So take a look at this image.
On your left, we have a nice sunny day
and you can see color and light and
everything seems really great.
And then we know that dust time at
night where things start to get dark.
On the right you notice,
you don’t see color.
All you see is borders and shade, but
you don’t see clear color, right?
And that is because of the differences
between what’s being activated
in terms of the rod cells
versus cone cells.
So color is cones and darkness
and light detection is rods.
here is an image illustrating
the difference between the two.
We can see where
it gets its name.
The rod has this rod shaped, you know,
cricket or baseball bat formation.
And the cone, I’m thinking
ice cream cone upside-down.
So they have this unique shape,
which is where they get their name.
And you look at where the lights, so the
arrow at the top of the diagram is showing
where the light interacts
with these cells.
And there are certain
features that they share.
So they both have a nucleus, they both have
mitochondria which is an energy source.
And they have an outer segment
that contains this chemical
and this photosensitive chemical or pigment
is what allows us to actually
engage and detect this light.
So let’s get in to more
detail around that.
First, we’ll take a look at sort of the
characteristics of rods versus cone.
So rods or scotopic vision is where we
have these rod cells being activated.
Now in the human retina, we have
about 120 million rod cells.
And for sure, the MCAT exam is going to
make you, force you, one way or the other,
to differentiate between
which one is more abundant.
And that would be the rods
and maybe even numbers.
So 120 and we’ll see how many
cones are in just a sec.
So they’re strongly photosensitive,
meaning light dark.
So less detail, no color,
concentrated at the periphery.
So they’re not at the fovea centralis,
they’re more around the
edges of the eye, okay?
So they’re using low
So a little, you know, a little,
I don’t want to say trick, but
a little, a little behavior
you can do is when you’re
out and it is dark,
if you’re looking directly
at something that’s dark
and that’s what you’re trying to
detect, you won’t see it very well.
But if you move your eye a little
bit to the side, you’ll actually be
bringing the light to the edges where a lot
of these rod cells are actually focused.
A lot of astronomers when
they’re looking at stars
and they’re having trouble
detecting a star,
look a little bit askew to the star
that you’re actually trying to detect
and you’ll see it a
little bit better.
Cone cells are found a
lot less abundantly.
It’s around six million.
And they’re weakly photosensitive so
they are okay with light, not great.
They’re more about like high
acuity, more resolution,
more detail and
So in order to remember cones, color
that sees, I think that might help you.
Concentrated at the fovea
like we’ve mentioned
and they’re great at bright
both respond to light and there
has to be something else
that they have in common that allows them
to sort of do their function
in a similar fashion.
Because really, I mean,
they sort of look the same
but they have all these
They’re both responding to light,
but what’s driving that difference?
Well, the difference lies
into the actual processing
and some of the proteins
that they contain.
So they each have a pigment protein called
opsin that changes in response to light.
So it’s a photosensitive pigment.
Now opsin contains a pigment
molecule called retinal,
which is expressed as one of two things
depending on which cell you’re in.
The rods have rhodopsin.
So that might help you remember.
Row, rhodopsin, rod, right?
So rhodopsin and rod cells
and photopsin in cone cells.
So depending on the presence
of which molecule is there,
that will determine what type of
processing is going to happen.
So at rest, opsin keeps the sodium
channels open making the cell depolarize
which causes glutamate release.
So we should kind of
try and grasp that.
At rest, opsin keeps the sodium channels
open keeping the cell depolarized.
And if you remember from our
previous lectures that we’ve done,
we were talking about
resting membrane potential
and depolarization versus
Now phototransduction is the
process of how we convert this
photon of light into
So the photon of light converts
retinal from its all-trans form.
So again, we don’t want to go deep
into chemistry but you can have a
“trans” or “cis”
form of a molecule.
So everything right now would be all
transformed and closes the sodium channels,
which hyperpolarizes the cell, okay?
So light, converts retinal to
all-trans form and hyperpolarizes the cell.
Glutamate then inhibits bipolar cells.
And once the hyperpolarization
is removed by the inhibition,
this activates the ganglion cells, okay?
So activation of the ganglion cells
then together sends information
through the optic
nerve to the brain.
Now the optic nerve does a lot
of different really cool things.
It can activate and --
a lot of different parts of your brain.
They do different things other than
just straight visual acquisition.
But for our discussion here,
we’re talking about how this
optic nerve projects to
the visual centers of the brain.