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Photoreceptors (rods vs cones)

In this video, I explore the similarities and differences between rods and cones. By Ronald Sahyouni. . Created by Ronald Sahyouni.
Video transcript
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.