 Welcome back to the lecture series in bioelectricity. So, this is lecture number 19. So, in the previous lecture we talked about the basic structure of the retina and the eye and there we talked about the complex layer of different cell types in the retina which ensures that the image which is formed on the retinal plate is conveyed correctly to the brain and the decoding takes place. So, our eyes human eyes can distinguish light wavelength from the range of 400 to 700 nanometers. So, if you recollect your previous lecture where I have shown on the retina on the bed of the retinal pigment epithelial cells you have these rods and the cone cells. So, the rods are the ones which could distinguish the different intensities of light whether it is in during the dark conditions or during heavy light conditions likewise and the cone cells are the ones which could distinguish the different colors. So, essentially cones have cones are of different types it could be a red cone, it could be a blue cone, it could be a green cone depending on the specific ability of that cone cell to distinguish a specific color type. So, today what we will do we will be talking about the electrical activities of these rods and the cones under one broad heading the photoreceptor electrical activity of the photoreceptors and then we will be talking about the current stage of the prosthesis where people have lost their complete vision or partial vision. So, essentially there are two kinds of blindness as I have previously described either the blindness could be at the level of the lens where the there is a damage on the lens and the lens has to be replaced by an artificial lens. Those are curable blindness, but there is another level of blindness which arises at the level of the photoreceptors where the photoreceptors gets damaged either because of old age or because of some both defects or because of some injury or some other path of physiological situations. So, let us first of all discuss the structure of the rods and cones and then we will talk about the electrical activity in terms of the dark currents which are generated and why these are called dark currents and then we will briefly talk about some specific aspects of the cones and then we will talk about the prosthesis. So, start off with so we are into lecture 19. So, basically we will be dealing with the photoreceptors and within the photoreceptors we have the rods and the cones and their electrical activities in the concept of the dark currents and then followed by that we will be moving on to the prosthesis. So, moving on to the next slide. So, let us talk about the structure of the rods and the cones. So, these are very unusual kind of a structure. These are almost like you know fairly elongated structure like that and if you could see a cone pretty much a cone looks like this. So, this is the part say for example, if you look at this pen. So, you could see that there is this kind of you know light black color or ash color body and there is underneath there is a lighter color. So, this is the part essentially where all the light receptive pigments are present and this is the cell body. So, if I draw it it looks something like this. So, for example, cones because of their whole structure they are more like a structure which similar to this and this is the classic structure of a cone like cell and within this structure you have these what you see this cap like structure what I have just now drawn for you. So, this is where all the light sensation or different wavelength distinction takes place. Whereas, if you look at the rod they are more like more almost like a rod only here you have. So, this is where. So, they are called cone they are for the color perception. So, these are the cones here for the colors and these are the rods for different light intensity. So, this is where they have the nucleus and the way it works is this is where the light falls. This is the one the cone is the one which could distinguish between you know the green it could be a green cone it could be a blue cone or it could be a red cone almost like you know R G V coding. So, the different wavelengths of these this could distinguish. So, these are the different wavelengths of the specific cones are of a specific type they could be excited. So, that they could emit they could they could absorb light at the blue wavelength or they could absorb light at the red or they could absorb the green wavelength. So, for example, this kind of color vision is only functional in high intensity light because if you walk in the dark you realize that you cannot distinguish color what you essentially see a shade of black and white. So, that means the cones are only active when you are in a bright light and where you could distinguish the different color red green likewise and there are people who suffer from color blindness where they could not distinguish there is a different kind of color. So, that means such individual suffers from certain genetic disorders where they are color distinction the specific type of cones which could distinguish color does not develop properly or mal-deformed or mal-formed. So, there is another kind of blindness which is called a night blindness which is essentially as it becomes darker your unable to really distinguish objects properly that essentially happens because of the rods and it is not a genetic disorder it is mostly because of the malnutrition of lack of vitamin A in the food and will come to that where it exactly happens in the case of rods if you look at this structure. So, out here any kind of like you know lights which are falling here and they respond to the light. So, essentially what happened when the light falls. So, these light or these photons binds to specific sites on the membrane and once they bind to the membrane this light energy is translated into an electrical impulse this is what essentially happens. So, here you have the light signal falling on photoreceptor which includes rods and cones depending on the situation it could distinguish the color component and if it is dark it is not going to do. So, and the intensity and these are coded as electrical signals electrical signals and these electrical signals are essentially sent to the brain for further processing and in between the rods and the cones are the whole network of amacrine cells horizontal cells which ensures that the exact depth of the object what we are looking at the exact the movability and all these features are being added to this. So, what the brain receives is a very complex electrical signal which it has to you know distinguish in terms of colors intensity depth frame and all those things. So, coming back again coming back to the structure. So, how this electrical signals are being received. So, it is very interesting that on the photoreceptor we there is one term which I am going to introduce now is called dark currents. So, what does dark current means. So, there is a common way we look at it. So, we believe when there is a intense some signal is coming our sensory cells respond to that and generate action potentials and this action potential eventually is conveyed to the brain and we decipher the object or what is over we are looking or we decipher the information fine. In the case of rods and cones this thing is just reversed something like that when the light falls on the photoreceptor layer they do not generate any signals and when the light does not fall they generate signals it is just the reverse it means when the light is not falling suppose my eyes are closed. So, it is actually conveying the signal. So, essentially what happens suppose you have a matrix like this this is how to conceptualize in your brain then only will be able to understand. So, for example, this is the matrix of say you know say for example, these are different matrix of say rods and cones which are setting there I am just putting different colors for your understanding. So, imagine this is a photoreceptor layer and these individual circles are different photoreceptors. So, say for example, when the light is falling here like this and this is the wave of light which is falling. So, when the light is falling differentially at some point this will one second at one point this one will get shut off then is the next point this one will get shut off and this one will get shut off this one will get shut off this one will get shut off, but this all shutting off is taking place if this is taking place at t 1 this will take place at t 2 this will take place at 3 3 t 4 t 5 likewise you know I am just randomly putting all the different numbers at different t stands for the different time. So, essentially what is happening you are getting a matrix over a period of time where initially this one switched off there is one frame you are getting where one say for example, think of a flood light one light goes off there is one image next light goes off you get next three light goes off you have another image next four light goes off you have another set of image essentially what is happening you are recording this frame by frame and eventually the whole image is formed in front of you or say for example, in a city you are looking from the top all of a sudden I you know switched off all the lights in such a way that you make a figure. So, I just selectively put certain lights on and switch off all the lights and then I change the figure I again switched off certain lights and gets a figure likewise I can really frame by frame I can actually convey the message this is exactly how some of these the retina actually this is kind of an analogy how it works. So, for example, at the frame one when if you talk about the frame let us let us try to you know see how the let us draw six frames what is happening. So, this is say for example, time t 1 this is time t 2 this is time t 3 this is say t 4 this is t 5 this is t 6 and this is let us assume that this is our. So, in the first when the first shot of light comes this one this one this one goes off the next frame. So, for example, this one sorry this one this one and this one goes off. So, now between these two you will see initially you saw a image like this what I am circling something like this with a contrast here there is a contrast here then you see an image like this with a contrast here you are seeing you are kind of seeing a shape now likewise frame by frame and this is all happening within a frame of you know nano or femtosecond you have lot of frames and these frames are the ones which eventually form the image this is very essential for you to understand before I go to the individual at the cellular level. So, it is a frame by frame images which are formed because of shutting off of say one photoreceptor or some population of photoreceptors the other photoreceptor layers are active then another set of photoreceptors is active the other set of photoreceptor goes off likewise. So, whenever you have to imagine think of a flood light in a in a velodrome or in a stadium where if one light goes off you have one frame then few lights goes off you have another frame then few lights goes off another frame or you can like think of it somebody is showing you the sky and slowly one by one you know showing you the star follow the stars it is almost like that one switch off switch on switch off switch on likewise something of that or suppose you are seeing a city from the top all the lights are on then somebody selectively switches off a light and make a figure then switches of the light makes a figure switches of the light make a figure and then that whole image will come exactly that is how the retina functions. Now, coming at the cellular level what is happening? So, I can I can use this. So, what we can do what I will be doing actually now I will give you the membrane structure here now let us see the structure of the membrane. So, we will be talking mostly about the rods at this time rods I mean the signal processing is fairly the same. So, here the here the rods. So, if you really have blow up this image it is almost like this this is the membrane structure which remember in previous one of the previous lectures we have talk about the membrane this is the lipid bilayer and on the lipid bilayer you have something like a protein sitting there which is called the rhodopsin protein the rhodopsin is a huge molecule rhodopsin which is essentially made up of two one is called opsin the other one is called retinal and the retinal fragment is sitting somewhere I will just put it in blue now somewhere here the retinal fragment is sitting. So, this is how it looks like. So, there is an opsin moiety and there is this retinal molecule. So, this is the key molecule which ensures or this is the one which receives the light. So, it receives the light. So, this has a light this is a light sensitive protein it is this molecule which ensures it has variants of course which ensures that at different intensities how it will function with different wavelength how it is going to function. So, this key molecule now we will be talking about what exactly happens with this molecule in the next slide the next slide what I will do I will talk about. So, let us again get back to the structure of let us take the example of a rod now. So, here you have all those rhodopsin molecules which are sitting here and here you have the nucleus and now say for example, you put an electrode inside the cell and you put another electrode of course the outside the cell like this and you have volt ammeter sitting here what will you see. So, initially what will you see when there is no light falling in this. So, at that time your volt ammeter reading will tell you round minus 40 millivolt. This is a situation when once again there is no light let me put that there is no light and at that point there is a flux of the sodium which is getting in as the sodium is getting in it is ensuring all the sodium ions are ensuring that this electrical impulses are being transmitted. So, this is the kind of the axon axonal end of a rod or a cone cells. So, because when the light is falling a light is not falling sorry. So, market light is not falling there is profuse entry of the sodium ions and this ensures that it generates axon potentials which are transmitted to the next layer what you could see. So, at this stage the electrical signal is travelling like this. Now when the light falls. So, this is basically what I termed as what I was trying to discuss with you is the dark current there is no light yet it is generating axon potentials. Now next let us go to the next situation the next situation what is happening here you have again the rod cells. Now again the same you have a electrode inside you have the volt ammeter sitting here and you have the other end of the volt ammeter. Now light is falling on this when light is falling on this what you see the membrane potential goes to minus 70 millivolt. Now what is happening here when minus 70 millivolt is here it means it is no more sending any signal out here just reverse to the situation where during no light it was sending signal and you see out light. So, basically the signals are being there is no signal what exactly happened this is a very very interesting phenomenon. So, let us dissect the problem at different level first of all when the light was falling sodium channel was not falling sorry excuse me when the light was not falling sodium channels are open when the light started falling sodium channels were closed. So, what is regulating the sodium channels first question and how that regulation is modulated by light because when the light falls sodium current stops. So, it means light and the gating of sodium channel is linked. So, these are the two things which now you will be discussing light and gating of sodium channels are linked and now what we will do we will establish that linkage what essentially is happening. So, there are two things which are happening out there. So, the first thing what happens is now let us go back to the membrane structure what is happening remember. So, here is the rhodopsin molecule and here lipid bilayer rhodopsin molecule is sitting out there with its opsin moiety and the retinal moiety. So, I told you said somewhere here is the retinal fragment is sitting and this is the protein which is with all its complex structure out there which is shown in green. Now, when the light falls. So, this moiety what you see here this retinal moiety this remain as 11 cis retinal with an aldehyde group this is in this stage and of course, you have the once again opsin moiety and all together it is a rhodopsin. So, now the light is falling at this stage there is no light. Now, here the yellow thing what you are showing showing my h nu is the when the light is falling when the light is falling what happens this 11 cis retinal what you see here transformed into 11 trans retinal. So, the cis bond becomes a trans bond when the light falls on it. So, now what you see there is a change in the molecular architecture of that retinal molecule what are the consequences of that change this is the first change. So, within the rhodopsin. So, rhodopsin is having this opsin and the retinal moiety attached on it it is a complex protein when the light is not falling on it the retinal moiety remain in 11 cis retinal form when the light falls on it 11 cis retinal become 11 trans retinal, but then what happens next let us move on to the next slide once again save it. So, the next thing what happens is this 11 cis retinal is formed. So, let us go back. So, here the photon is falling on here is the moiety sitting there. So, photon has formed now you have 11 trans retinal this 11 trans retinal then dissociates from this structure. So, what you are left with is now you have this opsin protein coming out plus you have 11 cis sorry 11 let me just rub it off and you have this 11 trans retinal now this opsin protein now remain it is pretty much orphan at this stage and whereas, 11 trans retinal through an enzymatic action again become cis ok. So, there is an complete enzymatic action where ATP is being used to convert it into 11 trans retis cis retinal now this cis form again assemble with the orphan protein or the orphan component which is the opsin and again form this structure. So, this is basically what is being called is a bleaching phenomena there is a kind of continuous bleaching taking place within the eye ok, but during that process something else has happened that is what we are going to discuss how that channel remains open. So, what essentially happens in a step 1 we come back to step 1 step 1 was opsin activation because when the light falls I told you the opsin becomes opsin comes out and the 11 trans retinal dissociates then what the opsin does the step 2 opsin does something very interesting opsin activates and second enzyme called transducin and this transducin which in turn activates another enzyme called phosphodiesterase and this phosphodiesterase then does something very interesting what it does is now just before that let me talk about little bit about the transducin what is exactly just giving one minute transducin. So, transducin is basically a G protein is a series of proteins G protein it is a it is a membrane bound enzyme and it activates by activates by interaction with receptor proteins bound in the cell on in the cell membrane in this case what transducin does transducin is of course activated by opsin and transducin in turn activates your phosphodiesterase what I have talked about and phosphodiesterase what essentially does is very interesting this phosphodiesterase disintegrates a molecule called cyclic GMP then where what is the role of cyclic GMP. So, coming back to the slide now you have phosphodiesterase I am showing by PDE this PDE then acts on cyclic GMP. So, what is the role of cyclic GMP in this whole context of things what I was trying to tell you. So, let us come back to the first question here out here what is the question what we ask once again let me go back to that slide where I talked about the dark current introduce it to the dark current yes when there is no light I showed you this picture when there is no light you could see the sodium channels are open up to this concept is clear then what essentially happens when the light falls sodium channels become closed. So, the fundamental question comes here we ask two fundamental question one was this one light and gating of sodium channels are linked we will take it up from here in this light two situation no light sodium channel or sodium current sodium channel closed no light sodium channel open fine. Now, the next challenging question was what keeps these sodium channel open at this stage as if a door is kept open. So, how you keep your door open if there are three ways how you can keep a door open think of the room wherever you are sitting and listening to this lecture you can keep the door open by using like something underneath the door stopper or you have something by which you keep the door like this. So, these molecules have something like a door stopper molecule and the door stopper molecule or door opeder molecule of sodium channel is cyclic g m p. In other word what is essentially is happening if say for example, this is let us take an analogy that you know this is the door of a room this is the wall and this is the door this is the wall and this is the door which is locked. So, this there is cyclic g m p if this is this is say for example, the sodium channel and this is in a closed state or this is the door. Now, this could be kept open like this by holding it with something like this or to hold it it could be a door stopper or anything in the case of channel here you have this cyclic g m p setting. So, cyclic g m p acts as a door stopper keeps the door open. So, now the door will only close when you remove that door stopper that door stopper cyclic g m p is being removed by the molecule called phosphodiesterase which is activated by transducin. Transducin is further activated by opsin and opsin is disintegrated in the presence of light does it make sense. So, when the light falls. So, let us summarize what exactly happened now coming back to the next slide. So, light falls on the retina what you essentially see no current no sodium flux. So, it means sodium channels are closed. So, what is happening at the molecular level during this whole thing light falls 11 says a retinal become 11 trans retinal on the rhodopsin molecule. 11 trans retinal dissociates out and this process dissociates the opsin molecule. This opsin molecule then activates a protein called transducin. This transducin further activates a molecule called a phosphodiesterase. Phosphodiesterase then dissolve the cyclic g m p which was keeping the sodium channel in open state. This is exactly what is happening in this whole process of signal transduction within the rods and the codes where when the light falls the electrical signal seizes when the light is not falling the electrical signal passes on. That is why this is called a dark current which is also called cyclic g m p regulated current. Depending on the wavelength of the light in the case of cones these electrical currents are being stimulated or you know stopped. So, this is a very fundamental concept which I wish or wanted that you people it is should be clear to you. It is this layer which is the most critical one. So, say for example, now some way or other if this layer. So, let us go back and see the global picture out here. So, out here just give me one minute. So, here you have the brain spinal cord here spinal cord here you have the eyes. So, the electrical signal moves like this left to right right to left. Now, at this level the first level where the image processing takes place is the PR or the photoreceptor layer which is essentially the rods and the cones. So, the electrical signals are traveling from the eyes all the way to the brain and the processing takes place. Now, imagine some way or other rods and the cones layer goes back what will happen. So, now there would not be any further image processing taking place. You will see it, but there would not be any further message going on. This is all blocked. What intervention can happen? One option is that if you could replace these eyes with a camera or with two cameras on two sides. So, you or you have a goggles like this you know something like this where you have the cameras sitting out here and this goggle is connected interface directly to the brain. So, camera instead of this now they are directly cross talking with the brain. This is the approach which inspire people for last one century that could we really bypass the whole vision by putting cameras instead of ISO. So, basically what is happening this circuit is no more functional. This is a defunct circuit out here. So, what you are doing you are interfacing the camera with the brain. So, images which are forming analog images are digitized and the digitize signal in the form of electrodes are being fed to the brain. So, in this context I will recommend you to read some of the papers and of course at the end of the course I will be providing all the references which I wish. So, one of the person who had made really good contribution is from university of southern California should read is Mark Humayun spelling may be wrong sorry Mark Humayun. This paper from university of southern California USC. He has made some very interesting cameras where he has interface them with the brain and this is really a very interesting piece of work which is going on. So, where the prosthesis has kind of a success of process, but there is lot of improvement which has to be done. So, what I will be essentially doing I will be giving you these references of Mark Humayun and I expect you to read through them because that is where lies most of the future of prosthesis. So, again to tell you what is the major challenge is this game in this game the major challenge how we interface the electrodes with the brain because you are putting some foreign material in the form of electrode. So, that has to be biocompatible not only it has to be biocompatible it has to be ensured that all the signal over a period of time continuously reaches because you cannot continuously open the system and you know fix the electrodes it has to be done. So, say for example, here essentially what we are talking about if this is the individual and if this is the brain. So, that is what you are saying is essentially this person has this goggles which they are putting. So, here is the retina which is receiving the light and this goggle which is a synthetic retina is essentially interface with the visual cortex out here in the brain. So, there is enormous amount of electrode implantation which has to be done to ensure all the message reaches the brain at the visual cortex and then of course, this person will be able to you know visualize if this person is unable to see, but this needs not only I mean of course, the camera technology is very well advanced, but the interface technology which is the whole area of neural engineering or you know you can talk in a much more specific way at the cell electrode interface or neuron electrode interface. This is in terms of the cell here this is the very very challenging area where lot of research is going on for last one century that how really we can develop extremely high biocompatible material which will convey the signal and help in this processes business. So, I will close in here with this and in the next class we will talk about the year and the year processes and so on and so forth we will finish this whole processes part with the basic structure. So, they are also following the same strategy. So, and I will be adding all these references for you and please kindly go through some of the work of Mark Hume and that is very inspiring pieces of work which is happening across the globe. Thank you.