February 11, 2011

032 Visual Processing in the Retina

After the rods and the cones, there are a few other important cells involved in visual processing. In this video, Leslie explains about how the bipolar cells and ganglion cells contribute to this process.


Transcript of Today’s Episode

Hello and welcome to another episode of Interactive Biology TV, where we’re making biology fun! My name is Leslie Samuel. In this episode, Episode 32, I’m going to be talking about the visual processing that happens in the retina. We’ve spoken a little bit about this in the previous episode, and I want to take this to the next step, the next step in the process.

First, we’re going to start with this animation that’s kind of a review of what we spoke about in Episode 31. There are a few things I’d like to highlight. First of all, if we look here, we have these little round balls, and those are sodium ions. They can be calcium ions also, but for the most part, those are sodium ions. Here we have a channel. In Episode 31, we spoke about this cyclic GMP-gated sodium ion channel. What we said was that if there’s cyclic GMP around, which there normally is in the dark, that channel is going to be open. If that channel is open, as you can see right here, you’ll see sodium is normally able to rush into the cell, and the rod is usually depolarized, and neurotransmitter is being released. So that’s what happens in the dark.

We also have some other things. Here we have phosphodiesterase, as you can see right here. Then we have the trimeric transducin molecule that normally has GDP attached. We have rhodopsin, which is the visual pigment in the rod. And here we have one that we didn’t speak about before, guanylate cyclase, and that’s basically the enzyme that makes the cyclic GMP.

So, in review of what we said before, cyclic GMP is normally around in the dark, depolarization happens. Then, when light comes, we said that a photon of light comes in, and we’ll see that right now, activates the rhodopsin, replaces a GTP, that activates the phosphodiesterase, and when that phosphodiesterase is activated, it converts the cyclic GMP into regular GMP. When that happens, you’ll see here, it closes this gate, and the rod hyperpolarizes. It becomes more negative, it goes toward the membrane potential for potassium.

So there’s a lot of stuff going on in this animation right now and you can see that. My recommendation would be to pause this and go back and look at it again, and kind of try to visualize what’s happening. Look at the details and make sure you understand this, because this is a good overview of what happens inside the rods. As I said in the last episode, the process that happens in the cones is relatively similar to this. So if you get this, you have a good idea of what happens.

So the result of this is, if we’re looking at the membrane potential, or the receptor potential in this case, when the light comes, that is not going to cause the regular action potential. It’s going to cause the membrane potential to go down until that light is gone again, and then it comes back up. This process, as with before with the neuron, this is called hyperpolarization. I’m going to write it as two words because I’m running out of space. Hyperpolarization, so that is what happens in response to light. I hope that review makes sense. If not, go back to the previous episode and you’ll get more of those details.

Now let’s move on and take it from what’s happening inside the rods to what happens with some of the other cells that we have around. So this is looking at the cells in the retina. If light is coming into the eye and it’s hitting the retina, the light is coming in this direction. So it’s coming from up here and it’s going down. There’s an interesting way in which this is arranged in that the rods are actually at the end here. So these are the rods, and here we can see an example of a cone.

There are few other important cells that we need to know about. One would be this red cell here, and that would be a bipolar cell. The next important one that you need to be aware of would be this guy here, these cells here, and those are called ganglion cells. So these are the major players:

  1. The rods and the cones
  2. The bipolar cells
  3. The ganglion cells

You can see here we have some other cells. For example, this one here is called a horizontal cell. We’re not going to focus on the horizontal cells in this video. This is called an amacrine cell. We’re also not going to focus on that in this video, either. So the major ones we’re going to talk about are the rods, and when I say the rods, of course, you can assume that the process is somewhat similar with the cones. We’re going to talk about the bipolar cells, and we are going to talk about the ganglion cells. So let’s look at what happens.

We looked at the process that’s happening inside of the rods before, and we say the net result of that light coming in, so here we have light that’s coming in, and the net result, the overall result, in the rods is that the membrane hyperpolarizes. So here we have a hyperpolarization of the rods. When that happens, neurotransmitter release slows down. It basically shuts off the neurotransmitter release.

Now, the rods are connected, you can see here there are a number of connections, to the bipolar cells. The neurotransmitters that are normally released here are inhibitory, so when it stops releasing those neurotransmitters, it is no longer inhibiting the bipolar cells, and what’s going to happen to the membrane of the bipolar cells is that we’re going to get a depolarization. So it’s basically the exact opposite of this hyperpolarization. Here, we’re going to get in the bipolar cells, we’re going to get depolarization. So let me write here bipolar cells.

Now, bipolar cells, just like rods, don’t have an axon. So this is the entire bipolar cell, but this is not an axon, so we don’t get an action potential. We do get this depolarization, and that is going to cause an increase in neurotransmitter release at these synapses here. So bipolar cells gets depolarized, more neurotransmitter is going to be released, similar to what happens in a regular axon.

Then we have this ganglion cell here, and all of these are ganglion cells. What’s going to happen there in response to more neurotransmitters being released, the ganglion cells are going to also depolarize, but since here we have axons, that’s going to cause a burst of nerve impulses, so a burst of action potentials. These are my spikes, these are my action potentials, these are signals that are travelling along the axons going to the brain.

Let’s look at that again. In response to light, the rod hyperpolarizes, and we looked at the process as to how that happens. That reduces neurotransmitter release, so let me draw here or write here there’s a decrease in neurotransmitter release. Those neurotransmitters are normally inhibiting the bipolar cells. Now, they are no longer being inhibited. They are going to depolarize. That’s going to cause an increase in neurotransmitter release here to the bipolar cells, and that’s going to stimulate the ganglion cells, causing depolarization and a burst of impulses.

So the final result, in terms of what’s happening in the retina, is that these ganglion cells are firing a burst of nerve impulses that are sent to the brain. I hope that makes sense. You can kind of go through that again if anything is unclear so that you can get a better understanding of that.

There’s one more thing I want to talk about. We spoke about the thalamus in the brain, and we said that the thalamus is kind of like the regulatory gateway of the brain. All of the sensory information comes into the thalamus, and then goes to the rest of the brain, or the motor output that’s coming from the brain goes via the thalamus to the rest of the body. There’s no exception here. When the ganglion cells are stimulated and they send their signals, they’re going to send their signals to a specialized region in the thalamus called the lateral geniculate. So this is in the thalamus, and then, that can go on to other parts of the brain for further processing. We’re going to talk about that in some later episodes.

So that’s really it for this episode. Hope everything makes sense. If you have any questions, of course you can ask them in the comments section below, or if you have any comments or anything of that sort, please give me feedback, because I love to get feedback from you guys and be able to help you even better. So that’s it for this video, and I’ll see you in the next one.

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