episode32

032 Visual Processing in the Retina

After the rods and the cones, there are a few other important cells involved in visual processing. In this video, Leslie explains about how the bipolar cells and ganglion cells contribute to this process.

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 32, I’m going to be talking about the visual processing that happens in the retina. We’ve spoken a little bit about this in the previous episode, and I want to take this to the next step, the next step in the process.

First, we’re going to start with this animation that’s kind of a review of what we spoke about in Episode 31. There are a few things I’d like to highlight. First of all, if we look here, we have these little round balls, and those are sodium ions. They can be calcium ions also, but for the most part, those are sodium ions. Here we have a channel. In Episode 31, we spoke about this cyclic GMP-gated sodium ion channel. What we said was that if there’s cyclic GMP around, which there normally is in the dark, that channel is going to be open. If that channel is open, as you can see right here, you’ll see sodium is normally able to rush into the cell, and the rod is usually depolarized, and neurotransmitter is being released. So that’s what happens in the dark.

We also have some other things. Here we have phosphodiesterase, as you can see right here. Then we have the trimeric transducin molecule that normally has GDP attached. We have rhodopsin, which is the visual pigment in the rod. And here we have one that we didn’t speak about before, guanylate cyclase, and that’s basically the enzyme that makes the cyclic GMP.

So, in review of what we said before, cyclic GMP is normally around in the dark, depolarization happens. Then, when light comes, we said that a photon of light comes in, and we’ll see that right now, activates the rhodopsin, replaces a GTP, that activates the phosphodiesterase, and when that phosphodiesterase is activated, it converts the cyclic GMP into regular GMP. When that happens, you’ll see here, it closes this gate, and the rod hyperpolarizes. It becomes more negative, it goes toward the membrane potential for potassium.

So there’s a lot of stuff going on in this animation right now and you can see that. My recommendation would be to pause this and go back and look at it again, and kind of try to visualize what’s happening. Look at the details and make sure you understand this, because this is a good overview of what happens inside the rods. As I said in the last episode, the process that happens in the cones is relatively similar to this. So if you get this, you have a good idea of what happens.

So the result of this is, if we’re looking at the membrane potential, or the receptor potential in this case, when the light comes, that is not going to cause the regular action potential. It’s going to cause the membrane potential to go down until that light is gone again, and then it comes back up. This process, as with before with the neuron, this is called hyperpolarization. I’m going to write it as two words because I’m running out of space. Hyperpolarization, so that is what happens in response to light. I hope that review makes sense. If not, go back to the previous episode and you’ll get more of those details.

Now let’s move on and take it from what’s happening inside the rods to what happens with some of the other cells that we have around. So this is looking at the cells in the retina. If light is coming into the eye and it’s hitting the retina, the light is coming in this direction. So it’s coming from up here and it’s going down. There’s an interesting way in which this is arranged in that the rods are actually at the end here. So these are the rods, and here we can see an example of a cone.

There are few other important cells that we need to know about. One would be this red cell here, and that would be a bipolar cell. The next important one that you need to be aware of would be this guy here, these cells here, and those are called ganglion cells. So these are the major players:

  1. The rods and the cones
  2. The bipolar cells
  3. The ganglion cells

You can see here we have some other cells. For example, this one here is called a horizontal cell. We’re not going to focus on the horizontal cells in this video. This is called an amacrine cell. We’re also not going to focus on that in this video, either. So the major ones we’re going to talk about are the rods, and when I say the rods, of course, you can assume that the process is somewhat similar with the cones. We’re going to talk about the bipolar cells, and we are going to talk about the ganglion cells. So let’s look at what happens.

We looked at the process that’s happening inside of the rods before, and we say the net result of that light coming in, so here we have light that’s coming in, and the net result, the overall result, in the rods is that the membrane hyperpolarizes. So here we have a hyperpolarization of the rods. When that happens, neurotransmitter release slows down. It basically shuts off the neurotransmitter release.

Now, the rods are connected, you can see here there are a number of connections, to the bipolar cells. The neurotransmitters that are normally released here are inhibitory, so when it stops releasing those neurotransmitters, it is no longer inhibiting the bipolar cells, and what’s going to happen to the membrane of the bipolar cells is that we’re going to get a depolarization. So it’s basically the exact opposite of this hyperpolarization. Here, we’re going to get in the bipolar cells, we’re going to get depolarization. So let me write here bipolar cells.

Now, bipolar cells, just like rods, don’t have an axon. So this is the entire bipolar cell, but this is not an axon, so we don’t get an action potential. We do get this depolarization, and that is going to cause an increase in neurotransmitter release at these synapses here. So bipolar cells gets depolarized, more neurotransmitter is going to be released, similar to what happens in a regular axon.

Then we have this ganglion cell here, and all of these are ganglion cells. What’s going to happen there in response to more neurotransmitters being released, the ganglion cells are going to also depolarize, but since here we have axons, that’s going to cause a burst of nerve impulses, so a burst of action potentials. These are my spikes, these are my action potentials, these are signals that are travelling along the axons going to the brain.

Let’s look at that again. In response to light, the rod hyperpolarizes, and we looked at the process as to how that happens. That reduces neurotransmitter release, so let me draw here or write here there’s a decrease in neurotransmitter release. Those neurotransmitters are normally inhibiting the bipolar cells. Now, they are no longer being inhibited. They are going to depolarize. That’s going to cause an increase in neurotransmitter release here to the bipolar cells, and that’s going to stimulate the ganglion cells, causing depolarization and a burst of impulses.

So the final result, in terms of what’s happening in the retina, is that these ganglion cells are firing a burst of nerve impulses that are sent to the brain. I hope that makes sense. You can kind of go through that again if anything is unclear so that you can get a better understanding of that.

There’s one more thing I want to talk about. We spoke about the thalamus in the brain, and we said that the thalamus is kind of like the regulatory gateway of the brain. All of the sensory information comes into the thalamus, and then goes to the rest of the brain, or the motor output that’s coming from the brain goes via the thalamus to the rest of the body. There’s no exception here. When the ganglion cells are stimulated and they send their signals, they’re going to send their signals to a specialized region in the thalamus called the lateral geniculate. So this is in the thalamus, and then, that can go on to other parts of the brain for further processing. We’re going to talk about that in some later episodes.

So that’s really it for this episode. Hope everything makes sense. If you have any questions, of course you can ask them in the comments section below, or if you have any comments or anything of that sort, please give me feedback, because I love to get feedback from you guys and be able to help you even better. So that’s it for this video, and I’ll see you in the next one.

163 comments
Eoghan o connor
Eoghan o connor

That's actually the best Youtube science video I have ever seen. V well explained! :D

Josh mith
Josh mith

Your videos are amazing! Savin' my Ass again and again ;)

EdOn Berlsha
EdOn Berlsha

Why are the neurotransmitters released by the rods at minus 30 mv and when it gets hyperpolarized to 70 mv it stops the submission of ntransmitters ??

James Kundart
James Kundart

Not exactly. In birds, the retinal blood vessels are bunched up in a structure called the avian pecten.

amrutvani2
amrutvani2

Thanks Can u tell me whether such inverted mechanism also exists in lower animals ?

James Kundart
James Kundart

Sounds like a dark adaptation problem -- especially if it's better after 15-20 minutes in the dark.

Xthalimic UXO
Xthalimic UXO

Thanks. I have a phenomena happening to me. When I go into my bedroom at night with the lights off and my eyes open I find it hard to navigate but when I close my eyes as if I am blind I can feel and sense my way around the room better. What is causing this in the brain? thanks.

James Kundart
James Kundart

Great observation! As I mentioned above, the retina is backwards, likely because the photoreceptors are highly metabolically active and need to be adjacent to the opaque retinal pigment epithelium. The retinal vasculature does block vision to some of the rods and cones -- a definite downside.

James Kundart
James Kundart

You'd think so, but no! The dark current flows only in the dark, when the receptors are depolarized. In the light, the receptors are hyperpolarized and the dark current shuts off, likely to conserve neurotransmitter.

James Kundart
James Kundart

80% of the optic nerve first synapses in the thalamus -- this is the retino-cortical pathway. The other 20% goes to the superior colliculus and pineal gland via the retino-tectal pathway. When dark adapting, the so called "dark current" begins to flow. The horizontal cells allow the rods to take over and the cones to slough their discs, while the photoreceptors depolarize as shown in this video.

James Kundart
James Kundart

Great observation! The retina is backwards, likely because the photoreceptors are highly metabolically active and need to be adjacent to the opaque retinal pigment epithelium.

James Kundart
James Kundart

I can give this old question a shot. It has to do with the dark current in the retina, that is, how it hyperpolarizes in the light. Having two types of bipolar cells allows each to respond to the neurotransmitter glutamate in different ways. Off bipolars have cone input only, and treat glutamate as an excitatory neurotransmitter, as it usually is used elsewhere in the CNS. On bipolars have both rod and cone input, and use glutamate is inhibitory, allows them to conserve it in the light.

amrutvani2
amrutvani2

Nice presentation .It is strange that light has to travel all the way from ganglion and bipolar cells all the way to reach rods and cones which fires action potential again in reverse direction of light! Why this "inverted" mechanism in retina ? Harish

Saved2theLight
Saved2theLight

does the signal go to the thalamus first then the visual cortex? also what happens to the rods and cones when we enter a dark or dim lit room with our eyes open in terms of the neuro-transmission. thanks

Reito Shion
Reito Shion

It is CLEAN and CLEAR ! Very easy to understand ~ You knows these things on your finger tips !! Thank you :)

Karan Bhandari
Karan Bhandari

YOur Awesome Mahn !!! Great Effort ! But What about Cones ? I wanna know about their functions too... Please upload something on cones

Hannah Patro
Hannah Patro

wow!!! why didnt i know about this guy last quarter of A &P 1? What a great help!! Thank you so much, oh my gosh,,hours and hours of studying and i watch these videos and it all clicks! :)

Thriptathapalama
Thriptathapalama

Thank you so much, my friend and I could not understand this process until we watched episode 31 and 32.....now we can enter our final feeling better about this! I always watch your videos for anything I don't understand. Mostly my teacher sucks, lol.

Roberto Ballarino
Roberto Ballarino

Nice video, you misspoke near the end though. You call the ganglioncell a bipolar cell. (might want to place an annotation)

Minoo B
Minoo B

I just dont get one thing and that is why the whole process is upside down. we know that light is absorbed by the receptors which are rods and cones. but they are located way at the back and all other cells are on the way. so how it works? light passes all those cells without distortion and reaches the receptors. there light goes through some process and the return back to ganglion cells and exit the eye?

sonea al hason
sonea al hason

thank you to much can you show more vedio aboute phosphodiestreas inhibitor and drug thate inhibit it

Adam Neita
Adam Neita

Great video sir, thank you for your efforts!

Hannah Taylor
Hannah Taylor

OFF bipolar cells express AMPA/KAINATE glutamate receptors, which behave as you would typically expect: More GLU = more depolarisation, and less GLU (i.e. in the light) = less depolarisation. This is why ON and OFF bipolar cells behave differently to the same neurotransmitter. Summarised: Light - Hyperpolarises rods/cones - Rods/cones release less GLU - ON bipolar cells become more depolarised (less mGluR6 activation) & OFF bipolar cells become less depolarised (less AMPA/KAINATE activity).

Hannah Taylor
Hannah Taylor

ON bipolar cells depolarise in the LIGHT ON condition. OFF bipolar cells depolarise in the LIGHT OFF condition. So, ON bipolar cells DEPOLARISE in the light condition in response to LESS glutamate (released by hyperpolarised photoreceptors). Confusing - usually we think that MORE glutamate depolarises a cell. BUT it depends on the type of receptor. ON bipolar cells express metabotropic glutamate receptor 6 (more GLU = more hyperpolarisation in this case. So less GLU = less hyperpolarisation).

Hannah Taylor
Hannah Taylor

Sorry, but this video is inaccurate. Yes, photoreceptors (rods/cones) hyperpolarise in response to light due to a reduction in cGMP and Na+ channel closure. BUT photoreceptors DO NOT release inhibitory neurotransmitters. They release glutamate, which is excitatory. So, when rods/cones are hyperpolarised they release LESS glutamate onto bipolar cells. The downstream response to LESS glutamate depends on whether the bipolar cells are the ON or OFF type. See my next comment...

sfboas
sfboas

i love ALL your videos!! it makes me really easy to understand.. :) tq soo much! keep it up! it is very helpful..thanks a lot!.

samasoftley
samasoftley

I am a student studying Zoology in the UK at Newcastle University. You explain this more concisely than my lecturer. You are my life saver.

Samira Nademi
Samira Nademi

I love this! Thank you so much! u just cleared this whole thing up for me.. By the way at the end (around 9:42) u mistakenly said BCs instead of GCs)..Thanks a lot

sarahxho
sarahxho

These videos are great, but it'd be even better if you could add the option (under settings) to be able to watch them at 1.5X or 2X the speed!

dumarcus110
dumarcus110

Thank you for what you are doing! Thank you

kima3maki
kima3maki

thanks for the video. it was a great use for may ophthalmology class... :)

burndtjam1
burndtjam1

when I combo this video with my Physiology book from Linda Costanzo I have a great understanding of how vision works. Thank you

lucas aragao
lucas aragao

how neurotransmissor is release on the bipolar cell from the fotorrepceptor to inibition of the bipolar cells??

Cian Noone
Cian Noone

I could really do with a picture of the above reaction you have on the video! Anything online is far more complicated!!

qngo1
qngo1

definitely best videos ever! I am about to take my medical board test in 2 weeks and it is very helpful. I have seen some of Leslie's videos before and of course, it always great. One thing for sure is that Fovea is mainly composed of cones predominately and very few rods. Thank you so much for contributing. plz continue to do so. God bless you!

ExplosiveFilms2007
ExplosiveFilms2007

This was extremely explanatory!! Thanks so much! I love making biology fun :D

Rockinperiwinkle
Rockinperiwinkle

I agree. He either meant neurotransmitters FROM bipolar cells or to GANGLION cells.

Rockinperiwinkle
Rockinperiwinkle

I think the cells are hyperpolarized in light due to the closing of the sodium channels. That blocks the release of the neurotransmitters.

studentsong83
studentsong83

Thanks so much!! it really helps for my exams!

pixiesashes
pixiesashes

dude! you are the COOLEST! thanks :)

kmac0258
kmac0258

Just so you know, I could be wrong, but at 9:41 you said bipolar cells. did you mean Ganglion cells?