February 16, 2011

033 The Receptive Field of a Ganglion Cell

In this episode, Leslie explains more about the connections between rods and cones to bipolar cells, and between bipolar cells and ganglion cells. He also describes how these connections determine the receptive fields of each ganglion cell. Enjoy!

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 33, I’m going to be talking about the receptive field of a ganglion cell. Let’s get right into it.

In order to understand the concept of the receptive field of a ganglion cell, we need to revisit the structure of the eye. I want to point out that we have this layer of tissue here that we call the retina. In the retina, we have the rods and the cones, and we said that the rods are for black and white vision and just for detecting light, and the cones allow us to see detail, it allows us to see color.

There’s another area, a specific region of the retina that we call the fovea, and this is a very important region because we have a large amount of cones, so much so that we can have up to as many as 150,000 cones per square millimeter. As you can see, that’s a high density of cones. We said that when light comes in, what the lens does is the lens actually focuses that light onto the retina. Specifically, if you’re focusing on an object, it focuses the light onto the fovea, and that allows you to see a lot of detail.

Now, if I am looking directly at my object, and let’s say this is my object here, and you have these photons of light coming off of that object, and that’s being focused onto the fovea. I can see the details of that object. If there’s an object that’s kind of to the periphery, over to the side, so let’s say this is the object over here that I’m not focusing on, that object is also going to be reflecting photons of light, but the photons of light won’t be focused directly onto the fovea. For example, it might be that the photons of light are focused up here.

Now, we still have some pigment here, we still have some rods and cones, not as many as we have in the fovea, so we’re not going to see as much detail, but I can see the object that’s to the left of me right now. I can see that there’s a keyboard to my left, I can see there’s a door to my right. I can’t see all the details of that keyboard and that door, but I can see them.

So, if the light is being focused on the fovea, with such a high amount of cones, a high density of cones, I’ll be able to see more detail. We’re going to look a little bit at some of the processing that happens that makes that possible. Don’t take these arrows as set in stone, I’m just trying to give you a general idea of how this works, but I hope that makes sense.

With that concept, let’s go on now and talk about the receptor field of a ganglion cell. To illustrate that, I’ve simplified the drawing of a ganglion cell, so here we have a ganglion cell, and here we have a cone. So these are all cones, and this is the same for cones or for rods, but we’re going to focus on cones because that really gives us a lot of detailed vision. The cones are connected to bipolar cells, and those bipolar cells are connected to ganglion cells. Here you can see we have one cone that’s connected to the ganglion cell via a bipolar cell. I don’t show that here, but just assume that a bipolar cell is in between here. Here we have another cone that’s connected via a bipolar cell to one ganglion cell. Here we have the same thing.

Over here, we have something a little different. We have 3 cones that are connected to one ganglion cell. Now, from the ganglion cell, we said that the ganglion cells have axons and they send signals to the brain. So, I’m just going to draw an arrow going to the brain from each ganglion cell, and I’m going to draw an arrow going to the brain from this ganglion cell over here also. This is the situation that we have in the fovea. Because we have so many cones, and in some cases, we have one cone that’s connected to a ganglion cell, when the brain is receiving information from these 3 ganglion cells, it’s getting a lot more detail. When it’s getting information from this one ganglion cell that’s kind of trying to summarize all of the information that it’s getting from multiple cones, it’s not going to be as detailed as the situation that we have over here.

So here, the receptive field of this ganglion cell is one individual cone, which gives it a lot of detail. So I’m going to say here, I’m going to say lots of detail. And here, it’s not as much detail. So I can still see what is being picked up by these cones, but I’m not going to see as much detail. So the receptive field of this ganglion cell includes these 3 cones. The receptive field of this ganglion cell just includes one cone. And this can go for cones, it can go for rods, and I draw this simply with 3 cones, but you can get thousands of cones that are connected to one ganglion cell. Of course, that’s not going to give you as much detail, but you’re still going to get some information that can go to the brain and be interpreted so you can see what the objects are with some detail, but not as much.

That’s all I want to talk about in this video. I hope this gives you a good idea of what we mean when we say the receptive field of a ganglion cell, whether you have very detailed receptive field, or not so detailed. This receptive field is larger because it’s connected to more cones, but you get less detail. The receptive field here is smaller, but because you have so many individual ganglion cells that are connected to individual cones and individual rods, that’s going to give you more detail.

If you have any questions about that, you can go ahead and leave them in the comments section below, and I’ll be happy to answer your question. That’s it for this video, and I’ll see you in the next one.

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