As we continue to follow the pathway of light through the eye, we've now reached the inner layer, and this is where we have the photoreceptors. So this is where light gets absorbed, and we start to send an image to the brain. Alright. So remember, the inner layer is also known as the retina, and you're going to see it called the retina far more than you'll ever see it called the inner layer. And the retina functions for what we're going to call phototransduction, and phototransduction just means the conversion of light energy into an electrochemical response. And by electrochemical response, I mean a nervous signal. So light comes in the eye, what goes out to the brain, an electrochemical response or a nervous signal. Alright. We're going to say that there are 2 major layers of the retina that we're going to talk about. And when we do this, we want to first just look at our diagrams here. Over here, we have this cross section, this transverse section of the right eye. And you can see here in yellow, we have the retina, and the retina is really covering the majority of the back of that eyeball there. And then those neurons from the retina actually go out and they become the optic nerve. Now zoomed in here, we've taken a section sort of about right here and we'll pull that out. That would be what we're looking at here. So this is a section of the retina, and you can see color coded we have all the different cells that we're going to talk about. And when you're looking at this, the back of the eye is back here and light is coming in in this direction. Alright. So our 2 layers. Our first layer that we're going to talk about is the pigmented layer. And this is a single cell layer at the back of the retina, and it is going to support and protect those photoreceptors, And it's also going to absorb excess light. So as we look at our diagram here, that's these cells all back here. A single cell layer sort of dividing the end of the retina from what's behind it, which, remember, that is the choroid. So I'll just write that in here, the choroid. And it's also supporting those photoreceptors. Right? Remember, all the blood supply for the back half of the retina is in this vascular layer in the choroid, So that blood supply or the nutrients from that blood supply actually has to come up through this pigmented layer. It's also going to do other things like, absorbing excess light. That's another job of the choroid as well. Alright. That pigmented layer, extremely important to make sure that those photoreceptors work and for functioning of the eye. In terms of understanding how the eye perceives light though, not that important. We're not really going to talk about the pigmented layer really much at all anymore. We're now moving on to the neural layer, and we're going to spend a lot of time going forward talking about what's going on in this neural layer. In the neural layer you have the photoreceptors and the neurons. Alright. Photoreceptors absorbing the light, the neurons passing that information on so that you can perceive it into the brain. Alright. So first up, we'll look at the photoreceptors. And the photoreceptors, we have 2 types. We have the rods and the cones, and we'll go into how these are different from each other coming up later on. And these rods and cones are going to be excited by different wavelengths of light. Right? So remember, light is coming in this way, and then all the way in the back of the retina, we have here in orange. Those are those rods. And then we also have in red a few cones drawn in as well. So those absorb the light and that's where that phototransduction happens. It's going to convert it into this electrochemical signal, this nervous signal, and then it's going to pass that signal back up through the retina in this direction. The first place that signal goes is into this layer of pink cells that we see here. Those pink cells are what we're going to call the bipolar cells. The job of the bipolar cells is to connect those photoreceptors and pass the signal from the photoreceptors to the ganglion cells. And the ganglion cells are these cells in blue here in the front of the retina. So bipolar cells get that message from the photoreceptors, pass it on to the ganglion cells. The ganglion cells then generate the action potential. So in the photoreceptors back here and in the bipolar cells here, so far we've just had graded potentials. Once it reaches the ganglion cells, we start an action potential because the ganglion cells have these really long axons that travel along the retina and then travel out through the optic nerve to the brain carrying that signal to the brain. So we're going to say they generate an action potential and from there run along the retina and form the optic nerve. Alright. So again, light comes in this way. The(signal is sent back out this way from the photoreceptors to the bipolar cells, to the ganglion cells, along the retina, and then it can come out the eye through the optic nerve. Now you'll notice we have 2 other cell types here. In green, we have amacrine cells, and in blue, we have horizontal cells. You can see these amacrine cells here, and we have one horizontal cell down here. It's very unlikely that you need to know the details of what these cells are doing, but you should probably know that they are in there. These cells aren't really in the direct pathway of this electrochemical signal of this nervous response. They're sort of going horizontally or sort of across the retina and doing sort of crosstalk between the different cells. And what they're really doing is sort of that first, first pass of visual processing, kind of turning up and turning down and modulating some of the signal based on what's happening in the retina. So some of the initial visual processing actually happens in the eye before it leaves, and that's what those cells are doing. But, again, it's really unlikely that you need to know the details of that. Just know that they're in there and they're sort of doing crosstalk between the cells. Alright. So those are the structures that we need to know about, but before we go on, I just want to note that we have light coming into the eye. I've said this a few times. The light comes in the eye this way, the receptors are all the way in the back, and then it sends a signal back out this way. So the light has to actually sort of pass through what I often refer to as kind of the wiring of the retina, those bipolar cells and the ganglion cells. That kind of feels like the backwards way to do it. Right? If I were to design an eye, I would probably put the photoreceptors as the first thing the light hits. And so a lot of people wonder why that is, and no one really knows, but a lot of the consensus is that it's just kind of an accident of evolution. In our ancient ancestors, this was the developmental process that got set up and now this is just how the eye is built. And if you were to really design an eye, maybe you would build it the other way. Some eyes are built that way. The octopus eye, which evolved completely separately from the human eye, but works very similarly, It has actually those photoreceptors in the front and the wiring behind. The way I kind of think of our eye is if you imagine like a flat screen TV, and you think of, like, where the picture is on the TV, I'd liken that to the photoreceptors. Well, in the back of a TV, you'd have to have all this wiring connecting all those things. Now I don't actually know how a flat screen TV works, but just go along with me for this example. The way the Retina is built, you kind of put all the wiring in the front of the TV and you have to look through it to see the picture, so you just kind of try and make it clear and minimize it as much as possible. Again, it kind of feels like the backwards way to do it. Remember, our eye is a marvel of biological engineering. It doesn't mean though that it's perfect. It works really, really, really well. That's good enough. Alright. Again, we're going to talk about a lot of what's going on in this neural layer a lot more going forward. We'll see you there.
Table of contents
- 1. Introduction to Anatomy & Physiology5h 40m
- What is Anatomy & Physiology?20m
- Levels of Organization13m
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- 22. The Respiratory System3h 20m
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- Cellular Respiration: Chemiosmosis7m
- Review of Aerobic Cellular Respiration18m
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- Gluconeogenesis16m
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- Amino Acid Oxidation17m
- 25. The Urinary System2h 39m
- 26. Fluid and Electrolyte Balance, Acid Base Balance Coming soon
- 27. The Reproductive System2h 5m
- 28. Human Development1h 21m
- 29. Heredity Coming soon
15. The Special Senses
Inner Layer of the Eyeball
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