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031 How Rods and Cones Respond to Light

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.

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Leslie Samuel is the creator of Interactive Biology. His mission is to use this site to Make Biology fun for people all over the world.

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Leave An AWESOME Comment

140 Responses to “031 How Rods and Cones Respond to Light”

  1. dsbllr0 February 10, 2011 at 5:06 pm #

    @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 March 7, 2011 at 4:05 pm #

    @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 March 7, 2011 at 4:48 pm #

    @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. joinmebowser9000 March 7, 2011 at 5:31 pm #

    @InteractiveBiology I’m also glad I understand it now ;)

  5. Malikorous April 7, 2011 at 12:17 pm #

    WHERE WERE YOU WHEN I WAS DOING MY BIOLOGY A LEVEL… but yeah great videos thanks :)

  6. InteractiveBiology April 11, 2011 at 11:02 pm #

    @Malikorous LOL. Not sure, but glad you are finding value in them. All the best!

  7. valarmanwe June 23, 2011 at 6:33 pm #

    @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 !

  8. valarmanwe June 23, 2011 at 6:33 pm #

    @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 !

  9. ximepipitas June 28, 2011 at 4:39 am #

    awesome!!! love your vids!

  10. ximepipitas June 28, 2011 at 4:39 am #

    awesome!!! love your vids!

  11. SuperLLL July 1, 2011 at 10:35 am #

    How does the retinal get back to its original position?

  12. InteractiveBiology July 3, 2011 at 6:51 pm #

    @SuperLLL Once the energy from the photon is used up, the retinol goes back to it’s original configuration. It takes energy from the photons to keep it in the modified configuration.

  13. miles2707 August 31, 2011 at 9:47 pm #

    GDP does not get phosphorylated to become GTP, it is exchanged. This is a G-protein.

  14. Djalitana September 16, 2011 at 6:09 am #

    I could watch the half of it, not sure why, maybe my internet connection has a problem. will come back later. thanks.

  15. InteractiveBiology September 16, 2011 at 7:34 am #

    @Djalitana Probably the connection yes. Try again later and let me know

  16. rajhetti6 September 23, 2011 at 1:54 pm #

    thanks for the vid but my lecture says there are no rods in fovea

  17. rajhetti6 September 23, 2011 at 3:59 pm #

    my book also says in fovea there r just cones

  18. rajhetti6 September 23, 2011 at 3:59 pm #

    my book also says in fovea there r just cones

  19. franciscodorey1 October 25, 2011 at 4:51 pm #

    @valarmanwe Sorry but glutamate is not a neuroinhibitor, it is a excitatory neurotransmitter

  20. Francisco d'Orey October 25, 2011 at 4:51 pm #

    @valarmanwe Sorry but glutamate is not a neuroinhibitor, it is a excitatory
    neurotransmitter

  21. valarmanwe October 26, 2011 at 10:57 am #

    @franciscodorey1 Quite right.

  22. valarmanwe October 26, 2011 at 10:57 am #

    @franciscodorey1 Quite right.

  23. valarmanwe October 26, 2011 at 11:13 am #

    @franciscodorey1 Ps. I was thinking of glycine.

  24. GoldenPhoenix223 November 6, 2011 at 1:50 am #

    Great video, thanks! Helped a lot for my Systems Neuroscience exam.

  25. InteractiveBiology November 7, 2011 at 9:32 am #

    @GoldenPhoenix223 Glad to know the video helped. Stay tuned for more Biology videos coming very soon!

  26. InteractiveBiology November 7, 2011 at 9:32 am #

    @GoldenPhoenix223 Glad to know the video helped. Stay tuned for more
    Biology videos coming very soon!

  27. bizz76 November 16, 2011 at 10:19 am #

    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??

  28. InteractiveBiology November 16, 2011 at 2:54 pm #

    @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! :)

  29. InteractiveBiology November 16, 2011 at 2:54 pm #

    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! :)

  30. InteractiveBiology November 16, 2011 at 2:54 pm #

    @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! :)

  31. yazzhopper864 December 7, 2011 at 6:55 pm #

    This video is so helpful! It’s really nice to see the process instead of just reading about it!

  32. InteractiveBiology December 8, 2011 at 3:13 pm #

    @yazzhopper864 I know, it makes it all easier, right? Glad to know you find some value in it. We have more Biology videos in the site. Stay tuned for new ones coming very soon! :)

  33. InteractiveBiology December 8, 2011 at 3:13 pm #

    I know, it makes it all easier, right? Glad to know you find some value in it. We have more Biology videos in the site. Stay tuned for new ones coming very soon! :)

  34. Ckaotenkind December 12, 2011 at 4:37 pm #

    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

  35. Ckaotenkind December 12, 2011 at 4:37 pm #

    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

  36. InteractiveBiology December 13, 2011 at 7:39 am #

    @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 :)

  37. InteractiveBiology December 13, 2011 at 7:39 am #

    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 :)

  38. InteractiveBiology December 13, 2011 at 7:39 am #

    @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 :)

  39. joinmebowser9000 December 13, 2011 at 9:51 am #

    @InteractiveBiology Can’t wait for that video. I remember this video was why I subscribed. You really saved my day!

  40. joinmebowser9000 December 13, 2011 at 9:51 am #

    Can’t wait for that video. I remember this video was why I subscribed. You really saved my day!

  41. InteractiveBiology December 14, 2011 at 3:29 pm #

    @joinmebowser9000 THank You! Keep on watching :) We’ll have more added to the site very soon!

  42. InteractiveBiology December 14, 2011 at 3:29 pm #

    THank You! Keep on watching :) We’ll have more added to the site very soon!

  43. EphraimHeinrich December 18, 2011 at 3:00 pm #

    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.

  44. InteractiveBiology December 18, 2011 at 5:35 pm #

    @EphraimHeinrich 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!

  45. InteractiveBiology December 18, 2011 at 5:35 pm #

    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!

  46. EphraimHeinrich December 18, 2011 at 8:00 pm #

    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.

  47. InteractiveBiology December 18, 2011 at 10:35 pm #

    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!

  48. bhatiakaran71 December 26, 2011 at 1:20 pm #

    Very simply explained…
    Great video…Thanx…

  49. bhatiakaran71 December 26, 2011 at 6:20 pm #

    Very simply explained…
    Great video…Thanx…

  50. xxnaruto1234xx January 27, 2012 at 10:54 pm #

    @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.

  51. xxnaruto1234xx January 27, 2012 at 10:54 pm #

    @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.

  52. xxnaruto1234xx January 27, 2012 at 10:54 pm #

    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.

  53. xxnaruto1234xx January 27, 2012 at 10:54 pm #

    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.

  54. xxnaruto1234xx January 28, 2012 at 3:54 am #

    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.

  55. mp5yourmp3 February 14, 2012 at 9:20 pm #

    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.

  56. mp5yourmp3 February 15, 2012 at 2:20 am #

    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.

  57. dearizqi February 22, 2012 at 8:30 am #

    Great video! very simple explained. It helps me to understand my medical study about the eye. Thank youuuu soo much.

  58. dearizqi February 22, 2012 at 1:30 pm #

    Great video! very simple explained. It helps me to understand my medical study about the eye. Thank youuuu soo much.

  59. 88rampitup88 February 24, 2012 at 12:40 am #

    @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.

  60. 88rampitup88 February 24, 2012 at 12:40 am #

    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.

  61. mp5yourmp3 February 24, 2012 at 2:31 am #

    @88rampitup88 and you think thats enough to say “the fovea contains a lot of rods and cones?” no. its very misleading

  62. mp5yourmp3 February 24, 2012 at 2:31 am #

    and you think thats enough to say “the fovea contains a lot of rods and cones?” no. its very misleading

  63. 88rampitup88 February 24, 2012 at 5:40 am #

    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.

  64. mp5yourmp3 February 24, 2012 at 7:31 am #

    and you think thats enough to say “the fovea contains a lot of rods and cones?” no. its very misleading

  65. belvidere123 February 28, 2012 at 2:41 pm #

    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.

  66. belvidere123 February 28, 2012 at 7:41 pm #

    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.

  67. AbdullahDiaa2 March 2, 2012 at 8:47 am #

    @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

  68. AbdullahDiaa2 March 2, 2012 at 8:47 am #

    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

  69. AbdullahDiaa2 March 2, 2012 at 9:29 am #

    @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 !!

  70. AbdullahDiaa2 March 2, 2012 at 9:29 am #

    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 !!

  71. AbdullahDiaa2 March 2, 2012 at 1:47 pm #

    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

  72. AbdullahDiaa2 March 2, 2012 at 2:29 pm #

    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 !!

  73. cherrybopbop March 22, 2012 at 5:41 pm #

    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.

  74. cherrybopbop March 22, 2012 at 9:41 pm #

    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.

  75. 05bebop March 25, 2012 at 2:39 pm #

    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?

  76. 05bebop March 25, 2012 at 6:39 pm #

    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?

  77. Linkfan07 March 26, 2012 at 9:34 pm #

    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! :)

  78. Linkfan07 March 27, 2012 at 1:34 am #

    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! :)

  79. Jusplais March 28, 2012 at 9:28 am #

    object being yourself.

  80. Jusplais March 28, 2012 at 9:28 am #

    object being yourself.

  81. Jusplais March 28, 2012 at 9:30 am #

    the image produced from the image of the subject of the lady. Going to another video.

  82. Jusplais March 28, 2012 at 9:30 am #

    the image produced from the image of the subject of the lady. Going to another video.

  83. Jusplais March 28, 2012 at 9:39 am #

    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?

  84. Jusplais March 28, 2012 at 9:39 am #

    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?

  85. Jusplais March 28, 2012 at 1:28 pm #

    object being yourself.

  86. Jusplais March 28, 2012 at 1:30 pm #

    the image produced from the image of the subject of the lady. Going to another video.

  87. Jusplais March 28, 2012 at 1:39 pm #

    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?

  88. ClassAencounter April 7, 2012 at 9:13 am #

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

  89. ClassAencounter April 7, 2012 at 1:13 pm #

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

  90. MultiBrando88 April 18, 2012 at 12:06 am #

    …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?

  91. MultiBrando88 April 18, 2012 at 4:06 am #

    …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?

  92. Nadia2Lise April 23, 2012 at 11:43 pm #

    thank you so much! amazing how you made this so much simpler than the textbook !

  93. Nadia2Lise April 24, 2012 at 3:43 am #

    thank you so much! amazing how you made this so much simpler than the textbook !

  94. Theabidal92 May 8, 2012 at 6:09 pm #

    Dude the fovea doesn’t have a lot of rods

  95. Theabidal92 May 8, 2012 at 6:09 pm #

    Dude the fovea doesn’t have a lot of rods

  96. Theabidal92 May 8, 2012 at 6:09 pm #

    Dude the fovea doesn’t have a lot of rods

  97. asgher07 May 20, 2012 at 2:35 pm #

    simply awesome

  98. asgher07 May 20, 2012 at 2:35 pm #

    simply awesome

  99. Sunny1Tuesdays June 20, 2012 at 12:48 pm #

    The fovea consists only of cones !

  100. Sunny1Tuesdays June 20, 2012 at 12:48 pm #

    The fovea consists only of cones !

  101. MrHassanleb June 28, 2012 at 10:09 am #

    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!

  102. MrHassanleb June 28, 2012 at 10:09 am #

    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!

  103. dilqnaslavova June 30, 2012 at 4:37 pm #

    @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

  104. dilqnaslavova June 30, 2012 at 4:37 pm #

    @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

  105. dilqnaslavova June 30, 2012 at 4:37 pm #

    @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

  106. JDomenex August 10, 2012 at 4:46 am #

    Most of the light refraction occurs in the cornea not the lens :)

  107. Evil915Frog September 6, 2012 at 6:58 am #

    no sound??

  108. sid123453 September 16, 2012 at 3:54 am #

    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.

  109. RidleyE September 23, 2012 at 2:07 am #

    so you big and bad now huh

  110. DJPoloMan22 November 2, 2012 at 5:06 am #

    Great Video. Thanks for your effort and time creating this!

  111. thealmassi1 November 25, 2012 at 1:00 am #

    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!

  112. Mohammad Al Hasan December 7, 2012 at 5:09 am #

    really informative video, good job :)

  113. Bco978 December 7, 2012 at 1:27 pm #

    No rods exist in the fovea, they are spread out into the retina. Fovea is basically packed with cones.

  114. Lazy Orc December 26, 2012 at 1:10 pm #

    amazing love the animations!

  115. cu99460 December 29, 2012 at 8:39 am #

    Thanks you interactiveBiology…. What does ‘Em’ mean? just confused with the little graph at 10:30?

  116. samasoftley January 6, 2013 at 6:05 pm #

    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!

  117. Anton Krivosheyev January 12, 2013 at 3:27 am #

    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.

  118. TheLilads January 24, 2013 at 3:48 am #

    The next person who comments will be murdered by a pack of oily sex cabbages with massive dicks.

  119. ePRTN January 25, 2013 at 6:44 am #

    JESUS CHRIST SAVES – For God so loved the world that He gave His only begotten Son, that whoever believes in Him should not perish but have everlasting life

  120. Snozzberries Wonka February 6, 2013 at 5:22 am #

    i loveeee this :)

  121. Tessa Dean February 7, 2013 at 7:56 pm #

    im going to pass this section of my pscyh exam thanks to you

  122. Shawn Harris March 30, 2013 at 5:40 pm #

    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!

  123. austmcc April 20, 2013 at 3:59 pm #

    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.

  124. Kenpow93 April 25, 2013 at 3:01 am #

    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!

  125. Dennis Don Karunakaran April 29, 2013 at 3:39 pm #

    Bless you Sir! Thank you

  126. Reotshepile Kgoro May 19, 2013 at 3:23 pm #

    Wow this video helped me a lot and i really appreciate it… GOD bless you

  127. juniecr34 May 21, 2013 at 10:32 pm #

    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 :)

  128. lmbarak May 24, 2013 at 1:20 am #

    Where is Leslie from? Being from the US I hear his accent and it’s really cool.

  129. priyanshurocks June 21, 2013 at 12:38 pm #

    coursera rocks.Steve Joorden brought me here

  130. studentquestor July 23, 2013 at 3:50 am #

    Cognitive Cannibals unite!

  131. studentquestor July 23, 2013 at 3:51 am #

    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.

  132. studentquestor July 23, 2013 at 3:52 am #

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

  133. studentquestor July 23, 2013 at 3:53 am #

    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.

  134. austmcc July 23, 2013 at 4:31 am #

    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.

  135. nikhil desai July 30, 2013 at 2:20 pm #

    nice video……….. good job

  136. Yingqiu Cao September 28, 2013 at 5:34 pm #

    Nice video. I’m an ECE student doing biophysics. your videos really gave me insights into biological processes.

  137. Tom Green September 30, 2013 at 5:14 pm #

    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.

  138. Taoran Yu October 7, 2013 at 2:30 pm #

    Theres no rods in fovea

  139. hopkin104 October 20, 2013 at 9:53 pm #

    Thanks a lot big hommie.

  140. joe martin November 1, 2013 at 3:59 pm #

    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)…