also activate this TrpVI receptor.
So if you've ever eaten a jalapeno
or any other type of really spicy pepper,
you know that it's very spicy and sometimes
it can cause you to feel a little bit of pain.
When you bite into a jalapeno you're breaking open the cells
that make up the jalapeno and these cells
actually contain a molecule known as capsaicin.
So when you bite into this jalapeno,
the capsaicin will exit the jalapeno
and will actually bind to TrpVI receptors in your tongue,
so this molecule will go over here
and it will bind right over here,
so now when the capsaicin binds it causes
this TrpVI receptor to fire an action potential
which reaches your brain, and your brain
will interpret that as pain,
and will also interpret that as heat.
In fact, if you were to eat a whole bunch of jalapenos,
over time, this receptor would stop firing.
It would become desensitized to the capsaicin
and if you kept eating jalapenos past the point
where this receptor was desensitized,
this receptor can actually die.
So this nerve can actually die off.
And doctors use this in the clinic.
So if somebody has arthritis and it's
a very painful arthritis in their knee,
they can actually inject high levels of capsaicin,
and this high level of capsaicin will actually go in
and kill off these nerve fibers, these pain receptors
that are causing the person to experience pain
at this site of the knee due to the arthritis.