Photochemistry of Vision

Hello everyone, welcome back and in this video we are going to do the photochemistry of vision. First we will look at the layers of retina and then we will understand what are rods and cones and then we will deal with the photochemistry of vision. How does the light activates the rods and cone and how it initiates to convert the light into electrical activity which will be then conveyed through the optic nerve to the brain. Okay, so let us start with retina. As you can see here, this portion here is the retina, the yellowish color structure here is the retina. And this is the receptor or the sensory aspect of the eye. As we considered eye as an optical system in the previous videos, Here this optical inside this optical system resides a sensor and that sensor is retina and this in this retina only the sensory and the neuronal network is found which will then convert the light into electrical activity or the action potential and which will be conveyed to the brain with the help of this optic nerve. Now coming to the layers of retina as you can see there are several layers of retina one thing most important to see in this diagram is the layers are arranged in a certain order so the pigmented layer will be the outermost layer so what do we call outermost layer is the one which is present outside the outermost layer And the inner layer the innermost layer will be the one on which the light falls first. So this is the inner aspect of the or the inner layer of the retina and the outside one which is here. So this is the outermost aspect or the pigmented layer what we call is the outermost layer. So this layers of retina are arranged in order. This one is the outermost layer and going through consecutively this is the innermost layer. of retina and the light which is coming from the optical system falls on the innermost layer that is the inner limiting membrane and then goes through various layers of cells and then reaches the rods and cones and the outermost pigmented layers so this is how the arrangement is let us see in details what are those layer made up of so when we consider this pigmented layer this was the outermost layer as we discussed before and it is made up of melanin pigment. What is the importance of this pigmented layer is it absorbs the light so that it does not allow its reflection. If the light is being reflected inside the eye itself the visual equity will be disturbed so much and that is the importance of the pigmented layer. It is made up of melanin. It contains melanin and absorbs most of the light does not allow the light to reflect inside the eye. One more important aspect of pigmented layer is it is having vitamin A so vitamin A is very important in the photochemistry as we will see it is the pigmented layer is having a large amount of vitamin A which will help in the vision so this is the first outermost layer then coming towards inside as you can see the outer the inside to the outermost layer is rods and cones so as you can see we will further see in detail what are the structures of rod and cones but the second layer formed is the rod and cones the layer of rod and cones which is the outer nuclear layer as you can see here the nucleus of the Rods and cones are present in the outer nuclear layer. And then comes the outer plexiform layer. So plexiform means it is a connection. Connection between the outer cells, the rods and the cones and the inner cells. that is the bipolar cells and the exons in the dendrites are connecting here in the outer plexiform layer. Then comes the inner nuclear layer. So in this layer lies the nucleus of various inner cells that is amacrine cell, the bipolar cell, the horizontal cells and that is why it is called the inner nuclear layer. Then comes the inner plexiform layer that means it will be having the connection between the inner cells and the ganglionic cells. So the axons of the inner cells and the dendrites of the ganglionic cells are forming the inner plexiform layer. Then comes the layer of ganglionic cells or the ganglion cell layer and from there it will be coming the inner plexiform layer. From this layer these ganglion gives axon which forms the optic nerve. And limiting this is the inner limiting membrane and as we see in the previous diagram as we saw it that the direction of the light will be the light will be coming from the optical system to the eye to the retina in the. direction of this from inner to the outer aspect. So as you can see when the light is falling the main receptors of the lights are rods and cones. So the light which is coming to rods and cones are coming from or passing through these many layers first and then it is coming to the rods and cones. So in this case the visual equity will be less but there is a important aspect important. region which is present in the retina that is called as fovea and there is very important very much importance of fovea is the visual acuity the detailed vision is very high in the foveal region okay so the foveal region when the light falls whichever the image is falling on the foveal region which has a diameter of 0.3 millimeter it has a very detailed picture of it on when the light falls on the foveal region and what is the reason behind that since the light is traveling the layers the inner layers what we have seen previously as you can see in other regions of the retina the inner layers the light will be traveling through the inner layers and then it will be falling on rods and cones but in case of fovea the inner layers are displaced it is an a vascular zone also and the inner layers are also displaced so that the light directly falls on the rods and the cones so in this case especially the cones okay so here in the foveal region as you can see there are much cones and very few rods okay so especially in the cones the light directly falls on them and that is how it increases the detailed image of the light and detailed vision in the fovea so what is the important aspect it is that it is avascular as well as the inner layers of the cells are displaced outwards so that the light directly falls on the photoreceptors and that is why the light which is falling on the fovea has the image of a detailed image of the object okay so that is the importance of foveal region. Now let us begin with the important aspect of our lecture that is the rods and cones so as we can see here this is the general structure of rods and cones and both of them are made up of this similar segments so first look at the outer segment why it is called as outer segment because it is facing towards the outer layer of the retina and it is embedded or near to the pigmented layer pigmented layer which is the outer segment of the retina so this is the outer segment of the rods and and And as you can see here in the outer segment of the rod there are various disc like structures which are just the extension of the cell membrane. As you can see the extension of the cell membrane forms a disc like structure. In this extension of the cell membrane or in this disc are present the color pigments in the cones or rhodopsins in the rods. So the photosensitive pigments rhodopsin or color pigments. rhodopsin for rods and color pigments for cones are present in these disc-like structures of the outer segment of the rods and cones then coming to the inner segment why it is inner segment because it is present towards the inner side of the retina okay so in this inner segment are present mitochondria another cell organelles this important aspect is mitochondria is very necessary for the chemical reactions of the photochemistry of light so the mitochondria provides the energy the ATP so that the reactions can take place which are occurring in the photochemistry of vision or the photochemistry to convert the light energy into the electrical signals then coming to the nucleus this is a nucleus and from there arises the axon which synapse with the inner cells of the retina so this this is the general structure of rods and cones. One difference will be between rods and cone is as you can see the rods are in this shape. They are slender, cylindrical in shape whereas the cones are, the outer segment of the cones are conical in shape. The diameter of rods can be from 2 to 5 micrometer whereas the diameter of the cones can be 5 to 8 micrometers. But the cones can have even smaller size because the cones which are present as we have seen in the foveal region, the cones which are present. are of 1.5 micrometer in diameter. Smaller cones as compared to the cones which are present on the periphery of the retina. The cones which are present on the fovea of the retina are very smaller and those are of 1.5 micrometer in diameter. So these are the general structure as you can see the disc like structures are present here also. The cut section and in the both rods and cones. In this disc. are present the color pigments and the rhodopsins color pigments in case of cones and rhodopsins in case of rod which are the one which are the photosensitive pigments now coming to rhodopsin as we discuss that rhodopsins are present in the rods and color pigments are present in the cones those are the photosensitive pigments in this video we are going to discuss you the photochemistry of rods and rhodopsin only the color vision and about the color pigments will be dealt in the next video. So let us begin with the rhodopsin. As you can see these are the extension these discs are the extension of the cell membrane and in this diagram you can see the cell membrane this is the molecular structure of rhodopsin. In case of rhodopsin, the opsin is called as scotopsin. And the combination of scotopsin and 11-cis-retinal forms the rhodopsin. So what is rhodopsin? It is an integral protein which is present in the disc-like extension of the cell membrane. As you can see, these are the cell membrane and the integral protein. which is combination of scotopsin and 11-cis retinal. So the combination of scotopsin and 11-cis retinal is called as rhodopsins and these are the integral protein which are present in the cell membrane the extension of the disc-like structure cell membrane. So this is rhodopsin. Now coming to the how rhodopsin is activated by light. and its visual cycle. So let us deal with the visual cycle and let us see how Rhodopsin can then activate or convert the light energy into the electrical energy which will be then converted into action potential and sent to the brain. So let us start with the visual cycle. So when we see the visual cycle the Rhodopsin will be present on the rods. on the disk like structure cell membrane of the rods because it is an integral protein in that cell membrane. So whenever the light passes through the optical system of the eye and then falls on the inner layers and passes through and then falls on the rods or cones in that case because the cycle will be same in both the rods and the cones and the activity of rhodopsin and color pigments are same. So we will discuss just the visual cycle of rhodopsin and it will be same for the color pigments also. So when the light falls on rods it converts the rhodopsin into all trans retina. So when the light falls on rhodopsin it converts the as you remember that the The rhodopsin is made up of two individual structures that is 11-cis retinal and scotopsin. So the scotopsin here and 11-cis retinal forms rhodopsin. When the light passes through the retina, the light passes through the retina. through the optical system and falls on rhodopsin it gets split up into the all trans retinal and scotopsin why this gets split up because the all trans retinal the the confirmation the what to say the structure of all trans retinal is different as compared to the structure of 11 cis retinal 11 cis retinal perfectly fits on the scotopsin and the combination you is formed which makes the rhodopsin but when the light energy converts the 11th cis retinal into all trans retinal this all trans retinal is the confirmation of all trans retinal does not fits in this codopsin and thus they split up and this all trans retinal is how it is formed by the light energy and these all of them the intermediate byproducts which are forming are the intermediate byproducts or intermediate products of that cycle that means rhodopsin directly does not gets converted into all trans retinal what happens is rhodopsins get converted into bathorhodopsin, lumirhodopsin, metarhodopsin 1 and metarhodopsin 2 and then the all trans retinal splits out from the scotopsin so that is how the cycle occurs once again rhodopsin is present on the cell membrane as we have seen made up of two components that is scotopsin as you can see here scotopsin and 11 cis retinal and they both combine to form rhodopsin whenever the light energy falls on rhodopsin it get converted into bathmore rhodopsin, lumi rhodopsin, metarhodopsin 1, metarhodopsin 2 and then it gets split up in all trans retinal and scotopsin. So basically light energy converts the 11th cis retina. to all trans retinal and why these split up why all trans retinal cannot stay with the scotopsin is because the conformation of the all trans retinal is different from the conformation of 11 cis retinal retinal and does not fit on the spotopsin that is why the splitter and these are the products which are formed in that process in that one thing what we have to consider here is the metarotopsin is the active byproduct which is the one which stimulates the visual cycle which stimulates the transformation of light energy into electrical energy. So in this cycle the metarhodopsin is the one active ingredient which will be important for activation of certain receptor what we will discuss that is transducin okay so metarhodopsin activates the transducin which is the product in the visual cycle so i I hope this would help you to understand the visual cycle and ultimately when the all trans retinal is formed by the light energy it gets restored or it gets restored back to the 11 cis retinal with the help of two process okay as we will see the reformation of rhodopsin the all trans retinal gets converted into 11 cis retinal inside the rods itself with the help of enzyme isomerase or outside the second pathway which has the role of vitamin A in that. So what is the second pathway? The first pathway is inside the rods itself with the help of isomerase enzyme whichever amount is getting formed here get converted into 11 cis retinal with the help of isomerase enzyme inside the rods itself but outside in the pigmented layer what can happen is the vitamin A the all trans retinal get converted into all trans retinal. retinol. This is a form of vitamin A and this then gets converted into 11 cis retinol with the help of isomerase enzyme and this 11 cis retinol gets converted into 11 cis retinol and that is why the vitamin A, the availability of vitamin A inside the rods and inside the pigmented layer is very important for the fast conversion of the retinol. all trans all trans retinol back to the 11 cis retinol and the deficiency of vitamin a can cause night blindness okay so the deficiency of vitamin a can cause night blindness or it can diminish the vision of which are the diminish the vision in the dark light which is sensed by rhodopsins okay so that is the manifestation of deficiency of vitamin a that is the night blindness. Okay, so we were talking about metarhodopsin and how does it affects in the conversion of light energy into electrical energy. So something has to occur in... conductance of ions so to produce or so to transform the light energy into the electrical energy. So how does that happens in case of rods in case of rods or cones in the eye. So let us see here when the light falls on rhodopsin we have seen in the previous slide that the cycle the visual cycle takes place and the rhodopsin gets split up with the rhodopsin which is made up of 11 cis retinal and the scotopsin protein these get split up because the 11 cis retinal is get got converted into all trans retinal and that all trans retinal does not fit inside the scotopsin protein that is why the this split up and in that process one one intermediate is formed that is called the metarhodopsin 2 so what is the function of metarhodopsin 2 this metarhodopsin 2 active activates the transducin. What is transducin? In that case transducin is just a G protein. Okay. So this rhodopsin is acting as a G protein coupled receptor. All right. Understand here in the cell membrane in the disc whatever you are seeing here is the disc inside the rod and this is the cell membrane. Okay. The disc is made up of the extension of the cell membrane and in that cell membrane is present a G protein couple receptor which is rhodopsin as we have seen the integral protein ok and this rhodopsin is integrated with a G protein that's why it is called as G protein couple receptor because it is integrated with a G protein so that G proteins name is transducin alright so the transducin is then when the light energy falls when the the light falls on rhodopsin it gets converted into metarhodopsin 2 this metarhodopsin 2 activates the transducin and this activated transducin the alpha subunit goes and combines with the cyclic gmp phosphodiesterase okay so it goes and combines with cyclic gmp phosphodiesterase what is the function of this cyclic gmp phosphodiesterase it breaks down the GMP okay cyclic GMP because phosphodiesterase hydrolyses the cyclic GMP there occurs a reduced concentration of the cyclic GMP okay. So cyclic GMP are being broken down by phosphodiesterase and the concentration of cyclic GMP is reduced. Now what is the importance of this concentration of cyclic GMP? This cyclic GMP when it was present it was combining with the sodium channel and when it is combined with the sodium channel it keeps the sodium channel open we will discuss in the next slide what is the importance of the sodium channel but just see for now is when the cyclic gmp is combined with the sodium channel it keeps the channel open and sodium keeps coming inside the cell okay so when the rhodopsins get activated it activates the phosphorous diastase and phosphodiesterase breaks down the cyclic GMP into GMP okay and then the concentration of cyclic GMP decreases resulting into closing of sodium channel and this closing of the sodium channel is the one which will produce the the required change in the membrane potential so to stimulate the next layer of the cells So this is how the excitation of the rods occurs when rhodopsin is activated by light. What happens is the light energy activates transducents. Transducents activates the phosphodiesterase. Phosphodiesterase breaks down the cyclic GMP. This cyclic GMP was keeping the sodium channel open. Since it got broken down, what is happening? The sodium channel is getting closed. So up to now we have to understand. that the sodium channel gets closed when the light is coming. Now let us understand what is the importance of this sodium channel. So in normal condition, this is the condition in the dark when there is no light is coming. This is the condition in the dark. What is happening is as you can see here, the sodium potassium ATPase pumps the sodium outside and gets the potassium inside but the number is 3. 3 positive ions or 3 sodium ions. outside and 2 potassium inside creating the negativity inside ok and at the same time what is happening is there is potassium leaky channels in which the potassium is going outside and there is cyclic GMP gated sodium channels through which the sodium is coming inside. So that is what is happening normally and maintaining the negative membrane potential around minus 40. in case of rods. So this is what is happening here in the outer segment and inner segment and this resultant current flow is maintaining a negative potential of minus 40. Now what is going to happen is when the rhodopsin decomposes it decreases the rods membrane conductance for sodium as we have seen in the previous diagram. So the sodium ions will decrease the influx of the sodium and will decrease so what will happen as the sodium is pumped out and potassium is caught here if this is decreased and still this is open what will happen more and more potassium will keep going outside okay and as a result it will create hyperpolarization inside the cell and that is how this hyperpolarization gives the signal of light to the next layer of cell in most of the cases in most of the receptors depolarization is the one which gives signal to the next cell but in the case of rods and cones it is very important to understand that hyperpolarization is the one which gives signals to the next layer of the cells so hyperpolarization is the one which is giving the signal not the depolarization as in the case of other sensory receptors So how does the hyperpolarization is being formed here? As you can see when this is dark in the case of dark when there is no light is coming to the rods the current flow is maintaining it the membrane potential to minus 40 millivolts. And this is because of the continuous flow of these three channels sodium potassium ATPase sodium leaky channels I mean the potassium leaky. channels and the cyclic GMP get its sodium channels but when the light comes the rhodopsin does action on transducin transducin on phosphodiesterase and phosphodiesterase on cyclic GMP which degrades the cyclic GMP and thus these channel closes once these channel closes what happens only the channels open are these channels and allowing more and more potassium to go outside will reduce the amount of positive ions inside causing causing hyperpolarization. So this is how the hyperpolarization is created inside the rods and which will be giving the signal to the next layer of the cells. So this is again the same diagram showing that in dark the The process is going on the sodium channel is open but when the light occurs the sodium channel gets closed and the positive ions does not come inside resulting in the more negative potential as you can see the negative potential. becomes more or hyperpolarization. So this is a main feature of rods and this is how the rods converts the light stimulus into the electrical stimulus light energy into the electrical signal and this is multiplied to so much time because a very less amount of light even in the dark situation when you are seeing in that case also the the amount of photons which are activating the rods the signal is less but multiplication is occurring because of the multiplication in the signal pathway as we have seen or would have studied before that the g protein couple receptors and amplifies the signal. So that same is occurring here also Z protein couple receptors amplifies the signal and very small amount of light can still produce a large amount of electrical energy and you are still able to see in the dark. So that is how the amplification also is taking place. This is it for this video and in the next video we are going to learn about the color vision and the theory of color vision. in the next video lecture thank you

And this is the receptor or the sensory aspect of the eye. As we considered eye as an optical system in the previous videos, Here this optical inside this optical system resides a sensor and that sensor is retina and this in this retina only the sensory and the neuronal network is found which will then convert the light into electrical activity or the action potential and which will be conveyed to the brain with the help of this optic nerve. Now coming to the layers of retina as you can see there are several layers of retina one thing most important to see in this diagram is the layers are arranged in a certain order so the pigmented layer will be the outermost layer so what do we call outermost layer is the one which is present outside the outermost layer And the inner layer the innermost layer will be the one on which the light falls first. So this is the inner aspect of the or the inner layer of the retina and the outside one which is here. So this is the outermost aspect or the pigmented layer what we call is the outermost layer.

So this layers of retina are arranged in order. This one is the outermost layer and going through consecutively this is the innermost layer. of retina and the light which is coming from the optical system falls on the innermost layer that is the inner limiting membrane and then goes through various layers of cells and then reaches the rods and cones and the outermost pigmented layers so this is how the arrangement is let us see in details what are those layer made up of so when we consider this pigmented layer this was the outermost layer as we discussed before and it is made up of melanin pigment. What is the importance of this pigmented layer is it absorbs the light so that it does not allow its reflection.

If the light is being reflected inside the eye itself the visual equity will be disturbed so much and that is the importance of the pigmented layer. It is made up of melanin. It contains melanin and absorbs most of the light does not allow the light to reflect inside the eye. One more important aspect of pigmented layer is it is having vitamin A so vitamin A is very important in the photochemistry as we will see it is the pigmented layer is having a large amount of vitamin A which will help in the vision so this is the first outermost layer then coming towards inside as you can see the outer the inside to the outermost layer is rods and cones so as you can see we will further see in detail what are the structures of rod and cones but the second layer formed is the rod and cones the layer of rod and cones which is the outer nuclear layer as you can see here the nucleus of the Rods and cones are present in the outer nuclear layer. And then comes the outer plexiform layer.

So plexiform means it is a connection. Connection between the outer cells, the rods and the cones and the inner cells. that is the bipolar cells and the exons in the dendrites are connecting here in the outer plexiform layer.

Then comes the inner nuclear layer. So in this layer lies the nucleus of various inner cells that is amacrine cell, the bipolar cell, the horizontal cells and that is why it is called the inner nuclear layer. Then comes the inner plexiform layer that means it will be having the connection between the inner cells and the ganglionic cells.

So the axons of the inner cells and the dendrites of the ganglionic cells are forming the inner plexiform layer. Then comes the layer of ganglionic cells or the ganglion cell layer and from there it will be coming the inner plexiform layer. From this layer these ganglion gives axon which forms the optic nerve.

And limiting this is the inner limiting membrane and as we see in the previous diagram as we saw it that the direction of the light will be the light will be coming from the optical system to the eye to the retina in the. direction of this from inner to the outer aspect. So as you can see when the light is falling the main receptors of the lights are rods and cones.

So the light which is coming to rods and cones are coming from or passing through these many layers first and then it is coming to the rods and cones. So in this case the visual equity will be less but there is a important aspect important. region which is present in the retina that is called as fovea and there is very important very much importance of fovea is the visual acuity the detailed vision is very high in the foveal region okay so the foveal region when the light falls whichever the image is falling on the foveal region which has a diameter of 0.3 millimeter it has a very detailed picture of it on when the light falls on the foveal region and what is the reason behind that since the light is traveling the layers the inner layers what we have seen previously as you can see in other regions of the retina the inner layers the light will be traveling through the inner layers and then it will be falling on rods and cones but in case of fovea the inner layers are displaced it is an a vascular zone also and the inner layers are also displaced so that the light directly falls on the rods and the cones so in this case especially the cones okay so here in the foveal region as you can see there are much cones and very few rods okay so especially in the cones the light directly falls on them and that is how it increases the detailed image of the light and detailed vision in the fovea so what is the important aspect it is that it is avascular as well as the inner layers of the cells are displaced outwards so that the light directly falls on the photoreceptors and that is why the light which is falling on the fovea has the image of a detailed image of the object okay so that is the importance of foveal region. Now let us begin with the important aspect of our lecture that is the rods and cones so as we can see here this is the general structure of rods and cones and both of them are made up of this similar segments so first look at the outer segment why it is called as outer segment because it is facing towards the outer layer of the retina and it is embedded or near to the pigmented layer pigmented layer which is the outer segment of the retina so this is the outer segment of the rods and and And as you can see here in the outer segment of the rod there are various disc like structures which are just the extension of the cell membrane.

As you can see the extension of the cell membrane forms a disc like structure. In this extension of the cell membrane or in this disc are present the color pigments in the cones or rhodopsins in the rods. So the photosensitive pigments rhodopsin or color pigments. rhodopsin for rods and color pigments for cones are present in these disc-like structures of the outer segment of the rods and cones then coming to the inner segment why it is inner segment because it is present towards the inner side of the retina okay so in this inner segment are present mitochondria another cell organelles this important aspect is mitochondria is very necessary for the chemical reactions of the photochemistry of light so the mitochondria provides the energy the ATP so that the reactions can take place which are occurring in the photochemistry of vision or the photochemistry to convert the light energy into the electrical signals then coming to the nucleus this is a nucleus and from there arises the axon which synapse with the inner cells of the retina so this this is the general structure of rods and cones.

One difference will be between rods and cone is as you can see the rods are in this shape. They are slender, cylindrical in shape whereas the cones are, the outer segment of the cones are conical in shape. The diameter of rods can be from 2 to 5 micrometer whereas the diameter of the cones can be 5 to 8 micrometers. But the cones can have even smaller size because the cones which are present as we have seen in the foveal region, the cones which are present. are of 1.5 micrometer in diameter.

Smaller cones as compared to the cones which are present on the periphery of the retina. The cones which are present on the fovea of the retina are very smaller and those are of 1.5 micrometer in diameter. So these are the general structure as you can see the disc like structures are present here also.

The cut section and in the both rods and cones. In this disc. are present the color pigments and the rhodopsins color pigments in case of cones and rhodopsins in case of rod which are the one which are the photosensitive pigments now coming to rhodopsin as we discuss that rhodopsins are present in the rods and color pigments are present in the cones those are the photosensitive pigments in this video we are going to discuss you the photochemistry of rods and rhodopsin only the color vision and about the color pigments will be dealt in the next video.

So let us begin with the rhodopsin. As you can see these are the extension these discs are the extension of the cell membrane and in this diagram you can see the cell membrane this is the molecular structure of rhodopsin. In case of rhodopsin, the opsin is called as scotopsin.

And the combination of scotopsin and 11-cis-retinal forms the rhodopsin. So what is rhodopsin? It is an integral protein which is present in the disc-like extension of the cell membrane. As you can see, these are the cell membrane and the integral protein. which is combination of scotopsin and 11-cis retinal.

So the combination of scotopsin and 11-cis retinal is called as rhodopsins and these are the integral protein which are present in the cell membrane the extension of the disc-like structure cell membrane. So this is rhodopsin. Now coming to the how rhodopsin is activated by light. and its visual cycle. So let us deal with the visual cycle and let us see how Rhodopsin can then activate or convert the light energy into the electrical energy which will be then converted into action potential and sent to the brain.

So let us start with the visual cycle. So when we see the visual cycle the Rhodopsin will be present on the rods. on the disk like structure cell membrane of the rods because it is an integral protein in that cell membrane.

So whenever the light passes through the optical system of the eye and then falls on the inner layers and passes through and then falls on the rods or cones in that case because the cycle will be same in both the rods and the cones and the activity of rhodopsin and color pigments are same. So we will discuss just the visual cycle of rhodopsin and it will be same for the color pigments also. So when the light falls on rods it converts the rhodopsin into all trans retina.

So when the light falls on rhodopsin it converts the as you remember that the The rhodopsin is made up of two individual structures that is 11-cis retinal and scotopsin. So the scotopsin here and 11-cis retinal forms rhodopsin. When the light passes through the retina, the light passes through the retina.

through the optical system and falls on rhodopsin it gets split up into the all trans retinal and scotopsin why this gets split up because the all trans retinal the the confirmation the what to say the structure of all trans retinal is different as compared to the structure of 11 cis retinal 11 cis retinal perfectly fits on the scotopsin and the combination you is formed which makes the rhodopsin but when the light energy converts the 11th cis retinal into all trans retinal this all trans retinal is the confirmation of all trans retinal does not fits in this codopsin and thus they split up and this all trans retinal is how it is formed by the light energy and these all of them the intermediate byproducts which are forming are the intermediate byproducts or intermediate products of that cycle that means rhodopsin directly does not gets converted into all trans retinal what happens is rhodopsins get converted into bathorhodopsin, lumirhodopsin, metarhodopsin 1 and metarhodopsin 2 and then the all trans retinal splits out from the scotopsin so that is how the cycle occurs once again rhodopsin is present on the cell membrane as we have seen made up of two components that is scotopsin as you can see here scotopsin and 11 cis retinal and they both combine to form rhodopsin whenever the light energy falls on rhodopsin it get converted into bathmore rhodopsin, lumi rhodopsin, metarhodopsin 1, metarhodopsin 2 and then it gets split up in all trans retinal and scotopsin. So basically light energy converts the 11th cis retina. to all trans retinal and why these split up why all trans retinal cannot stay with the scotopsin is because the conformation of the all trans retinal is different from the conformation of 11 cis retinal retinal and does not fit on the spotopsin that is why the splitter and these are the products which are formed in that process in that one thing what we have to consider here is the metarotopsin is the active byproduct which is the one which stimulates the visual cycle which stimulates the transformation of light energy into electrical energy. So in this cycle the metarhodopsin is the one active ingredient which will be important for activation of certain receptor what we will discuss that is transducin okay so metarhodopsin activates the transducin which is the product in the visual cycle so i I hope this would help you to understand the visual cycle and ultimately when the all trans retinal is formed by the light energy it gets restored or it gets restored back to the 11 cis retinal with the help of two process okay as we will see the reformation of rhodopsin the all trans retinal gets converted into 11 cis retinal inside the rods itself with the help of enzyme isomerase or outside the second pathway which has the role of vitamin A in that.

So what is the second pathway? The first pathway is inside the rods itself with the help of isomerase enzyme whichever amount is getting formed here get converted into 11 cis retinal with the help of isomerase enzyme inside the rods itself but outside in the pigmented layer what can happen is the vitamin A the all trans retinal get converted into all trans retinal. retinol. This is a form of vitamin A and this then gets converted into 11 cis retinol with the help of isomerase enzyme and this 11 cis retinol gets converted into 11 cis retinol and that is why the vitamin A, the availability of vitamin A inside the rods and inside the pigmented layer is very important for the fast conversion of the retinol.

all trans all trans retinol back to the 11 cis retinol and the deficiency of vitamin a can cause night blindness okay so the deficiency of vitamin a can cause night blindness or it can diminish the vision of which are the diminish the vision in the dark light which is sensed by rhodopsins okay so that is the manifestation of deficiency of vitamin a that is the night blindness. Okay, so we were talking about metarhodopsin and how does it affects in the conversion of light energy into electrical energy. So something has to occur in... conductance of ions so to produce or so to transform the light energy into the electrical energy. So how does that happens in case of rods in case of rods or cones in the eye.

So let us see here when the light falls on rhodopsin we have seen in the previous slide that the cycle the visual cycle takes place and the rhodopsin gets split up with the rhodopsin which is made up of 11 cis retinal and the scotopsin protein these get split up because the 11 cis retinal is get got converted into all trans retinal and that all trans retinal does not fit inside the scotopsin protein that is why the this split up and in that process one one intermediate is formed that is called the metarhodopsin 2 so what is the function of metarhodopsin 2 this metarhodopsin 2 active activates the transducin. What is transducin? In that case transducin is just a G protein. Okay.

So this rhodopsin is acting as a G protein coupled receptor. All right. Understand here in the cell membrane in the disc whatever you are seeing here is the disc inside the rod and this is the cell membrane. Okay. The disc is made up of the extension of the cell membrane and in that cell membrane is present a G protein couple receptor which is rhodopsin as we have seen the integral protein ok and this rhodopsin is integrated with a G protein that's why it is called as G protein couple receptor because it is integrated with a G protein so that G proteins name is transducin alright so the transducin is then when the light energy falls when the the light falls on rhodopsin it gets converted into metarhodopsin 2 this metarhodopsin 2 activates the transducin and this activated transducin the alpha subunit goes and combines with the cyclic gmp phosphodiesterase okay so it goes and combines with cyclic gmp phosphodiesterase what is the function of this cyclic gmp phosphodiesterase it breaks down the GMP okay cyclic GMP because phosphodiesterase hydrolyses the cyclic GMP there occurs a reduced concentration of the cyclic GMP okay.

So cyclic GMP are being broken down by phosphodiesterase and the concentration of cyclic GMP is reduced. Now what is the importance of this concentration of cyclic GMP? This cyclic GMP when it was present it was combining with the sodium channel and when it is combined with the sodium channel it keeps the sodium channel open we will discuss in the next slide what is the importance of the sodium channel but just see for now is when the cyclic gmp is combined with the sodium channel it keeps the channel open and sodium keeps coming inside the cell okay so when the rhodopsins get activated it activates the phosphorous diastase and phosphodiesterase breaks down the cyclic GMP into GMP okay and then the concentration of cyclic GMP decreases resulting into closing of sodium channel and this closing of the sodium channel is the one which will produce the the required change in the membrane potential so to stimulate the next layer of the cells So this is how the excitation of the rods occurs when rhodopsin is activated by light.

What happens is the light energy activates transducents. Transducents activates the phosphodiesterase. Phosphodiesterase breaks down the cyclic GMP. This cyclic GMP was keeping the sodium channel open. Since it got broken down, what is happening?

The sodium channel is getting closed. So up to now we have to understand. that the sodium channel gets closed when the light is coming. Now let us understand what is the importance of this sodium channel. So in normal condition, this is the condition in the dark when there is no light is coming.

This is the condition in the dark. What is happening is as you can see here, the sodium potassium ATPase pumps the sodium outside and gets the potassium inside but the number is 3. 3 positive ions or 3 sodium ions. outside and 2 potassium inside creating the negativity inside ok and at the same time what is happening is there is potassium leaky channels in which the potassium is going outside and there is cyclic GMP gated sodium channels through which the sodium is coming inside. So that is what is happening normally and maintaining the negative membrane potential around minus 40. in case of rods.

So this is what is happening here in the outer segment and inner segment and this resultant current flow is maintaining a negative potential of minus 40. Now what is going to happen is when the rhodopsin decomposes it decreases the rods membrane conductance for sodium as we have seen in the previous diagram. So the sodium ions will decrease the influx of the sodium and will decrease so what will happen as the sodium is pumped out and potassium is caught here if this is decreased and still this is open what will happen more and more potassium will keep going outside okay and as a result it will create hyperpolarization inside the cell and that is how this hyperpolarization gives the signal of light to the next layer of cell in most of the cases in most of the receptors depolarization is the one which gives signal to the next cell but in the case of rods and cones it is very important to understand that hyperpolarization is the one which gives signals to the next layer of the cells so hyperpolarization is the one which is giving the signal not the depolarization as in the case of other sensory receptors So how does the hyperpolarization is being formed here? As you can see when this is dark in the case of dark when there is no light is coming to the rods the current flow is maintaining it the membrane potential to minus 40 millivolts.

And this is because of the continuous flow of these three channels sodium potassium ATPase sodium leaky channels I mean the potassium leaky. channels and the cyclic GMP get its sodium channels but when the light comes the rhodopsin does action on transducin transducin on phosphodiesterase and phosphodiesterase on cyclic GMP which degrades the cyclic GMP and thus these channel closes once these channel closes what happens only the channels open are these channels and allowing more and more potassium to go outside will reduce the amount of positive ions inside causing causing hyperpolarization. So this is how the hyperpolarization is created inside the rods and which will be giving the signal to the next layer of the cells. So this is again the same diagram showing that in dark the The process is going on the sodium channel is open but when the light occurs the sodium channel gets closed and the positive ions does not come inside resulting in the more negative potential as you can see the negative potential. becomes more or hyperpolarization.

So this is a main feature of rods and this is how the rods converts the light stimulus into the electrical stimulus light energy into the electrical signal and this is multiplied to so much time because a very less amount of light even in the dark situation when you are seeing in that case also the the amount of photons which are activating the rods the signal is less but multiplication is occurring because of the multiplication in the signal pathway as we have seen or would have studied before that the g protein couple receptors and amplifies the signal. So that same is occurring here also Z protein couple receptors amplifies the signal and very small amount of light can still produce a large amount of electrical energy and you are still able to see in the dark. So that is how the amplification also is taking place. This is it for this video and in the next video we are going to learn about the color vision and the theory of color vision.

in the next video lecture thank you

Transcript for:Photochemistry of Vision

Transcript for:
Photochemistry of Vision