episode31

031 How Rods and Cones Respond to Light

Leslie Samuel IBTV, Physiology, Sense Organs 141 Comments

In this video, Leslie explains how rods and cones work, using the rods as an example. Watch to find out how rhodopsin, transducin, and phosphodiesterase, all play a major role in the process of vision.

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 31, I’m going to talk about how rods and cones respond to light.

Now let’s do a little bit of a review. We’ve been talking about the eye. We said that in order for us to see something, what happens is light bounces off an object, and in the previous episode, we said that that object was an attractive young lady. So let’s stick with that, and let’s say that light is bouncing off that young lady. It goes through the pupil, and then we have the lens that it goes through. The lens focuses the light onto the retina, and we said especially in the fovea where we have a lot of rods and cones, so that light comes in and it’s focused onto the retina via the lens.

Now, in the retina, as I said, we have rods and cones. These are the receptors that allow us to see. Now what we’re going to do is we’re going to look at what’s happening inside these rods and cones. We’re going to take the rods and use that as an example. However, I want you to know that the process that happens in the rods and the cones are relatively similar. Yes, there are some nuances and differences between the two, but by understanding what happens in the rods, we’re going to also have an idea of what happens in the cones. So let’s look inside the rods right now.

So here we are inside the rods. I know it doesn’t look like it’s inside the rods, but that’s fine. It has the major things that we need. There are 3 main things that I want you to pay attention to. Here we have visual pigment that’s found in the rods, and that’s rhodopsin. Now, rhodopsin is made up of the protein opsin and retinol. The specific form we have this is cis-, it’s a cis-form, so it’s called cis-retinol. Now retinol is basically a slightly processed version of Vitamin A. This explains part of the reason why Vitamin A helps with vision. So here we have cis-retinol and opsin, and together, that makes up rhodopsin.

We also have this molecule over here. It’s a trimeric molecule, meaning it has 3 subunits. This one, this one, and this one. This is called transducin. And then over here, we have this guy here, you see I have PDE, and that basically stands for phosphodiesterase. So these are the major players inside of the rods: rhodopsin, transducin, and phosphodiesterase.

Now, to keep the bigger picture in mind, the phosphodiesterase is the one that really does the damage, and I mean damage in a good way. However, it cannot do what it needs to do because it has these two alpha subunits attached. So these are inactivating alpha subunits. In order for this to do its job, and we’re going to talk about its job in a little while, these alpha subunits need to be removed. Keep that in mind as we go through this process.

So this process all starts with light. We’re going to take one photon of light, and let’s say a photon of light comes in and strikes rhodopsin. So this right here is light, and it comes in and it strikes rhodopsin. When the photon of light strikes rhodopsin, what happens is the cis-retinol changes into a different form, and that different form is called trans-retinol. So it’s no longer in the cis-form, it’s now trans-retinol. When that happens, that causes it to lose its attraction for the opsin molecule, and once that connection breaks, that retinol leaves and what that does is it exposes a binding site on the opsin. That’s what we’re going to use next as we go to the next part of the process.

So what we’ve accomplished so far is we’ve freed up this binding site on the opsin. The next stage in the process involves opsin going over to transducin. Since the binding site is exposed, that can catalyze a reaction. Now, I want you to pay attention here because here, on this subunit of the transducin molecule, you see we have GDP. Once this binding site is exposed, this active site is exposed, that can then catalyze a reaction that converts that GDP into GTP. And you can see here now, we no longer have GDP, we have GTP. So it basically adds a phosphate group on. Instead of GDP, diphosphate, it now becomes triphosphate.

Once that happens, that subunit is activated and that subunit actually leaves the other two subunits behind and goes over to the alpha subunit of the phosphodiesterase. And then, it removes that alpha subunit. So you can see, we said that the goal was to free up this phosphodiesterase. We’re almost there, we have one alpha subunit removed, as you can see here, but we still have one more alpha subunit.

In order to remove this second alpha subunit, this entire process has to happen again, with light coming in, changing the retinol from cis to trans, the retinol leaving, opsin coming over, then opsin comes and catalyzes the reaction to have another GTP, and then we get another subunit. I’m not going to go through the animation of all of this because it’s the same process. But basically here, you can see we have another subunit of the transducin molecule that comes. That can remove this alpha subunit from the phosphodiesterase. So let’s go ahead and remove the second one, and now we have exactly what we wanted, we have this phosphodiesterase and it’s by itself.

I know there are a lot of complicated details in here, but if you keep in mind that this was our goal, it should make sense. So now we have this PDE, this phosphodiesterase, and it can go and do what it does. So what does it do? Well, it converts cyclic GMP into GMP. So it changes this from a cyclic molecule and now it’s just GMP. This is the step that leads to vision. This is how we’re able to detect light.

Now, let’s put this in perspective. We said that we’re inside the rods. And here, I have a picture of a rod, and you can see this is a rod. Here we have a cone, but we’re going to pay attention to the rod since that’s what we’re using as our model. Now normally, with the rod, we have cyclic GMP available. So if there’s no light, there’s no stimulation, there’s cyclic GMP. What that does is it opens up and this is going to sound a little different than what we’ve looked at in the past, but cyclic GMP-gated sodium channels. So this is not a voltage-gated sodium channel, this is a cyclic GMP-gated sodium channel.

So in the dark, we have cyclic GMP around, the cyclic GMP-gated channels, of course, those are going to be open. What’s going to happen is sodium is going to rush in, so we have sodium coming into the cell, Na+. If you remember from previous episodes when we spoke about depolarization, sodium rushes in, making the membrane potential more positive. So this is the exact opposite of what we’ve been looking at because when there’s no stimulation, when there’s no light, cyclic GMP-gated sodium channels are open, sodium is rushing in. As a result of that, the membrane is depolarized and neurotransmitters are being released.

I know what you’re thinking. Why are neurotransmitters being released when there’s no stimulation? It is true, this is exactly opposite to what we’ve looked at, but this is the process and this is how it works in the rods.

Now, once the phosphodiesterase is activated and it gets rid of this cyclic GMP and makes it GMP, what’s going to happen to these channels? These channels are going to close, sodium will no longer rush in, and neurotransmitters will no longer be released. If we were to look at the membrane potential, and since this is a receptor, we’re going to call this the receptor potential. Here we have time, of course, and here we have Em, but in this case, we’re dealing with a receptor potential. Normally, the membrane is depolarized, normally neurotransmitter is being released.

Once this entire process happens and we have GMP instead of cyclic GMP, the channels close, sodium no longer rushes in, and the membrane potential, the receptor potential, is going to go towards the equilibrium potential for potassium ions. So it’s going to become more negative, so it’s going to go down until that stimulation stops, and then it’s going to come back up.

So once again, I want to emphasize that this is exactly opposite because when we get a stimulus, we get a drop in the membrane potential, neurotransmitters are no longer being released, and that’s going to have an effect on the cells that it makes a synaptic connection to.

We’re not going to go beyond this point in this video, but I hope you have a better understanding of what happens in the rods. We’re not going to go into what happens in the cones because it’s a similar process. Yes, there are some differences, but this gives you a general idea. That’s it for this video, and I’ll see you in the next one.

Comments 141

  1. dsbllr0

    @InteractiveBiology What are you planning to make your next video about ? I
    am asking because I have physiology right now and so far you have covered
    similar concepts. If that continues on I will have to check back more
    often.

  2. joinmebowser9000

    @InteractiveBiology Thanks for the answer. Well I think that kinda answered
    my question, but I’m still a little bit confused. So the light enters our
    Rods at night, or in dark places, and then the procces begins, or does
    Rhodopsin first goes in contact in transducin at night. You helped me with
    some of the question, but in just 10 hours, I should tell something about
    Rods and Cornes. People in my class always puts “?” on everything (just
    like I do) I’m reallly glad I found your channel Subbed:)

  3. joinmebowser9000

    @InteractiveBiology Yeah I understood that about they nearly have the same
    process. And that about the attractive young lady. Oh wait NOW I
    UNDERSTAND!!! I totaly misunderstood it. The normal form is when it’s dark.
    I thought the normal form was in the day, that was why I couldn’t really
    understand it. I was wondering how the light could stop the light, if you
    can say so. And now your comments make perfect sense.

  4. valarmanwe

    @joinmebowser9000 Maybe it`s the nature of the neurotransmitter released by the rods that causes confusion. In the dark Na+ flows through channels that are held open by cGMP. The Na+ inward flow causes continual release of a neurotransmitter called glutamate at the synaptic terminals.
    But glutamate is a neuroinhibitor. It will therefore hyperpolarise the post synaptic cell and thus decrease post synaptic activity. Think that some neurotransmitters are actually inhibitory, and job done !

  5. valarmanwe

    @joinmebowser9000 Maybe it`s the nature of the neurotransmitter released by
    the rods that causes confusion. In the dark Na+ flows through channels that
    are held open by cGMP. The Na+ inward flow causes continual release of a
    neurotransmitter called glutamate at the synaptic terminals. But glutamate
    is a neuroinhibitor. It will therefore hyperpolarise the post synaptic cell
    and thus decrease post synaptic activity. Think that some neurotransmitters
    are actually inhibitory, and job done !

  6. bizz76

    Would it be the opposite in the cones? Rods are responsible for our night vision but when there is light rods arent stimulated.. are the cones stimulated??

    it would be the same process but the opposite?? light –> the cones takes the lead and dark –> the rods take the lead??

  7. InteractiveBiology

    @bizz76 I’m sorry, but Leslie won’t be able to answer your question as he is busy with a lot of stuff. He’ll be making more Biology videos for the site though tackling more systems and other topics, so stay tuned for more! :)

  8. InteractiveBiology

    I’m sorry, but Leslie won’t be able to answer your question as he is busy with a lot of stuff. He’ll be making more Biology videos for the site though tackling more systems and other topics, so stay tuned for more! :)

  9. InteractiveBiology

    @bizz76 I’m sorry, but Leslie won’t be able to answer your question as he
    is busy with a lot of stuff. He’ll be making more Biology videos for the
    site though tackling more systems and other topics, so stay tuned for more!
    :)

  10. Ckaotenkind

    Thank you a lot. I’m from Germany and although the videos are in english I understand it very well. You rescued me for my biology test tomorrow. And now I will be able to write it in english :) Unfortunately my teacher won’t understand one single word. THANK YOU, it’s very well explained

  11. Ckaotenkind

    Thank you a lot. I’m from Germany and although the videos are in english I
    understand it very well. You rescued me for my biology test tomorrow. And
    now I will be able to write it in english :) Unfortunately my teacher won’t
    understand one single word. THANK YOU, it’s very well explained

  12. InteractiveBiology

    @Ckaotenkind Well, we’re glad to know you’ve gained something from it. We
    hope you aced your test. Stay tuned because we have new Biology videos
    coming very soon :)

  13. EphraimHeinrich

    Interesting and well done video. But I miss some details to the reactions happening in the rods. For example the fact that the whole process is a cascade.

  14. InteractiveBiology

    Hi, thank you for watching. Leslie will be uploading more videos by next year. He just might or might not tackle this topic again with more clarifications. Please stay tuned for more!

  15. xxnaruto1234xx

    @bizz76 Yes, we entirely use cones in bright light since there are very low cGMP levels in rods and no further hyperpolarization can be done. Also, the process is same for cones as well and in response to light, they also go through hyperpolarization. Only difference is cones have different type/s of opsins present.

    I think you are confusing cones with the Bipolar cells? I hope InteractiveBiology can cover a topic on Bipolar cells and their receptive field.

  16. xxnaruto1234xx

    @bizz76 Yes, we entirely use cones in bright light since there are very low cGMP levels in rods and no further hyperpolarization can be done. Also, the process is same for cones as well and in response to light, they also go through hyperpolarization. Only difference is cones have different type/s of opsins present.

    I think you are confusing cones with the Bipolar cells? I hope InteractiveBiology can cover a topic on Bipolar cells and their receptive field.

  17. xxnaruto1234xx

    Yes, we entirely use cones in bright light since there are very low cGMP levels in rods and no further hyperpolarization can be done. Also, the process is same for cones as well and in response to light, they also go through hyperpolarization. Only difference is cones have different type/s of opsins present.

    I think you are confusing cones with the Bipolar cells? I hope InteractiveBiology can cover a topic on Bipolar cells and their receptive field.

  18. xxnaruto1234xx

    Yes, we entirely use cones in bright light since there are very low cGMP levels in rods and no further hyperpolarization can be done. Also, the process is same for cones as well and in response to light, they also go through hyperpolarization. Only difference is cones have different type/s of opsins present.

    I think you are confusing cones with the Bipolar cells? I hope InteractiveBiology can cover a topic on Bipolar cells and their receptive field.

  19. xxnaruto1234xx

    Yes, we entirely use cones in bright light since there are very low cGMP levels in rods and no further hyperpolarization can be done. Also, the process is same for cones as well and in response to light, they also go through hyperpolarization. Only difference is cones have different type/s of opsins present.

    I think you are confusing cones with the Bipolar cells? I hope InteractiveBiology can cover a topic on Bipolar cells and their receptive field.

  20. mp5yourmp3

    great video. but at the beginning you said the fovea contains a lot of rods and cones, which i believe is false. it ONLY contains cones.

  21. 88rampitup88

    @mp5yourmp3 You are incorrect. While the central 300 µm of the fovea, called the foveola, is totally rod-free, there are actually some rods scattered amongst the cones in the rest of the fovea.

  22. 88rampitup88

    You are incorrect. While the central 300 µm of the fovea, called the foveola, is totally rod-free, there are actually some rods scattered amongst the cones in the rest of the fovea.

  23. belvidere123

    If in the dark there, PDE converts CGMP into GMP, how is CGMP depoloarizing? Wouldnt there be less CGMP and more GMP in the dark.

  24. AbdullahDiaa2

    @belvidere123 Rod cells have a relatively high concentration of cGMP => the cGMP-gated Na Channels are open => Na enters rods => increase membrane potential in the DARK ,, once photons hit Rhodopsin => PDE will be activated => converting cGMP to GMP => closing the Na channels => decreasing membrane potential

  25. AbdullahDiaa2

    Rod cells have a relatively high concentration of cGMP => the cGMP-gated Na Channels are open => Na enters rods => increase membrane potential in the DARK ,, once photons hit Rhodopsin => PDE will be activated => converting cGMP to GMP => closing the Na channels => decreasing membrane potential

  26. AbdullahDiaa2

    @belvidere123 and cGMP is not depolarizing directly => it opens the cGMP gated Na channels => causing depolarization through Na entry to the cell // and in the dark there’ll be more more cGMP and less GMP as there’s NO light photons which activate PDE and as a result converting cGMP to GMP !!

  27. AbdullahDiaa2

    and cGMP is not depolarizing directly => it opens the cGMP gated Na channels => causing depolarization through Na entry to the cell // and in the dark there’ll be more more cGMP and less GMP as there’s NO light photons which activate PDE and as a result converting cGMP to GMP !!

  28. AbdullahDiaa2

    Rod cells have a relatively high concentration of cGMP => the cGMP-gated Na Channels are open => Na enters rods => increase membrane potential in the DARK ,, once photons hit Rhodopsin => PDE will be activated => converting cGMP to GMP => closing the Na channels => decreasing membrane potential

  29. AbdullahDiaa2

    and cGMP is not depolarizing directly => it opens the cGMP gated Na channels => causing depolarization through Na entry to the cell // and in the dark there’ll be more more cGMP and less GMP as there’s NO light photons which activate PDE and as a result converting cGMP to GMP !!

  30. cherrybopbop

    sorry if this is not what you mean but unlike usuall action potentials, photons create hyperpolorization which is the negative charge they need within the pigment to trigger sight. the depolorization is the inactive cones at night with heavy secreation of nurotransmitter to maintain this. completely backward design really.

  31. cherrybopbop

    sorry if this is not what you mean but unlike usuall action potentials, photons create hyperpolorization which is the negative charge they need within the pigment to trigger sight. the depolorization is the inactive cones at night with heavy secreation of nurotransmitter to maintain this. completely backward design really.

  32. 05bebop

    great video, very helpful! I am wondering, why do the rod cells have cGMP to begin with? My understanding is that for cGMP to exist, an enzyme must convert GTP to cGMP. This enzyme would have to be activated by an active G protein subunit, which would be active due to an extracellular signal binding to a cell-surface receptor. So, if this process is the same in rod cells, what is the extracellular signal that leads to a high concentration of cGMP in the first place?

  33. 05bebop

    great video, very helpful! I am wondering, why do the rod cells have cGMP to begin with? My understanding is that for cGMP to exist, an enzyme must convert GTP to cGMP. This enzyme would have to be activated by an active G protein subunit, which would be active due to an extracellular signal binding to a cell-surface receptor. So, if this process is the same in rod cells, what is the extracellular signal that leads to a high concentration of cGMP in the first place?

  34. Linkfan07

    thank you so much leslie! i really appreciate that u went through te process how we percieve light with every step and explained the little details too! I really understand it now! this video helped me so much thank you. visual aid is always helpful! :)

  35. Linkfan07

    thank you so much leslie! i really appreciate that u went through te process how we percieve light with every step and explained the little details too! I really understand it now! this video helped me so much thank you. visual aid is always helpful! :)

  36. Jusplais

    next time use an actual thing, i notice that uploaded object hid himself so i gues that means invisible light or away form lense She can not look at him, but hear. Wanna explain sound?

  37. Jusplais

    next time use an actual thing, i notice that uploaded object hid himself so i gues that means invisible light or away form lense She can not look at him, but hear. Wanna explain sound?

  38. Jusplais

    next time use an actual thing, i notice that uploaded object hid himself so i gues that means invisible light or away form lense She can not look at him, but hear. Wanna explain sound?

  39. ClassAencounter

    Does anyone know if there is a reaction during the rhodopsin cycle that would produce and emit light or perhaps a glow?

  40. MultiBrando88

    …doesn’t phosphodiasterase require two alpha subunits of transducin with GTP to be bound in order to convert cGMP to GMP? Aren’t alpha subunits GTPases?

  41. MrHassanleb

    u r great man…i really was in mess cramming of info worthlessly…know i understand tomorrow i have test..u’ve saved me..thanks!

  42. dilqnaslavova

    @qnuhcyu yeah. actually in the last 3 weeks i’ve put on 12 pounds!. Between I know why people are rushin to watch this video that definitely make you stronger with shaped muscles. lucky i could find this video ==> bit.ly/Ks5kC5?=xcisbf

  43. dilqnaslavova

    @qnuhcyu yeah. actually in the last 3 weeks i’ve put on 12 pounds!. Between I know why people are rushin to watch this video that definitely make you stronger with shaped muscles. lucky i could find this video ==> bit.ly/Ks5kC5?=xcisbf

  44. dilqnaslavova

    @qnuhcyu yeah. actually in the last 3 weeks i’ve put on 12 pounds!. Between I know why people are rushin to watch this video that definitely make you stronger with shaped muscles. lucky i could find this video ==> bit.ly/Ks5kC5?=xcisbf

  45. sid123453

    Sir I am very very thankful to you for clearing my concepts. Can you tell me if you have any video made on sympathetic and parasympathetic response.

  46. thealmassi1

    Very coherent, well explained, good diagrams, very good breakdown of the processes. Only one mistake, though a major one and basic still does not take away from what a great video this is. Thanks!

  47. samasoftley

    Cu99460 … Em is the membrane potential, usually measured in volts. No doubt you will have seen a graph that shows an action potential/spike before, when there is depolarisation above a threshold value due to sodium ions raising the membrane potential. On this graph, it is a similar phenomenon, but because there is less sodium ions, the membrane potential becomes lower, which is known as hyperpolatisation..hope you see this in time for when you need it, I cannot reply inline on my tablet!

  48. Anton Krivosheyev

    It would be nice to also visually explain how, say, a 450nm(blue) photon, striking a blue cone activates the chemical reaction explained in this video, and exactly WHY a 550nm (green) photon would not induce the reaction in a blue cone. I suspect the chemicals present only in a blue cone absorbs a photon of strictly ~450nm due to this chemical’s natural vibration frequency, which only responds to (or synchronizes with) light that carries photons of the same electromagnetic vibration frequency.

  49. ePRTN

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  50. Shawn Harris

    Fabulous clip. Youtube is great for this type of thing.My good friend was previously bullied. He said he was planning to get bigger muscle mass. I chuckled… Then he gained 40lbs of absolute muscle mass. He tried the Muscle Building Bible (Google it). No person dares to intimidate the man now. :) I actually signed up a few days ago. See what will happen. Not to mention the guys emails are cool!

  51. austmcc

    one major mistake, its 11-cis retinal. 11-cis retinol is an intermediate which is formed in the transformation from 11-trans retinal back to the light sensitive 11-cis retinal.

  52. Kenpow93

    Excellent video series. Not certain why persons are being so critical, when they are clearly made to give students a general understanding. Thanks, You’ve helped sooo much!

  53. juniecr34

    thanks so much for your help I bet this is a very good explanation but I am so confused maybe I need to watch video more than once. Thanks again for your hard work and interest in educating the world :)

  54. studentquestor

    I hope Leslie will get back to you on this. So I’m assuming you’re a chemistry or biology major or something? Perhaps, the video should be corrected. But then again, such stuff might “complicate” a general and short “introduction” to how rods and cones work.

  55. studentquestor

    I’m sorry, but how is bullying and muscle building connected to this biology video on rods and cones?

  56. studentquestor

    Correct me if I’m wrong, but isn’t Leslie’s explanation on rods and cones correspond to the fact that rods only detect black and white light? Thus, they only get activated in darkness. Otherwise, it’s the cones that are getting activated.

  57. austmcc

    I don’t think it will complicate things, just chemically speaking there is a big difference and writing the wrong one down in an exam is not the best. If you just kept it as retinal throughout the video, then i would consider it a general introduction and wouldnt criticise. in all fairness, its great that things like this are available on youtube and that people take the time to upload them. i would keep it as retinal for simplicity and only mention the retinol etc in a more in depth video.

  58. Tom Green

    This video really save my backside with a bit of revision so if I ever get in a tight spot again I think i’ll definitely be coming back to your channel.

  59. joe martin

    Hello everyone. Outstanding clip. My good friend was formerly an obese guy. He went from 283lbs of fat into 218 lbs of complete lean muscle mass. I came to be shocked. I just registered myself coz I wanna strengthen. He used the Muscle Building Bible (Search on Google)…

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