Our retinas have 3 types of nerve cells: photoreceptors, bipolar cells, and ganglion cells. They're arranged in 3 layers, kind of like a stack. So here we have the back of our retina, and here is the front where the light is going to be coming from. Now, you'd probably expect the photoreceptors 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 photoreceptors are actually found all the way at the back. I'm just going to jump out of the way here so you can see what I write. These are photoreceptors, 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 photoreceptors than 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 fovea, and the rods are more concentrated on the peripheries.
Now this middle layer, which has this yellow line through it here, is where our photoreceptors connect to the bipolar cells. And these bipolar cells, which you can see here, are going to bring information from the photoreceptors to the ganglion cells. And it should be noted that the ganglion cells take input from multiple bipolar cells. So in essence, ganglion cells are 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, you know, has some interesting facets to it. That is that the photoreceptors and bipolar cells actually have graded potentials, not action potentials. But the ganglion cells are what, say, send action potentials. Here we have graded potentials sent through the photoreceptors create graded potentials based on the 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 to actually produce an action potential. Now remember that the receptor potential is generated by hyperpolarization from the opening of ion channels, and that, this is, you know, a little strange, a little different from what we're used to. You know, normally we think of depolarization leading to an action potential, but in this case we actually have a hyperpolarization 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 going to be synapsing 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 a mishmash really. You know, here we have a really nice image, I'll jump out of the way, so you can see it better, that sort of illustrates how this happens. So, here we have, you know, what the eyes are looking at. And the rods and cones are going to produce, you know, different outputs of that image. The rods are going to sort of form a black and white version, and then our cones are going to show colored versions. This is going to have to be processed, to detect different types of color and edges, you can see. And that's going to be played with in many different ways, and all of these different facets of the image are put together to actually develop something that looks like this. Essentially, different parts of the visual cortex are going to play with this information in different ways, and then that all has to be integrated.
It should be noted that we actually have what's called binocular vision. Right? We have two eyes, so we actually get two images of the world around us. And it should be noted that these images aren't exactly the same. Because if you think of each of our eyes as a camera, then our cameras are in very slightly different positions. But that is the key to our ability to perceive the world in 3 dimensions, or perceive a sense of depth, I should say. You see, by taking essentially two pictures from slightly different angles, our brain can compare 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.