 So, welcome back to the lecture series in NPTELON bi electricity. So, in the previous class we talked about that within the nervous system how the electrical impulses are translated into mechanical stimulus and that inter conversion electrical to mechanical to electrical in the process how the stretch reflex circuits are functioning how the calcium dictates the electrical current and how the sarcoplasmic reticulum functions we kind of went in depth and discussed all that. So, now what we will do there are if you remember in the first lecture when I was telling you the outline of it. So, there are two sections one is the initial animal bioelectricity the other section is the prosthesis and they are interlinked with each other once you understand animal bioelectricity then you can understand the prosthesis. So, today we will talk about two special senses two to three special senses and we will talk about the prosthesis. So, talking about the prosthesis the neural prosthesis actually started apart from some deep brain stimulation all these things the real prosthetic devices started coming when we try to replace our sensory systems. So, what exactly I meant by that? So, think of a situation of a person who is blind and the form of blindness which cannot be cured with the known biomedical or biological intervention what to do. So, talking about the visual in the context of the visual system your eyes acquire the image and this image is through the circuit is transmitted in the brain where it gets interpreted and then you understand what you are seeing. So, say for example an apple is an apple for you orange is an orange for you because your eyes had made an image of that and it has a store in the brain that this is an apple. So, whenever you see an apple the electrical impulse the structure is translated into electrical impulses and this electrical impulses travel to the brain and in the brain it is processed and you appreciate this is an apple. So, there are two levels where the problem may arise the first level may be the image plate where you are seeing an apple is not functional that is level one level two is the conduit tube which is carrying the message from the eyes to the brain is not functioning or the third thing is that the area of the brain which processes the visual stimulus or visual map is not functional and based on that we will be looking into this problem. So, if you are so let us let me draw it for you. So, we are into lecture 18 and we will be talking about special senses like eyes, ears and nose and with its different sensing abilities eyes for the visual ears for the hearing and nose for the smell and for them there are special prosthesis which are involved in it. So, let us start with the eyes because that is much more well explored system, but to tell you very interestingly the whole prosthesis business did not start with the eyes actually it got started with the ears I will come just later as I will be moving through. So, let us draw what I have just discussed with you. So, here we have the brain which has different areas to process visual stimulus smell and we are talking about the three sense organs your eyes, ear and nose. So, from here a signal reaches the brain say for example this area we call this as say within rate as visual cortex and from here a response signal comes and you realize what you are seeing. Similarly, for hearing signal goes from here to an area which is called auditory cortex and the processing takes place. So, similarly let me just name it auditory cortex and these are the highways which are carrying the nerve conduit tubes and in this case this is the visual cortex and here you have the eyes, here you have the ears, here you have the nose and there are regions called olfactory cortical region within the brain where your olfaction is being processed. So, now possibilities are there that the damage may either take place out here one possibility out here another possibility or out here either in the eyes ears or nose or the damages are taking place out taking place here, here or here the highlighted regions or the damages are taking place here, here, here, here, here three levels. So, the first level what the eyes are doing? Eyes are essentially nothing but a simple camera which is capturing the image. Your camera has a image plate, eye also has a image plate, your camera has an aperture, your eye also has an aperture, your camera can control the amount of light which you should enter, your eyes can control the amount of light can enter. The only difference is that one is purely, purely mechanical and other is purely, purely biorganic. Your eyes depends on ionic currents, your camera depends on the pixel by pixel and the electronic components. Your eyes depends on excitation of a specific molecule called rhodoxin whereas, your camera depends on plates which could sense light and ejects out electrons. So, if you look at both of them the logic is the same. So, one of the thing comes is there a way by which you can replace a camera in the eye. In other word what we are talking about is could you interface your actual eyes with a camera? Say for example, a person is totally blind. So, essentially what we are talking about you will have a camera in front of your eyes that will be interface with the, coming back to the diagram if you look at it this diagram very carefully. So, here instead of the camera, now instead of the eyes what I am putting in yellow is, there will be a camera and the camera will be interface with this, with this pathways and they will be going to the visual cortex and the signal will be interpreted. Similarly, you instead of, instead of your ears suppose your ears are not functional, you all have seen a mic, the recording devices in tape recorders or any other audio visual devices. So, you replace your ear with a mic, simple mic. Let me, before I go in this let me say. So, your eye what you are trying to do is putting a camera here. Similarly, for a ear you are putting a mic and for your nose you are trying to have some smell device which could smell and this whole area when we try to replace biological systems or not replace exactly interface solid state electronic devices in the biological system falls under neural engineering and the specialized area. So, this whole area is called neural engineering and within neural engineering all these falls under very specific domain for neural prosthesis. In order to appreciate neural prosthesis what we needed to understand is how these different structures are in the biological systems and how we can replace them. So, in order to really appreciate first of all we will be discussing about the structure of the eyes, structure of the ear and structure of the nose and from there we will try to draw a one to one correlation of what kind of currents which are being sent through this conduit nerve tubes to the retina sorry to the brain and how these could be replaced. But remember one thing this is all about this like you know the sensory system what we are talking about to replace. But in case your visual cortex or the auditory cortex or the olfactory cortex are not functioning properly there are damage we essentially do not have much hope there we really cannot change much on that as of now with our known technologies that is a whole deep gamut of a very challenging area I mean how really we represent. So, you have to philosophize this whole thing say for example I see an say for example I see an apple out here. So, this apple for my eyes if I am looking through my eyes like this this apple is forming an image out there and this is being coded in form of an electrical signal basically this is your apple this is your eye and this is electrical coding for apple and this coding electrical coding for an apple and this coding is being decoded by your brain. Now what you can replace you can replace this part which we have discussed this part of course you can replace, but this part we really do not know in what we essentially understand as from the beginning I was trying to highlight is this part this is what we call as neural code the next challenging frontier for mankind. What is the coding? Why we like certain music? Why are you are like some of the symphonies of Beethoven so much? Why we like certain kind of songs? Why there is a sorrow certain kind of music beats revolves and given a larger extent what I wanted to say is say for example I feed this neural code this electrical code for an apple to instead of within a coding it putting it in the visual cortex I feed this signal feed this signal to the auditory cortex that essentially means I am trying to tell you could you hear an apple I may sound very crazy I am asking you to hear an apple because you always hear I want to see an apple I want to see an orange, but you never hear that could I hear an apple this is exactly what is trying to philosophize because of course I will be talking about all the processes, but what you have to understand that all these things back in the mind of as a race as a human race we are thinking much more in depth it is just not that you know I replace a camera in front of the eyes it is not that simple yes I mean it is not a big deal I mean there are people who are doing it for last 100 years, but there is a deep rooted philosophy and which is very essential for young minds like you to understand that this is not just putting on gadget absolutely not that it is far and beyond that that how we really could think could you see an apple fine I can see an apple could you hear an apple now you are bit you know shaky or say for example I have a Beethoven symphony going on now I put that symphony has say for example a trace like this this is the trace of a Beethoven symphony. Now I feed this trace into the visual cortex so this is Beethoven's symphony so now I ask you a question what is the trace question could you see the song you always have here that you know I could hear a song you know this is your ear and your listening to the song now I tell you could you see the song isn't that bit crazy yes it is, but that is exactly lies in the very core of the neural code these are simple electrical signals which are traveling down could we deconvolate them could we use the known techniques from mathematical repertoire to understand what this bits and codes are actually carrying in them then we may be able to answer some of these crazy questions could we hear an apple or could we see the symphony you could hear the symphony and you can see the apple and another knowledge but could you see the symphony and hear the apple is a challenge so with this small background about why these are so very important all these devices and everything what we know and what we cannot do I told you the auditory cortex or any damage in a visual cortex or in the alfactory cortex we cannot do anything we really do not know those extraordinary computations which are taking place in those areas we are absolutely helpless on that front. What we can do we told you we can at least replace at least last hundred years of dream of mankind is taking us to a point where we can replace some of these simple sensory systems with solid straight electronic devices so from here I will move on to explain the structure of the eyes and after structure after that we will move on to the structure of the ears and then we will talk about the different ionic currents which are involved in this process and then we will talk about the prosthetic devices and I will give you the references which I wish you people go through them very carefully because that will help you to know appreciate that what all extraordinary research which are going on all over the world to make our life much more you know much more comfortable especially for those people who are the children of the lesser god you know they have lost their vision or you know lost their hearing capabilities or lost their smelling capabilities so on and so forth. So let us talk about the structure of the eyes so talk about the structure of the eyes so first of all I will draw the gross structure of the eyes which most of you have seen any high school or middle school textbook and from there we will talk about the different forms of blindness and then we will talk about the exact image plate which is involved in it and from then from there we will talk about the different ionic currents which are involved in it and what could be done fine so let us draw the complete structure of the eyes so this is pretty much is there is something called lens here so this is from where the light is coming so the outermost layer which is called cornea this is I am just drawing a very simplistic picture if those of you who loves complex picture please go online you can find several very complex and very nice well colored pictures then this is the lens which is a transparent piece of tissue through which the light passes without any problem then you have out here you have iris you have aqueous humor the fluid which is filling this aqueous humor then you have very thin tissue here give me once again let me so this is highlighted red part what is coming is this is what we call in the back of the eyes is called the retina and this zone is filled with something called vitreous gel the back of the eyes is called sclera the l e r a sclera and out here you have like this so this is the conduit tube which is carrying this is the optic nerves here optic nerves the nerves which are carrying the message to the brain and there is a specific spot here somewhere out here which is called phobia centralis we will talk a letter about it then you have something called papilla here so this is the overall structure of the eyes slightly more spherical but I will just kind of drawing it if you see it so if you see from the front so this is the path of the light so light moves travels like this if you see the yellow arrow what I was drawing to draw so this is all the regions where this is the red region where the amount of light which will pass through is controlled the region what I have circled with the green highlighter and image processing takes place out here this region what we will do now first of all we will talk about the different kind of blindness could be described at the level of eyes could be described at two levels so the first level is so let me draw it that will make things clear level one is any blindness which is happening in this side level two any blindness happening in this side what does that mean this side and that side so this is the side where light is passing through the lens light is passing through the cornea you follow the image light is passing through cornea so there is a damage in the cornea that could lead to blind once again damage to cornea leading to blindness damage to lens leading to blindness so in case of cornea the option is that replacement of the cornea by the donation of the eyes somebody donates the eyes because cornea is a non vascularized tissue that means there is hardly any blood flow in that tissue so any corneal replacement would not lead to any kind of immune reaction and that is what essentially means when people say they are donating their eyes after death essentially it is the cornea which is being removed that is it so it is a cornea which is being replaced in place of the existing cornea of a person whose cornea is non functional that is the level one second one is the lens tissue which consists of specialized lens cells which are arranged in a spherical manner that is the reason why I drew it in a spherical concentric circles these cells during the process of development loses all their organelles and they become transparent and that whole process is being regulated by a specialized enzyme which execute that process when they lose their organelles 15 lipoxygenase so these cells have a specific protein called crystalline just write it down those who forget so there is special proteins within those cells called crystalline and these crystallines are of course these cells are arranged in a specific way and the crystalline proteins are stacked around in a particular way but during aging some of these crystalline protein starts misfolding individual proteins the way that three dimensional geometry is organized there is kind of gets disoriented and what essentially that leads to is this the situation when the light passing kind of becomes like this light could not pass properly it may reach but it will reach with staggered because the crystalline proteins are not arranged within the cells they are now misfolded that is the situation what we essentially called cataract all must have heard this several people in this country so a huge population in this country actually suffers from cataract which is the major from a form of blindness in India so for cataract the other or the option is that say for example your cornea is all fine so for cataract your option is that you replace that transparent tissue with an artificial lens or with an artificial lens or with an artificial glass okay which is transparent that is the only therapy what we have that that is what essentially means a cataract operation is all about when you replace it with an artificial this is the second this is within the within the eye this is the second level of blindness now comes on this side of the picture that is getting back to this side of the picture where we are let me pick up another color so this is the side which is the very critical one what form of blindness are we talking about here this is the blindness where this image plate or the retina is not functioning properly it means the cellular structure of the retina is compromised. There are damages or there are misformation or their arrangement is not right or something x y z this is the within the eye this is the second level of blindness if I of course categorized it just the way in the diagram I did it on coming back to the diagram on this side this is one form of blindness which has two level either it will be cornea or it will be the lens and this side of it either it will be at this or it will be on the optic nerves if there is a damage optic nerve your retina is all functional all fine but you have damage optic nerve and just for slightly digressing back a little bit where I say for one minute so that is where you see people put on specs because when this lens is not forming the image in the right place either it may form out here or it may form beyond it there are lens which are being put in front of the eyes which essentially ensures that your image actually forms on the image plate which is the retina. With this brief introduction what I will do I will move on to explain the cellular architecture of the eyes that is very essential to understand because once you understand the cellular architecture of the eyes we will be able to appreciate that what is the basic difference fundamental difference between image plate of the camera and the image plate of the eyes there are different biochemical events which leads to bioelectrical events we will be talking about the biochemical events we will be talking about the bioelectrical events and then we will move on to the what are the replacement which could be done in this. So, coming back to the structure of the retina so the structure of the retina is of very I should say it is complex yet it is very very well organized structure of six layers of cells the layers are rods and cone layer which is called the photo receptor layer photo receptor I will draw everything. So, do not worry photo means light receptor the once which receives light this is the most critical layer this is the layer which receives the light from x y z sources this is the layer which distinguishes colors the rods are for light dim light and cones are for colors it is very easy to remember C cones for colors and we follow a RGB red green so we have three different kind of cones red green blue RGB there are fissures which has another kind of cones additional kind of cone they have four different colors they can handle there may be other species which may have other different forms of we have to just understand that. So, we have the color distinction which is done by the cone cells we have the rods which works in the dim light. So, say for example, a species of fish which is living deep down into the ocean you will see that these fishes do not have any cones might wonder why it is very straight forward light never penetrates into the into the sea floor light hardly enters few feet after that light does not go. So, those fishes functions only with rods because they adapted themselves. So, again the reason why I am giving you people this kind of example is this biological cameras are very well adapted to the surrounding environment depending on the needs and requirements their needs and requirement is being modulated. So, if you do not need colors colors interpretation of colors they do not have colors they do not care they function with you know only with the dim light. So, you have the rods and cones which is the called the photoreceptor layer just beyond this photoreceptor layer is sitting on top of epithelial cell layer which is called retinal pigment epithelial cell R P E in short it is written retinal pigment epithelial cells on top of retinal pigment epithelial cell you have these rods and cones which are sitting on top of rods and cones there are layer of three different layers bipolar cells amacrine cells horizontal cell ganglionic cells bipolar cells amacrine cells horizontal cells ganglionic cells they all converge into optic nerves what I have shown you in the picture. So, now these different layers have different kind of functions and moreover they all integrate all different kind of signal the depth of the image the movement of the image say for example, the amacrine cells amacrine cells say for example, you are standing in a railway station you are standing in a railway station and there is a train which is approaching. So, you are seeing the train coming towards you. So, within the eyes you have the amacrine cells which could actually sense a moving object coming towards you. So, there is a continuous real time computational computation involved because this is like a video camera a video camera which is seeing the whole thing. So, it is adjusting it is adjusting exactly the same way or somebody is dog is running a coming towards you rushing towards you and you have to run away. So, you are continuously adjusting because the image is moving. So, my I am moving my my and is moving towards my eyes. So, you know or I pick this up is moving. So, I just move my head back. So, essentially that means my eyes are continuously adjusting just like a video camera you can see all the motion pictures. So, I am picturizing the motion. So, these amacrine cells they play a very critical role you have the horizontal cells bipolar cells ganglionic cells. And what is good about this what I told you though it is a very complex structure, but it is very well structured in the sense. If you take a section of the retina you can physically see of course, under the high resolution microscopes you can physically see the different layers as if they are stacked over one another like this very nicely stacked over one another. And among all the layers the most the first layer which is sensing are the only ones which has photoreceptors in them or in other word that is the layer which could translate light signal into electrical signal. All other layers are devoid of that they do not have any photoreceptors. So, even if the light passes through them it hardly matters for them you know. And arrangement is very interesting this is something people have to understand let me go back to that picture. The arrangement is like this the photoreceptors let me this is very clumsy other if I draw it on this. If this is the retina which I had drawn in red in the other one. So, within this the weight is arranged is. So, this is the back side. So, this is the black bordering what I am doing is the retinal pigment epithelial layer. And I just for those who this is the optic nerves which is going out. And on top of the retinal pigment epithelial layer you have this layer the second layer in the pink color what I am drawing now this is the rods and cone layer. And mind it light is entering like this light is entering like this on top of that you have the you have the horizontal cell layer which is this is the horizontal cell layer. Then comes the amacrine cell layer which I am drawing in the broken lines just for your understanding. After the amacrine cell layers you have in between you have the bipolar cells and the ganglionic cells you have the their interspersed out here bipolar and ganglionic cell. So, what I wanted to highlight here is and then all of this eventually moves to the. So, this is the optic nerve and this is the retinal pigment epithelial layer this is the P R layer or photoreceptor layer here you have the horizontal cell H C layer. Then you have the bipolar cells you can call this as the bipolar bipolar cell layer and then you have the amacrine cell layer very deep inside you have the ganglionic cell layer ganglionic cell layer fine. Let me just cross check bipolar cell cell. So, these six layers that might wonder why most of these other cells are in the front. The reason probably being that this darker layer which is in the on the back the on the on which the R P on which the rods and cones are setting is the one this retinal pigment epithelial layer is the one which is supporting nutrients to the rods and cones layer. So, if the structure is reverse the retinal pigment epithelial is opaque layer it does not allow the light to move through. So, if it is in the front the light would not pass through. So, that is why probably probably I mean this is all speculations what we have probably nature is designed in such a way the retinal pigment epithelial layer is in the back which is opaque does not allow light to pass on top of that you have the retinal pigment epithelial cell which have light sensitive pigments and on top of that you are stacked all the different layers and light is coming from the front. So, it is the back. So, what we will do now is after this we will talk about the cellular architecture of these individual layers how they are connectivity and then we will pick up most importantly the rods and cones layer and we will talk about the electrical signals which are generated out there. So, I will close in here and we will review with the different architecture and the cellular architecture and the electrical signals and how these are being transmitted and then of course we will want to prosthesis part of it because it is extremely essential that you will understand the anatomy and the physiology the electrophysiology of it before you understand or appreciate the real challenge of the prosthesis. Thanks a lot.