[MUSIC] Okay, so, now we're going to talk about photo receptors, and they come in two varieties. Before we talk about the two varieties, the rods and the cones, let's talk about when we use vision. We use vision over this incredibly wide range of brightness. We can see a single photon when we've been in the dark. It's a, let's say it's a new moon. There's no light out. It's pitch black, and if there was a single photon, we would be able to detect that. Very, very dark conditions, even a small moon can give you what's called scotopic conditions, where there's not very much light. And on the other hand, we still can see in bright sunlight, and that's called photopic. And in between is is called mesopic. So, how many, how, how, how big is this range? Well, it is many orders of magnitude, which means it's, they are log units about, I think it's about 12 log units from scotopic to photopic. And, and that means that you can go from pitch black to bright, bright sunshine, and you can still see. And how do we do that? Well, in part, we do it by assigning these dimmer conditions to a different set of photoreceptors than these brighter conditions. And there's an overlap. There's an overlap here somewhere in, in the mysopic range, where both the the rods, which are useful in dim light conditions and the cones, which are the only things offered of, in photopic condition. So, there's this range where they're both operating. And if we come over here, I've drawn out a model of a one rod and one cone. So, they look different. And just to orient you, this is the opposite orientation from what we just had on, on the screen. The light is coming up in the instance. The retinal pigment aphelion is up at the top. And this area here, this is the outer segment. And the outer segment of the rod and of the cone look a little different. This looks like a paddle. This looks like half half a tree, half a tree, half a Christmas tree, say. And along the the surface or within this outer segment are all of these molecules of rhodopsin. This is the, the molecule that's going to turn light into neural energy. And what's interesting about how it, that is done, is that, let's take the situation of a rod. This is the rod. In the situation of the rod, there, the rodopsin molecules, are actually internal. They're inside the cell. They're sitting on membranes that are inside the cell. And so, when light comes in here, we'll get, get a new color. So, light's going to come in here. It's going to activate this molecule, and this is actually a metabotropic receptor, which means it doesn't actually have a channel, doesn't have a pore. But what it does, is it changes an enzyme, which then goes, and has to travel to the edge of the cell, and open or close a cell an ion channel. So, as you may imagine, the amount of time that this takes is actually long. it takes much longer than it takes, for instance, just to change it, an ion channel directly. And that's one of the reasons why vision is a very slow process. It doesn't really matter, because in the natural world, our, what we look at doesn't change on less than say, two millisecond time scale. Things aren't changing at any time scale that this would this would effect. Okay, so, that's the photoreceptors work, and what's different about the rod and the cone? Well, the rod is extremely sensitive to light. So, it can respond to a very low number of photons hitting here. The cone is less sensitive. The, each of these has a preferred wavelength that it likes, and as it turns out, there is one type of rod, but in humans there are three types of cones. So, there are three different types, and each of those types has a wavelength of light that is preferred that it responds maximally to. And what that means is that the cones can be used. They don't produce color, but they can be used by the brain to enable us to perceive color. So, the cones are necessary for perceiving color. And so, it, if you're only using rods, could you use that information to perceive color? And the answer is no. And so, consequently, under scotopic conditions, there's no, there are no we cannot perceive color. Under photopic conditions, we see, see vibrant colors. Under mesopic conditions, we see muted colors. And so, actually, I use this every morning, when my cats want to go out, they want to go out before sunrise. And it's, it's mesopic. And I, the deal I have with them, is that once it gets light enough that I can see muted colors, we're out of scotopic. We're out of coyote land, into mesopic, and they can protect themselves, and they can go out. So so, that's one difference. There's a second difference, which is that the rods, not only are they sensitive to more sensitive to light, but they also gather information from a wider part of the, of the visual world. So if, let's just imagine that we're fixated, we're looking at this x, and there's a rod, and it might gather information from that much of the visual world. On the other hand, a cone of any type would gather information from a much smaller region. And therefore, which of these is going to be useful when you want to go and read a book? Okay, what's, what's the answer? And the answer is cones. Rods are not going to, you cannot read with rods. So, you cannot read under scotopic conditions. You need your cones. And in fact, what, the point I, I, the take home point here is that yes, cones are important for for, for perceiving colors, but the real importance to cones is to be able to see high acuity, to see fine details. We see that, because cones have these very small, what are called receptive fields, they receive information from small field, as opposed to the rods, which receive information from a very large field. So it's the, it's the cones that enable us to read a book. And that's probably much more, more important for us than seeing colors. Seeing colors is very important, or perceiving colors is very important, but being able to have high acuity vision is really what the, the cones are really spectacular at. In the next segment, we're going to look at the distribution of rods and cones across the retina, and we're going to look at central vision. [MUSIC]
