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Current time:0:00Total duration:5:06

Photoreceptors (rods vs cones)

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 around 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 disks 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 the 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 there's 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 foe Thompson 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 will actually trigger the phototransduction cascade that results in this rod firing 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 rena there are about a hundred and twenty million rods in contrast there are about six 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 coded by a membrane known as the retina so rods are actually found on 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 in this region is known as the fovea and combs 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 there are 1000 times more sensitive to light than cones 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 and 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