Let's examine the difference between rods and cones
in our eyes.
Let me draw a very simplified schematic of a rod just
to give you an idea of what it looks like.
So rods actually get their name because if you look at a rod
under a microscope, it actually has this elongated cell
body that kind of gives it a rod shape.
So a rod is a photoreceptor.
What exactly is a photoreceptor?
Is it a neuron?
Is it a type of nerve?
So, in fact, it is.
It's a very specialized type of nerve
that's able to take in light and convert it
into a neural impulse.
So inside a rod, there are a whole bunch
of structures known as optic discs.
And these optic discs are large, membrane-bound structures
inside the rods.
And there are thousands of them in an individual rod.
Embedded within the membrane of each optic disc
is a whole bunch of proteins, and these proteins actually
absorb light and begins a phototransduction cascade that
eventually leads this rod to fire an action potential that
will reach the brain.
Similarly, a cone gets its name because it's cone-shaped.
Cones are also photoreceptors.
So they're specialized nerves that
have the same internal structure as a rod.
So cones also have a whole bunch of these optic discs that
are stacked upon one another, and embedded
within each optic disc is a whole bunch of this protein.
So as I mentioned over here, the protein in a rod
is known as rhodopsin.
In cones, it's basically the same protein.
But it just has another name, and it's called photopsin.
So as I mentioned, as a ray of light enters the eye,
if it happens to hit a rod, and it happens to hit rhodopsin,
it'll actually trigger the phototransduction cascade
that results in this rod firing an action potential.
This exact same process happens in a cone.
So these are the major similarities
between rods and cones.
Now let's look at the differences.
So in an average retina, there about 120 million rods.
In contrast, there about 6 million cones per retina.
So there are about 20 times more rods
than there are cones in each eye.
Another big difference between rods and cones
is where they are located in the eyeball.
So if I draw a very simplified diagram of an eyeball, and this
is the optic nerve exiting the back of the eye.
So this would be the front of the eyeball.
This is the back of the eyeball.
And as I mentioned in a previous video, the back of the eyeball
is coated by a membrane known as the retina.
So rods are actually found in the periphery of the eyeball.
So they're found in this area over here
and in this area over here.
And there's actually a region of the retina, right about here,
that actually dimples in.
And this region is known as the fovea,
and cones are mostly concentrated
in this region in front of the fovea.
So rods are mostly found in the periphery of the eye,
whereas cones are mainly found near the fovea.
Another big difference between rods and cones
is that rods do not produce color vision, whereas cones do.
So rods are very sensitive to light.
In fact, they are 1,000 times more
sensitive to light than codes are.
For this reason, rods are really good at detecting light.
So they're basically responsible for telling us
whether or not light is present.
Another way to think of this would
be black and white vision.
On the other hand, cones are not as sensitive.
But they do result in the detection of light.
So they result in color vision.
And in fact, there are three different types of cones.
So there are red cones, which make up about 60%
of all cones in the eye.
There are green cones, which make up about 30%
of all cones in the eye.
And there are blue cones, which make up about 10%
of all cones in the eye.
Another major difference between rods and cones
is their recovery time.
So rods have a very slow recovery time,
whereas cones have a very fast recovery time.
So what I mean by slow and fast recovery times
is that as soon as a rod is activated by a ray of light--
so let's imagine that a ray of light
comes in and activates this rod, and it
fires an action potential.
It takes a lot longer for the rod
to be able to fire another action potential than it does
for a cone, and you've actually experienced this.
So if you've ever been outside, playing soccer or football,
and you run inside to get a cup of water,
there's a big change in illumination,
yet you don't stub your toe.
You're able to transition from outside
to inside really quickly.
That's because cones are able to rapidly adapt
to changes in illumination, whereas rods take a lot longer.
So at night, when you walk into a dark room,
it takes a while for your eyes to get adjusted to the dark.
And that's because the rods need to be reactivated, reset,
in order for you to be able to use them to see anything.