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Current time:0:00Total duration:5:43

Photoreceptor distribution in the fovea

Video transcript

let's look at how rods and cones are distributed in the retina let me begin by drawing a very simplified diagram of the eyeball so in the back of the eye we have the optic nerve exiting and going towards the brain the back of the eyeball is coded by specialized membrane known as the retina this dimpled portion of the retina is known as the fovea and the part of the retina directly in front of where the optic nerve exits the back of the eye is actually known as the blind spot this is known as the blind spot because no photoreceptors are present in this area let's go ahead and look at the distribution of rods and cones in the eye so rods are mainly found in the periphery of the eyeball so we're going to use this blue color to represent rods and they're found mostly in the periphery of the eye so up here and a little bit over here and as I mentioned there are no photoreceptors at the blind spot because that's where the optic nerve actually exits the eye so there's no photoreceptors their cones on the other hand which will represent in purple are actually found throughout the fovea so they're found in a really high concentration near the fovea and there are no cones at the blind spot and there are very few cones kind of sprinkled throughout the rest of the eye so they're found kind of sprinkled throughout the periphery of the eye so let's go ahead and zoom in on the fovea just to kind of make sense of what we're looking at so if we zoomed in on the fovea what we would see would be the retina so it dimples in at the region where the phobia is this region from here to here is d fovea so let's go ahead and draw in the rods and cones so rods are found as I mentioned outside of the fovea so there are a whole bunch of rods and the eye specifically in the periphery so outside of the fovea there are a whole bunch of rods and there are more rods over here and they kind of line the periphery of the eye and so on cones on the other hand are found in a really high concentration near the fovea so at the fovea there are a whole bunch of cones and in the periphery of the eye there might be a few cones every now and then so the reason that the fovea actually dimples in here is because these photoreceptors are connected to other neurons that actually send axons through the optic nerve into the brain so there are a whole bunch of neurons over here in this region and they all have axons that actually go to the optic nerve and exit the back of the eye and so when light enters the eye and hits the fovea what it actually looks like is like this and so the benefit of having no axons in the way of the light is that you actually get a higher resolution so you actually get more light is able to hit the cones rather than get absorbed by these axons so if light were to enter this way and hit the periphery of the eye what you would have you have light entering I would actually have to go through this bundle of axons and as it's going through some of the energy is actually lost so less light actually hits rods and cones in the periphery so at the fovea you actually have light directly hitting the cones rather than having to go through a layer of axons and neurons so let's look at this same picture in another way let's go ahead and look at it graphically so if I were to draw a graph and if I were to say that the zero point on the graph so let's go ahead and this is the x-axis this is the y-axis and the zero point is going to be where the fovea is so if we were to actually take the retina so in the eyeball the retina is curved like this and if we were to actually flatten it out there's a little dimpled region here that's known as the fobian we're going to set this to zero and then you can move a wave so this could be five degrees away from the fovea this could be ten degrees away from the fovea this is 15 and so on on both ends so that's what we're going to do here so this is going to be five degrees away from the fovea this is ten degrees and so on and we're going to have the same thing on the other side of the fovea as well so now on the y-axis what we're going to have is receptor density so this is the number of receptors found in the retina so over here we have a low receptor density and over here we have a high receptor density so as I mentioned this region right here is D fovea and this outside region from here to here and from here to here is the periphery so in the periphery there's a really high level of rods and then as we get towards the fovea the rods drop in numbers and then as we start to move away from the fovea again there's a really high number of rods and we actually reach a region which is the blind spot so there as I mentioned there are no photoreceptors where the blind spot is and then as we into the other side of the blind spot there again photoreceptors and so you kind of get this type of distribution of rods cones have a different distribution so cones there aren't very many of them in the periphery but when you get to the fovea there's a huge spike in the number of cones and as you again move away from the fovea the cones actually drop back down and then during at the blind spot there are no photoreceptors and on the other side of the blind spot there's a very low number of photoreceptors