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Photoreceptors (rods vs cones)

Rods and cones are two types of photoreceptors in the eye. Both are specialized nerves that convert light into neural impulses, but they differ in number, location, and function. Rods are more numerous, located in the periphery of the eye, and good for detecting light in general. Cones are concentrated near the fovea, responsible for color vision, and able to adapt quickly to changes in light. Created by Ronald Sahyouni.

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  • piceratops ultimate style avatar for user ∫∫ Greg Boyle  dG dB
    Why are the cones distributed as 60 % red, 30 % green, and 10% blue? Is there some advantage for our color vision to be this way?
    (29 votes)
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    • male robot hal style avatar for user Ognian
      From a quick web search, it appears that we don't know yet. It might be due, in part, to the fact that blue/green cones are more sensitive. However, the quantities of each color appear to vary widely from person to person, yet we all have more or less the same sensitivity to colors, so there must be a compensation mechanism, perhaps during the processing of the information by the brain...
      (15 votes)
  • leaf green style avatar for user Multitunk
    Hello. Correct me if I am wrong, but at it is said that photons hitting the opsin in a rod starts a cascade that leads to the firing of an action potential. I have been learned that the cascade leads to phosphodiesterase converting cGMP to GMP which in turn causes the level of cGMP in cytosol to drop, then cGMP will leave the Na+ channels in the outer segment membrane so that they close. This would mean that rods stop sending signals (glutamate) to postsynaptic cells when they are exposed to light. In other words the opposite of what is being said in the video?
    (23 votes)
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    • purple pi purple style avatar for user newrowlogik
      Correct. The information between the videos in this section in parts contradict themselves, so be careful. The last video (phototransduction cascade), however, does more or less correctly shows the rods being turned "off," that is, ceasing the glutamate signal. Ron probably meant to say a bipolar cell fires an action potential as a result (granted, the glutamate receptors are hyperpolarized by glutamate thus depolarizing in its absence).
      (15 votes)
  • starky ultimate style avatar for user Andrew
    Wouldn't the rod "fail" to fire an action potential if it is turned "off" by stimulation? Just figured that it usually sends an inhibitory signal to the bi-polar cell, and stops sending this signal when it is stimulated by light. Is this off?
    (5 votes)
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    • spunky sam blue style avatar for user ola
      The transduction cascade happens as follows
      when light hits the rods - retinal contained in rhodopsin (a protein in rods) changes from cis conformation to tans - this causes rhodopsin protein itself to change shape - this shape change cause activation of transducin (a G protein) - transducin converts cGMP to GMP - low levels of cGMP cause Na+ channels to close - this causes hyperpolarization - hyperpolarization inhibits the release of glutamate - since glutamate serves as a inhibitory for ON bipolar cells, it's inhibition causes bipolar cells to turn ON - this signals ganglion - signal received by axons - axon transmit info to brain - response.
      so although the rod are turned "off" by light, by being off, bipolar cells turn on and this basically signals transmission to brain. It's like a negative effect kinda thing. The rod is technically sending a signal by being off due to this cascade of events.
      Hope this helps :)
      (9 votes)
  • leafers ultimate style avatar for user Joseph Frimpong
    . Why would cones be responsible for the change in illumination in that example instead of rods. I thought that rods were the ones responsible for detection changes in illumination (like in the dark room example)
    (4 votes)
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    • leaf blue style avatar for user dysmnemonic
      Rods are much more sensitive to light, and can respond to single photons. The problem with this is that they saturate easily - if the light is any brighter than a candle, then rod photoreceptors can't recover between signals, so they can't communicate any light information.
      (4 votes)
  • leafers tree style avatar for user amy.hummel
    For other mammalian species that see in color, do they have a similar or different ratio of red, green and blue cone cells as humans? (Ie, is this something at has been evolutionarily conserved, or something that evolved in a species-specific way?)
    (1 vote)
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    • leaf green style avatar for user Daniel

      Many mammals are dichromats, meaning they only have two types of cones (e.g. dogs possess blue and green cones but lack red). However, many animals that lack color vision possess a higher concentration of rods in the fovea; i.e. they can see better in low-light conditions. Other mechanisms that help certain animals see better include substances such as the tapetum lucidum, a reflective layer on the back of the eyeball that allows their photoreceptors a second chance to capture light (this is what makes certain animal's eyes--e.g. cats' eyes--sometimes appear to glow in the dark).

      Among non-mammals you can find an even broader variety of cone types. Butterflies have five types of cones, and certain shrimp have more than a dozen!

      This leads me to speculate that some degree of color perception is selected for conservation, particularly among animals that rely on visual discrimination to survive in their environment. However, not all mammals have the same ratio of spectrally differentiated cone cells.


      Hope this helped,
      (6 votes)
  • blobby green style avatar for user Emelie
    I know that light "turns off" rods, but does this apply to cones as well? Are they normally in the on/ off state?
    (3 votes)
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  • leafers ultimate style avatar for user Laura Jin
    I thought that the optic disk is the part where all ganglion axons converge and thus creating a blind spot?
    (3 votes)
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  • male robot johnny style avatar for user Dawson
    What about rods make them so much more sensitive to light than cones?
    (3 votes)
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  • blobby green style avatar for user Ameen
    From what I understand so far is that the rods are the light sensors of the eye that would regulate the pupil size based on the exposure of light. The 6 million cones are the cells that would translate the different wavelengths of the light to colors for the space they occupy and thus forming an image . Therefore if rods are not functioning, which physiologically happens when readjusting, a person sees white not because the rods are perceiving that much "white" but because the cones are over exposed. so wouldn't the function of the rods be maintaining a black box that would allow the cones to translate the colors correctly and thus forming the 6 million cell image? and therefore wouldn't cons be considered more sensitive than rods? or do rods get activated by the lowest energy light wavelength and thus considered more sensitive?
    (2 votes)
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  • blobby green style avatar for user adrija adhikary
    doesn,t cone cells contain a pigment called iodopsin?
    (2 votes)
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Video transcript

Let's examine the difference between rods and cones in our eyes. Let me draw a very simplified schematic of a rod just to give you an idea of what it looks like. So rods actually get their name because if you look at a rod under a microscope, it actually has this elongated cell body that kind of gives it a rod shape. So a rod is a photoreceptor. What exactly is a photoreceptor? Is it a neuron? Is it a type of nerve? So, in fact, it is. It's a very specialized type of nerve that's able to take in light and convert it into a neural impulse. So inside a rod, there are a whole bunch of structures known as optic discs. And these optic discs are large, membrane-bound structures inside the rods. And there are thousands of them in an individual rod. Embedded within the membrane of each optic disc is a whole bunch of proteins, and these proteins actually absorb light and begins a phototransduction cascade that eventually leads this rod to fire an action potential that will reach the brain. Similarly, a cone gets its name because it's cone-shaped. Cones are also photoreceptors. So they're specialized nerves that have the same internal structure as a rod. So cones also have a whole bunch of these optic discs that are stacked upon one another, and embedded within each optic disc is a whole bunch of this protein. So as I mentioned over here, the protein in a rod is known as rhodopsin. In cones, it's basically the same protein. But it just has another name, and it's called photopsin. So as I mentioned, as a ray of light enters the eye, if it happens to hit a rod, and it happens to hit rhodopsin, it'll actually trigger the phototransduction cascade that results in this rod firing an action potential. This exact same process happens in a cone. So these are the major similarities between rods and cones. Now let's look at the differences. So in an average retina, there about 120 million rods. In contrast, there about 6 million cones per retina. So there are about 20 times more rods than there are cones in each eye. Another big difference between rods and cones is where they are located in the eyeball. So if I draw a very simplified diagram of an eyeball, and this is the optic nerve exiting the back of the eye. So this would be the front of the eyeball. This is the back of the eyeball. And as I mentioned in a previous video, the back of the eyeball is coated by a membrane known as the retina. So rods are actually found in the periphery of the eyeball. So they're found in this area over here and in this area over here. And there's actually a region of the retina, right about here, that actually dimples in. And this region is known as the fovea, and cones are mostly concentrated in this region in front of the fovea. So rods are mostly found in the periphery of the eye, whereas cones are mainly found near the fovea. Another big difference between rods and cones is that rods do not produce color vision, whereas cones do. So rods are very sensitive to light. In fact, they are 1,000 times more sensitive to light than codes are. For this reason, rods are really good at detecting light. So they're basically responsible for telling us whether or not light is present. Another way to think of this would be black and white vision. On the other hand, cones are not as sensitive. But they do result in the detection of light. So they result in color vision. And in fact, there are three different types of cones. So there are red cones, which make up about 60% of all cones in the eye. There are green cones, which make up about 30% of all cones in the eye. And there are blue cones, which make up about 10% of all cones in the eye. Another major difference between rods and cones is their recovery time. So rods have a very slow recovery time, whereas cones have a very fast recovery time. So what I mean by slow and fast recovery times is that as soon as a rod is activated by a ray of light-- so let's imagine that a ray of light comes in and activates this rod, and it fires an action potential. It takes a lot longer for the rod to be able to fire another action potential than it does for a cone, and you've actually experienced this. So if you've ever been outside, playing soccer or football, and you run inside to get a cup of water, there's a big change in illumination, yet you don't stub your toe. You're able to transition from outside to inside really quickly. That's because cones are able to rapidly adapt to changes in illumination, whereas rods take a lot longer. So at night, when you walk into a dark room, it takes a while for your eyes to get adjusted to the dark. And that's because the rods need to be reactivated, reset, in order for you to be able to use them to see anything.