Today is sensation. And specifically what
we're going to look at is our sense of
sight, so vision. So we know we have five senses. And each one of our senses
requires two things. The first is some sort
of physical stimulus. And in the case of vision that
physical stimulus is light. And then the second
thing that it requires is some sort of receptor. So some kind of
specialized cell that can take the physical stimulus. So in the case of vision,
that can take the light and convert it into
a neural impulse. So the receptor in
the case of vision is something called
a photoreceptor. So we're going to go into the
photoreceptor in a little bit. But first, let's just
focus on the light. So what is light? Light is an
electromagnetic wave that is part of a large spectrum. So there's something called
the electromagnetic spectrum. And it contains
everything from gamma rays and x-rays all the way
to AM and FM radio waves. And so light just
falls in the middle. And it ranges from
something like violet, which has a wavelength of 400
nanometers, all the way to red, which has a wavelength
of 700 nanometers. So the rest of the
light that we see is somewhere in the
middle over here. So basically a light is
this electromagnetic wave that gets emitted from a
bunch of different sources. So one of the most common
sources, the most well known, is our friend, the sun. So the sun emits a bunch
of light wave rays. These light rays come to
earth and some of them go into our eyeballs. So let's go ahead
and look at what happens when a light
ray from the sun-- let's keep them right here--
comes down and hits an eyeball. So a little light ray comes in. And there's just a little
guy standing over here. And let's just
zoom in on his eye. So let's pretend
this is his eyelid. Let's pretend he's
looking right at the sun. You normally don't want to
do that but let's go ahead and-- so this is the little
pupil, the little hole in the eye in which
the light enters. So this is the
front of his head. This is the back of his head. So light comes into the eye
and hits the back of the eye. So in the back of
the eye there's this really special and
interesting structure called the retina. So it just lines
the back of the eye. It's this membrane. Just coats the very
back of the eye. And it's composed of a
bunch of different cells. And so inside the retina there
are two really important cells. So let's go ahead and write the
names of those two cells down. So one of those cells
is called a rod. And it looks kind of like a rod. So it's got this rod shape. If you actually look
at a microscope, looks like a little rod. And the other cell
is called a cone. So inside the retina
there are these two cells. And they're rods and cones. And it's called a cone because
it looks like a little cone. And they're both smiling
because they're happy. So basically these guys
are all over this retina. So they're in there along
with a couple of other cells that we'll touch
on in a little bit. But basically they're
really important. Because what they do is
they actually take the light and they convert it
into a neural impulse. So these are the big players. These are the receptors that
we were referring to before. So let's talk a little bit
about what the difference is between a rod and a cone. So rods have-- there are
about 120 million of them. And they're really
sensitive to light. So they're really, extremely
sensitive to light. And they're really
good for night vision. So when there's not
a lot light out, they're really sensitive to
it, so the little bit of light allows you to see at night. And they're also found all
around the periphery over here. So there are a bunch of
rods over here and over here in the periphery
of your eye that allows you to see on the sides
and allows you to see at night. There are a lot less cones. So there are about six to
seven million cones per retina. And these cones, even though
there are fewer of them, they're really important
because they're responsible for color vision. So there are three
different types of cones. There are red cones. There are green cones. And there are blue cones. So write blue cones. And we split them up into
these three categories because red cones are really
sensitive to red light. And green cones are
sensitive to green light. And blue cones are
sensitive to blue light. So there are these
three main types of cones that absorb
light that ends up being red, green, and blue. So the cones are centered
in this little region of the retina right here. And we call that the fovea. So there are almost no rods
in this part of the eye. And there are a
whole bunch of cones. So much almost all the cones
are centered in the fovea. And the fovea is
basically the part of the eye that
let's us see really fine details in pictures. So if you're
searching for Waldo, the fovea is what
let's you find him. So what happens now that
the light enters the eye? It enters the retina. And it hits the back of the eye. What happens now? So basically the next
thing that happens is something called the
phototransduction cascade. So this
phototransduction cascade is basically a
set of things that occurs as soon as light
hits a rod or a cone. So as soon as light
hits this guy, the light wave triggers this
phototransduction cascade. And we're going to go
into the phototransduction cascade in the next video. But let's go ahead and
skip over it for now. Just so that we can
explain what occurs at the end of the
phototransduction cascade. So let's go ahead and focus
on just the rod for now. So here's a nice, happy rod. And light comes in from the
sun, goes through the pupil, hits the retina, and then
hits this little rod. So normally this rod
is actually turned on. So when there's no light
this rod is turned on. But when light
comes and hits it, it actually turns the rod off. So when the rod is turned
off, in a weird way it actually turns on this
other cell over here, which is called a bipolar cell. So basically by the rod turning
off when it's exposed to light, it actually turns
on a bipolar cell. And the bipolar
cell in turn, turns on another cell called
a retinal ganglion cell. And this retinal
ganglion cell basically goes into the optic nerve
and then enters the brain. So, to the brain. So basically the process
turning the rod from on to off is the phototransduction-- can't
spell-- transduction cascade. So this is generally
what happens when light hits the retina. It hits the rod, turns
on all these cells, and then enters the brain. And then your brain
goes and makes sense of what's happening by
creating a rich visual field, which we can enjoy
every single day.