 Light eyeballs, oh my gosh, it is so unbelievably cool. First of all, a little bit of eyeball anatomy. Eyeballs, we have to have some kind of receptor. Eyeballs are your receptor organ. The sensory receptor structure inside your eyeball is your retina. Your retina is made of like six layers of different kinds of cells, and we're going to look at the layers of cells in the next section because that's important for how information gets sent from the photoreceptor cells, which make up one of those layers. The photoreceptor cells in your retina are rods and cones, and I'm sure that you've heard of them already. Rods and cones are different. Rods pick up darkness. They don't pick up darkness. They allow you to see in black and white in darker conditions. They're more sensitive to low light situations. Cones let you see in color. C, c, cone, c, c, color. Cones need more light, brighter light to function. You can play with that to see how dim can you dim the lights and still really actually perceive different colors. Sometimes you look at something in a dim light, and you're like, oh, yeah, that's my blue purse over there. But really, you know it's blue already, so that's why you're perceiving it blue. In the dim light, it's actually kind of gray or black or whatever, some color that isn't color because you're picking up perceptions of your purse or your rods. Cones are most highly concentrated in a part of your retina called the fovea. And the fovea is the place where right now I'm looking at you and all the light that's bouncing off of you is bouncing back and hitting my fovea. Now I can see the other stuff around here. I can see you and the other stuff around here. And the other stuff are like my light. I can see that. I can see the notes that I put on my wall so I wouldn't forget anything. The door, which I'm really excited to go through eventually when this lecture is done. The cookies that are sitting over there, thanks Ash down for hooking me up with some cookies because as soon as I finish this lecture, the yumptualization of cookies will happen. All those things, the light from those things are not hitting my fovea. You are hitting my fovea. You could like read a book with your peripheral vision but it would be really hard and irritating. But all you have to do is turn your head and then voila. All the light bouncing off the book is now hitting my fovea. This isn't a book, it's my hand. And I can see it really clearly now. If you look at the structure of your fovea, that's because basically the light directly hits your cones which are the photoreceptors. And the light will stimulate those cones directly. You don't have to go through all these different layers of cells in order to hit those cones. Now, how cool is this? How great is it that all you have to do is turn your eyes and your fovea will be directly in line with the light bouncing off whatever you are directly looking at. Brilliant, what a great plan. In this section, all we're going to talk about are the photoreceptors, the rods and cones and how they function, how they turn light information into an action potential or lack thereof. That's a hint. All right, rods, there, okay, I got to write this down. Rods are one type of photoreceptor. There are three different flavors of cones. And the three flavors of cones, I have to go back to that thing because I've got it here. The three different flavors of cones pick up different wavelengths of light. It's almost like three different receptors because the blue cones, oh, respond to light in this wavelength, band. The green cones respond to light in this wavelength. The red cones respond to this light in this wavelength. Take a deep breath and imagine that if the combination of cones and rods that are fired when you look at something is going to send a really precise message of color to your brain, depending on who fired. Did 32 blue cones fire with a couple green cones and a red cone? That's going to give you a sense, or a perception of a certain bluish color. You can play with that, like whoa, it's really kind of wild. So cones, we know that there's three of them. Rods and cones have very similar functions or mechanisms. So I'm just going to do the rods. And I'm going to do draw the cell itself in black. And they had a really cool shape. That's awesome. But I'm just going to make it kind of a dull shape because really the bottom line is that all we're perceiving here, and I have to draw it in yellow even though rods don't pick up yellow colored light. The light is just going to pass into the cell itself. And once the light is in here, this is so cool. The light runs into a molecule called rhodopsin. I don't think I can fit it in that word. So this is a molecule called rhodopsin, rhodopsin. Rhodopsin is found in rods. In cones, there's another molecule that has a different name, three different named molecules, but they function very similarly. So let's undo our numbering again. In comes the light. The light connects to rhodopsin. Guess what happens next. Rhodopsin breaks apart into two new molecules. What? The light causes it to do this. It breaks into opsin and retinol. Seriously? Rhodopsin cracks in half and now we have opsin and retinol. And the breaking in half of rhodopsin, that process is called bleaching. So let's make number three that bleaching happens because rhodopsin broke into opsin and retinol. Now, think about this. As soon as a light molecule breaks rhodopsin inside a rod or a cone, a rod, because we're talking about rhodopsin, that's, that, you're going to have to put rhodopsin together again before that particular rod can be activated by a light molecule again. So that's going to take time. That's going to take a little bit of energy to do. That's an important concept. Once rhodopsin is broken, huh, what's this? We get a second messenger cascade. And the end result of the second messenger cascade is take a deep breath, sodium channels. What do you think? Well, of course, you think they open. No. They close. What? That was number four. So you bleach rhodopsin and you cause sodium channels to close. And guess what that does? That hyperpolarizes. This is number five. The cell membrane is hyperpolarized, which means are we going to get an action potential? No action potential. But guess what? The next cell in line. Do I want to draw a picture of it here? Hmm, like that. That looks totally like it. It's called a bipolar cell. And we're going to talk about that in the next one. The bipolar cell says, where's my neurotransmitter? Gone! So, wait a second. No action potential. So the bipolar cell, bipolar cell says, need neurotransmitter. And guess what it does? It sends the message along. It fires. What? It fires its own action potential because it did not get information. It did not get neurotransmitter from the rod. That's so awesome. So in this scenario, chronically, if you do not have light, if you don't get stimulated by light, no stimulus. If you don't have chronic leakiness, leaky neurotransmitter. And I can't remember. I think it's glutamate. Yeah, leaky neurotransmitter that's glutamate. That says glutamate. Glutamate is like chronically dumped on this bipolar cell. And the bipolar cell is like, awesome. I'm getting my neurotransmitter, so I don't have to freak out. I don't have to do anything. It's just coming like every little chunk of time. And then all of a sudden, light hits, rodopsin splits, bleaching happens, second message cascade, sodium channels close, bipolar cells say, what the hell just happened? And where's my neurotransmitter? I'm better fire a message and tell the dogs upstairs that something's going funky here. Oh my gosh. How amazing is that? Now, you think that's crazy. I think that's crazy. Surely you think that's crazy. Wait till you hear the rest of the story. Let's come back for the rest of the story.