The Retina

by Jason Amores Sumpter
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our retinas have three types of nerve cells photoreceptors, bipolar cells and ganglion cells. And they're arranged in three layers. Kind of like a stack. So here we have the back of our rat now, And here is the front where the light is going to be coming from now. You'd probably expect the photo receptors to be right up at the front to greet these waves of light coming in. But you'd be wrong. Evolution is not perfect, remember? So our photo receptors are actually found all the way at the back. I'm just gonna jump out of the way here so you can see what I write. These are photo receptors and you can see we have rods here on the outer edges of our retina and in the center is where we have cones. So obviously we have many more photo receptors that are being shown here. In this diagram. I just want to point out that the cones are concentrated in the center of the retina. Remember, in that area called the phobia and the rods air more concentrated on the peripheries. Now this middle layer, which has this yellow line through it here is where our photo receptors connect to the bipolar cells. And these bipolar cells, which you can see here, are going to bring information from the photo receptors to the ganglion cells. And it should be noted that the ganglion cells take input from multiple bipolar cells. So in essence, ganglion cells air really receiving input from multiple rods and cones at the same time because they synapse on more than one bipolar cell. So the way this communication works is also has some interesting facets to it. That is that the photo receptors and bipolar cells actually have grated potentials, not action potentials. But the ganglion cells are what send action potential. So here we have graded potentials sent through the photo wrists or the photoreceptors create graded potentials based on light coming in. They cause the bipolar cells to have graded potentials and multiple bipolar cells synapse on a ganglion cell and they can lead the ganglion cell tow actually produce an action potential. Now, remember that the receptor potential is generated by hyper polarization from the opening of ion channels and that, uh, this is, you know, a little strange, little different from what we're used. Thio. Uh, you know. Normally we think of deep polarization leading to action potential. But in this case, we actually have a hyper polarization that will then be translated into an action potential by the ganglion cells. Now the ganglion cells, axons, form the optic nerve and actually bring the information to the brain to be turned into something useful. Now, the reason I went with this image, even though this one looks a little nicer, is because I just wanted to convey that the ganglion cells are gonna be synapse ing on multiple bipolar cells, which you don't really see too much of in this image. So even though it's a little prettier, it labels everything nicely for you. I wanted to make sure that, you know, I stress the point that ganglion cells are receiving inputs from multiple rods and cones. Now, this information, when it comes to the brain, you know, it has to be interpreted. It's just mishmash, really. You know, here we have a really nice image. I'll jump out of the way. Eso you can see it better. That sort of illustrates how this happens. So here we have. You know what the eyes air looking at and the rods and cones. They're gonna produce different outputs of that image. The rods they're going to sort of form a black and white version, and then our cones, they're going to show colored versions. This is going to have to be processed to detect different types of color edges you can see. And that's going to be, you know, played with in many different ways. And all of these different, uh, facets of the image put together to actually develop something that looks like this, you know, essentially different parts of the visual cortex. They're gonna play with this information in different ways, and then that all has to be integrated. So each of these frames is just showing you sort of like a different version of what some group of cells is going to process and output in that all has to get eventually put together to create meaningful representations of the world around us. Now, it should be noted that we actually have what's called binocular vision, right? We have two eyes, so we actually get to images of the world around us. And it should be noted that, you know, these images aren't exactly the same because if you think of each of our eyes is a camera, then our cameras Aaron very slightly different positions. But that is the key to our ability to perceive the world in three dimensions or perceive a sense of depth. I should say, you see, by taking essentially two pictures from slightly different angles are brain concurrent pair. Those two images look at the differences between them and from that generate a sense of depth. So pretty sophisticated processing has to happen to this simple visual input in order to actually generate anything meaningful from it. With that, let's go ahead and flip the page.