I got my eye on you.
Let’s talk about vision.
We’re going to talk about structure
and function of the eye.
the eye is comprised
of three layers:
the sclera, the choroid,
and the retina.
The sclera is what gives
us the white in our eyes.
That’s the white portion of the eye
and that’s the outermost layer.
Then we have the choroid, which
is the darkly colored --
the darkly colored layer and that
absorbs all the excess light.
And then we have the retina which is
the one that you hear about the most,
and this is the surface where
light is actually focused
and a lot of that
So we can see all the three layers here.
And you’ll also notice that the sclera
continues to go over the front of the eye
and that’s a clear portion of the eye
that we’ll talk about in just a sec.
So the idea here is light is refracted
as it passes through the cornea.
So the outside light coming in
goes through to the cornea,
which is an extension of the sclera.
And its job is to actually control
where it’s going to go a little bit.
It enters the anterior chamber.
So there’s a chamber there that’s filled
with a fluid termed aqueous humor.
You’re going to need to know
that term for sure for the MCAT.
So one way you can try to remember
that is aqueous humor,
I don’t know. Sometimes that helps me.
Sometimes it’s stupid, but
I like stupid things.
So aqueous humor, funny, water.
Now that, that fluid
fills that chamber.
And what happens is as the
light enters, there’s an iris.
And the iris is the
colored part of the eye,
that’s what gives us the color of our eye,
and it has an opening called
the pupil which appears black
and that’s where the
light is going to enter.
the size of that pupil, we can
control and we can influence.
So the size of the pupil
controls the amount
of light which is going
to enter the eye.
And this is mediated by
two separate muscles.
We have the dilator muscle
and the sphincter muscle.
And so the way to remember
that, I think we all
heard the term before
dilation or to open.
So this is a muscle that
would open your eye.
And then the sphincter, you can think of
sphincters in your GI tract or your rectum.
That makes things smaller. Okay?
So dilator muscle opens,
sphincter muscle makes small.
So it’s kind of interesting to
note where these are coming from
and that we have constriction
and we have dilation.
Now, the constriction is controlled
by the parasympathetic system,
while the dilation is actually
mediated by the sympathetic system.
If you recall from previous lectures,
parasympathetic refers to
the rest and digest action,
and sympathetic refers
Now, if you think of the fight-or-flight
situation, that’s when you’re aroused
and you have a stressor in front of
you and you need to make a decision,
one of the actual physiological
effects that you see in individuals
is a dilation of their eyes.
And you might imagine this because they
need to take in as much information
and you get this tunnel vision where
they’re really focused at the task at hand
because that's their stressor.
So it’s not a coincidence that this is
controlled by the sympathetic system
while the rest and digest, you’re
relaxed, the eyes are droopy,
you don’t really need to be focused
that much, you see the constriction.
let’s take a look at once the light has
entered and passed into that chamber.
Behind that posterior chamber is the lens
which fine-tunes the angle
of the incoming light.
So this is really, really great.
If you ever played with a microscope
in class, you have two knobs;
you have your coarse adjustment
knob and your fine adjustment knob.
So we’re now entering the
fine adjustment portion.
So here what happens is that lens is going
to focus the light on a part of the retina,
the back of the eye
that we wanted to.
And so we can control.
We can control the
curvature of this lens.
Now the curvature of the
lens is normally convex.
If it was not convex, we’d be concave. We’d
be weird. We’d have weird-looking eyes.
So it’s actually convex
as you can see here.
And it controls refractive power.
So you should know that as well.
And it does that through a certain
muscle called the ciliary muscle.
And you can think of it
as something that’s --
these muscles when they contract,
they actually pull or they
pull the lens this way.
And as they pull, it can determine the
degree of curvature of that lens.
So, here’s a lot here to digest.
There’s a lot of
information, but we’re going
to walk through the major
You know you’re going to probably have to
spend some time studying this diagram.
But for now, we want to
appreciate the different layers
and the components that
make up this diagram.
I want you to notice at the top,
we have some structures that look like some
rods and some look like different cones.
Those are photoreceptors
called rods and cones
that we’re going to look at
in a little bit more detail.
They project to a bunch of
neurons, bipolar cells,
and we collectively call this
other layer interneurons
and these will then go on to
connect to retinal ganglion cells
which we’ll look
into more detail.
And those ultimately exit the
eye and go to the brain.
for the MCAT, you should be
well-aware of these different layers.
We should be aware of the
orientation as well.
And we should be aware of the fact that this
is part of the retinal layer of the eye.
Now, you might think
these rods and cones
need to detect the light and
so lights going to hit them.
It’s actually flipped.
The light comes in through this network and
hits, you can see the arrow pointing up,
that represents the light
going in and it actually is
as far possible from the rods and cones
and makes its way to the rods and cones.
the retina contains two types of
electromagnetic receptor cells,
rods and cones.
The rods and cones synapse
with bipolar cells
which then synapse with
retinal ganglion cells.
The cones and rods are
the most deep layer.
So light comes from the bottom
and they’re at the far end.
And the axons of the ganglion
cells from the optic nerve --
sorry form the optic nerve.
So all of the ganglion
cells coalesce together
and create a sort of
a bundle of fire.
Imagine a computer setup, a desk at
home with all your printer cables and
monitor cables and you
bundle them all out
and that’s what leaves your
desk sort of same idea.
let’s take a look at a blowup, a
small section of the retinal layer
and you can see the orientation
that I’m talking about now.
So we can see that we have the
rods and cones on the far end
and we can have the interior
as the retinal layer
and we have the interneurons in between
and these all interact in synapse with
the retinal ganglion cells which go on and
form the optic nerve exiting the eye.
Now, the point on the retina where many
axons from the ganglion cells converge
to form this optic nerve and that leave the
eye, that area is called the optic disk.
So there’s a spot within your eye
because it’s leaving the eye,
you won’t have any receptors there.
And that is what we call the blind spot.
So we’ve all experienced this before
and there’s lot of different test you can
do of putting something in front of you
and moving your head away and you’ll
notice that you can no longer see it.
If you’ve ever driven a car, you know you
have something called your blind spot,
and that’s where you have that spot
in your eye where you can’t look
or it’s a spot that you
can’t see anything.
That’s what they’re referring to.
So one of the simplest experiments is on a
piece of paper draw a black dot or an X.
And as you move it away, closer move
or it farther and closer to your face,
you’ll notice the spot where you have
that blind spot in terms of distance.
So the macula is an oval-shaped pigmented
area near the center of the retina.
And you got to imagine a 3D space.
In the center of that is something
called the fovea centralis,
which is the largest
concentration of cone cells,
and this is responsible for
central, high-resolution vision.
And we’re going to get into the
difference between rods and cones,
but cones are really, really high-resolution
cells and they detect color.
And so this is the spot in
the fovea centralis where
we have the highest
concentration of color vision,
which is what we use
most of the time,
and so that’s where a lot of our light
that’s entering the eyes is focused upon.