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Current time:0:00Total duration:8:02

Video transcript

today is sensation sensation and specifically what we're going to look at is our sense of sight so vision so we know we have five senses and each one of our senses requires two things and the first is some sort of physical stimulus and in the case of vision that physical stimulus is light and then the second thing that it requires is some sort of receptor so some kind of specialized cell that can take the physical stimulus so in the case of vision that can take the light and convert it into a neural impulse so the receptor in the case of vision is something called a photoreceptor photoreceptor so we're going to go into the photoreceptor in a little bit but first let's just focus on the light so what is light light is an electromagnetic wave electro magnetic wave that is part of a large spectrum so there's something called the electromagnetic spectrum and it contains everything from gamma rays and x-rays to gamma rays all the way to AM and FM radio waves and so light just kind of falls in the middle and it ranges from from something like violet which has a wavelength of 400 nanometers all the way to red which has a wavelength of 700 nanometers so the rest of the light that we see is somewhere in the middle over here so basically a light is this electromagnetic wave that gets emitted from a bunch of different sources so one of the most common sources the most well-known is our friend the Sun so the Sun emits a bunch of light weight rays these light rays I come to earth and some of them go into our eyeballs so let's go ahead and look at what happens when a light ray from the Sun to keep them right here comes down and hits an eyeball so a little right light ray comes in and there's just a little guy standing over here and let's just zoom in on his eye so let's pretend his eyeless pretend he's looking right at the Sun you normally don't want to do that but let's go ahead and so this is the little pupil the little hole in the eye in which the light enters so this is the front of his head this is the back of his head so the light comes into the eye and hits the back of the eye so in the back of the eye there's this really special and interesting structure called the retina so it just kind of lines the back of the eye it's this membrane just kind of codes the very back of the eye and it's composed of a bunch of different cells and so inside the Ranna there are two really important cells so let's go ahead and write the names of those two cells down so one of those cells is called a rod and it looks kind of like a rod so it's got this rod shape if you actually look at a microscope looks like a little rod and the other cell is called a cone so inside the retina there are these two cells and their rods and cones and it's called a cone because it looks like a little cone and they're both smiling because they're happy so basically these guys are all over this retina so they're they're in there along with a couple of other cells that we'll touch on a little bit but basically they're they're really important because what they do is they actually take the light and they convert it into a neural impulse so these are the big players these are the receptors that we were referring to before so let's talk a little bit about what the difference is between a rod and a cone so rods have there are about 120 million of them and they're really sensitive to light so they're really extremely sensitive to light and they're really good for night vision so when there's not a lot of light out they're really sensitive to it so the little bit of light allows you to see at night and they're also found all around the periphery over here so there are a bunch of rods over here and over here in the periphery of your eye that allows you to kind of see on the sides and allows you to see at night so cones there are a lot less cones so there are about six to seven million cones per retina and these cones even though there are fewer of them they're really important because they're responsible for color vision so there are three different types of cones there are red cones red there are green cones green and there are blue cones so right comes and we split them up into these three categories because red cones are really sensitive to red light and green cones are sensitive to green light and blue cones are sensitive to blue light so there are these three main types of cones that kind of absorb light that ends up being red green and blue so the cones are really are centered in this little region of the retina right here and we call that the fovea so there are almost no rods in this part of the eye and there are a whole bunch of cones so much almost all the cones are centered in the fovea and the phobia is basically the part of the eye that lets us see really fine details and pictures so if you're searching for Waldo the phobia is what lets you find him so what happens now that the light enters the eye red enters the retina and it hits the back of the eye what happens now so basically the the next thing that happens is something called the photo transduction option cascade so this phototransduction cascade is basically a set of set of things that occurs as soon as light hits a rod or a cone so as soon as light hits this guy this the light wave triggers this phototransduction cascade and we're going to go into the phototransduction cascade in the next video but let's go ahead and skip over for now just so that we can explain what occurs at the end of the phototransduction cascade so let's go ahead and focus on just the just the rod for now so here's a nice happy rod and nice and happy and light comes in from the Sun goes through the pupil hits the retina and then hits this little rod so normally this rod is actually turned on so when there's no light this rod is turned on but when light comes and hits it actually turns the rod off so when the rod is turned off it in a weird way it actually turns on this other cell over here which is called a bipolar cell bipolar cell so basically by the rod turning off when it's exposed to light it actually turns on a bipolar cell and the bipolar cell in turn turns on another cell called a retinal ganglion cell ganglion cell and this retinal ganglion cell basically goes into the optic nerve optic nerve and then enters the brain so to the brain so basically the process turning the rod off from on to off is the phototransduction trans photo can't spell transduction cascade so this is generally what happens when light hits the retina it hits the rod turns on all these cells and then enters the brain and then your brain goes and makes sense of what's happening by creating a rich visual field which we can enjoy every single day all right