 Welcome back to the NPTEL lecture series on bioelectricity. So, we are in module 2 and the module 2 we have done our first class which is on bioelectric potentials. So, from here we will move on to the next two topics and if you go back and just please refer to the screen. So, if you look back, so these are the two topics what we are going to deal with now, ion channels and action potentials. Let me just put it here, lecture 5, this is module 2, module 2 this is some total this is the fifth lecture. So, these are the things essentially we are going to deal in this class. We will start with the basic structure of the membrane which we have not done yet. From the basic structure of the membrane we will move on to the different transport phenomena taking place across the membrane. After that we will discuss about the Nernst equation and from the Nernst equation I will give you a little bit structural, generic structural detail of the ion channels and we will talk about the different kind of ion channels. From there we will move on to the action potential in that process while talking about the ion channels. Initially I will just give you a generic introduction of the ion channel and then as we will progress we will talk about more and more about the different kind of ion channel their subtype, the one which are voltage gated or which are open by the voltage difference, the one which are chemically gated, the one which are open after binding of a ligand. Likewise there are series then mechanosensitive ion channels which are sense the mechanical change and mechanical stimulation or the ones which are light sensitive ion channels. So, likewise so on and so forth there are a whole bunch of them and we will discuss one by one in different context as we will be talking about vision we will talk about light activated one as we will talk about hearing we will talk about the mechanoreceptive one as we will talk about different sensory circuit there will be mechanosensitive ion channels we will be discussing. So, let us start with the basic structure of the membrane very basic. I am not going to get into any in depth detail but just to give you an idea how ion channels are placed in those membranes and how the electrical properties of individual ion channels are being studied. So, as we will progress in the course we will talk about it. So, here is the section. So, today in the today's lecture we are dealing with as mentioned the basic to start off with basic structure of the membrane and this is the membrane we are talking about we are only taking into account in the animal kingdom. So, we are not talking about plant cell or any other cell and the generic structure. So, if you look at the basic structure. So, if this is yourself this is any kind of cell it could be a neuron it could be a muscle whatsoever. So, if I take a cross section of this and blow it up what I will essentially. So, this is basically your cell membrane or plasma membrane sometime it is called plasma membrane also. So, we go to the next page the essentially what you will see the membrane consist of bilipates likewise. This is how the membrane looks like and please refer to any standard text book as I was mentioning in the last class that you can refer to Stryer, you can refer to Leninger or any other standard physiology text book. You will get really very nice description about different kind of lipids which are constituting the membrane and here I will just draw for your understanding sake. So, my objective here is not to have an extensive discussion about the membrane structure on anything, but only the electrical phenomena. So, this is how it this is very tightly packed though the way I am drawing it there are gaps you are seeing. So, here you will see here there are proteins which are in the different in the yellow color or in the like this and there are proteins which are setting single handedly like this. There are proteins which are setting out here yet there are proteins which are setting like this. Again there are certain integral proteins which are moving through the membrane like this and apart from it you have whole bunch of carbohydrate molecules which are setting decorating the surface like this and let me let me label it that will make more sense to you people. So, coming back. So, this is the outside the cell this is the lipid bilayer out here the lipid bilayer and this is inside the cell as I was drawing you previous if you look. So, this is inside or this is also called intracellular and this is outside which is called extracellular and here since I am showing it like that I am let me introduce. If I electrode is placed here like this this is called intracellular electrode and if an electrode is placed outside the cell like this this is called extracellular electrode just in case come across because we will be coming across this pretty frequently. So, I just wanted to highlight that coming back to the structure of the membrane. So, this is inside outside the cell and this is inside the cell and these are the hydrophobic tails or non-polar tails they are also called non-polar tails of the lipids. And these are polar head groups of the lipids polar head groups of the lipids these are all the different integral proteins spanning the membrane these one these one these one these are all integral proteins spanning the membrane. And yet there are certain proteins which does not span the membrane and those are called peripheral proteins. The red ones what you see are the mostly carbohydrate your glycoprotein or someone some of them could be even glycolipid and in this membrane within this membrane you have there are whole bunch of colostrols which are embedded all over the place in yellow the way which enhances the fluidity of the membrane. In other words the fluidity what I meant by fluidity here. So, once again let me come back these are colostrol. So, what I essentially meant by the fluidity of the membrane means those colostrols allows the membrane to be much more you know flexible and bending and all those kind of things are being provided by the presence of colostrol at apart from it this kind of this is the membrane which basically this is semi permeable in nature as I was mentioning in one of the previous classes semi permeable it is selective about allowing things to pass through it selective to allow electrolytes to pass electrolytes or other chemicals to pass and essentially these kind of membranes are regulated by a whole series of if you see this structure these embedded among these are the different integral proteins which forms the ion channels and these are also these membranes the model which is being currently used for this is called fluid mosaic model. This is the very very basic basic very basic fundamental which you people needed to understand in order to understand the membrane. So, my request will be please go through any standard textbook or go online you will see this structure of the membrane even given in three dimension much more beautifully, but this much basic is essential for me to kind of communicate with you further about you know what all the processes which are taking place. Now across this membrane they are at transport phenomena of different kind of solute and solvents which are taking place next what we will do we will enumerate all the transport phenomena and then from there we will move on to the next part. Let us talk about all the transport phenomena which are happening out here. So, talking about the different transport transport mechanisms which are being dealt out here the basic transport mechanism. So, if I if I consider one second let me let me do it in slightly. So, if I make the membrane like this. So, this is the bilipid layer. So, there are essentially four different kind of transport phenomena which are taking place one is the free diffusion inside of a molecule and outside the molecule. The other set of diffusion are like this which has specific proteins which ensures I will come to that. So, essentially what is happening out here in the first case what you see is. So, what you essentially see out here this is a case of free diffusion here in blue what you see what I have drawn out here which I am circling now these are the proteins which allows a facilitated diffusion facilitated diffusion apart from it you have the other process which is called filtration. So, this is showing the filtration this is another transport phenomena then you have something called electrical conduction which is something like this based on potential difference conduction phenomena and then you have a energy driven process which are totally regulated by the availability of energy that is called active transport. So, this is in most brief manner all the different transport phenomena could be described you have conduction. Conduction is basically a function of the potential drop across the membrane which I have explained in terms of potential in the last class then you have facilitated diffusion where basically there are proteins which ensures the movement of a specific molecule electrolyte or organic molecule or whatsoever then you have filtration because then you need a mesh through which it filters through and then you have the active transport which essentially depends on supply of energy. These are the four basic mechanism by which a membrane acts in order to maintain its bioelectric potential and out here lies in this picture out here lies your whole series of ion channels and everything is kind of you know lying all over this this is the zone where all these events are taking place. So, now based on all these things I will come back to the nursed equation which I initiated in the last class and now I will move on to the nursed equation or nursed equilibrium. So, before I explain nursed equilibrium let us try to understand two simple situation. So, let us assume the inside the cell and outside the cell. So, basically what he proposed is he derived the whole condition and proposed under what condition what will be the exact potential drop which will be maintained across a membrane there will be a balancing act between the charges inside charges outside and along with the different concentration and that is essentially is called nursed equation. So, let us formalize this thing in terms of when the put it down in the slide how the nursed equation really looks like and how we kind of you know give it a give it a formal explanation of it. So, nursed equation could be essentially could be written as this is a state. So, please be very careful the state of equilibrium. Equilibrium exists when the flow of an ion down its voltage gradient down its voltage gradient is equal and opposite to the flow of the same ion flow of the same down its concentration gradient. So, if you read through this now carefully since I have now put it everything in word a state of equilibrium. So, your first keyword should be the word equilibrium which exists between the flow of an ion down its voltage gradient the second keyword which is equal and opposite to the flow of same ion down its concentration this is important. So, essentially you need to realize there are in between the line there are three important words there is an equilibrium which has to be established. So, let us see what exactly that mean that means if this is the membrane which is separating one side to the other side there are lot of positive charges likewise positive negative and these different positives have different nature I am just take care of that once again. So, essentially show different nature say for example, this is sodium here sodium here sodium here sodium here and say for example, this is potassium sorry this is potassium this is potassium say this is chloride chloride chloride chloride and then say for example, these are the anions anions of proteins whereas we call these also say anions there are lot of proteins inside and let us label them. So, this is sodium this is anion a minus this is chloride this is potassium. So, now let us do the same thing again. So, you have say this is the potassium and then you have these lot of sodium outside then you have say one more potassium out here then you have these whole bunch of chlorides which are present out there this is me facing it this is the left and this is right. So, once again across this membrane there has to be electrical neutrality which has to be established in terms of the charge and in terms of concentration. So, the blue is basically telling you the concentration gradient this is basically telling you the electrical charge. So, the balancing act between these two. So, there has to be equality if you go through this diagram. So, there has to be equality between the voltage gradient and the concentration gradient and this is essentially expressed by a simple equation basically is equal to and this is the most simplified form of it you will see minus 60 log concentration inside upon concentration outside. So, where C in C out represent concentration of. So, this is the part which is representing the concentration of freely diffusible positive or negative ions concentration of freely diffusible positive or negative ion from inside to outside from left or right to the membrane from inside which is represent by I n or outside which is O u outside the membrane and essentially E stands for the voltage gradient. And for individual ions you could calculate the values for potassium if you know inside and outside you can calculate for chloride you can calculate for sodium and calculate for pretty much anything and everything out there across the cell as long as you have the fundamental you have you know the values of inside and outside. So, this is the most fundamental equation which needed to be understand in order to understand membrane bioelectrical phenomena. So, from here after giving a very brief by the way this minus 60 what you see is actually has come from minus R T L N F L F and I will it is very fairly straight forward thing I am not getting in depth into this and you can really see any takes look you will see the whole basically there is faradis constant involved in it in absolute temperature at which it takes place and that also takes into account the in the valency of the particular ion which is freely diffusing across the membrane. So, from here what we will do I will based on my one of the slides so come back where we talked about the membrane structure I will introduce the structure of the membrane protein how it looks like. So, from here let us talk about the structure of the membrane protein and that will give you help you to visualize it much better. So, if this is the lipid bilayer we are talking about for example, this is the lipid bilayer where here you have the polar head groups which are sitting there the membranes kind of look like something like this other side that will be top of this you have this is the anchoring and these are being anchored this is basically called the selectivity pore. So, you can call this inside the cell and this is outside the cell and this is the selectivity filter and this is the aqueous pore. So, this is all protein this is essentially is the gate out here when we talk about voltage gating and all these things and you have a sensor element which is sitting there this is the sensor element this is lipid bilayer these are the channel proteins having introduce you to this structure. So, this is the generic structure what I just now drew for a channel protein or a membrane protein it could be either voltage sensitive or ligand sensitive. If it is a voltage sensitive this sensor element what you see out here this senses the voltage this is the voltage sensor sitting out there which I have just now highlighted fine. Then it has a gate element which is out here now I am highlighting this gate open or closes. So, in other word this gate has some kind of mobility molecular mobility fine. Then you have the selectivity filter this selectivity filter decides whether it will be sodium whether it will be potassium whether it will be chloride likewise and then you have this aqueous pore which is out here which is essentially this part of the through which it passes. So, and this is something we are talking about one of the most and of course, apart from it there are few other may be some kind of a glycoproteins which are sitting there out here out here likewise. So, this is the overall geometry and this gate what you see out here which is existing out here this gate is a mobile gate or this is a movable gate. So, this gate basically moves in both direction it can open according to the command it gets from the sensor element. So, this whole structure these are the most beautifully evolved structure of ion channels and now what will be essentially try to do from here is that after since we have done now this equation we will talk about in depth about sodium channel potassium channel and we will talk about the techniques which are being used to study these ion channels, but the very basic structure remains like this on top of that there are lot of modifications which takes place. So, I will close in here with this introduction. So, introducing you to the membrane structure. So, what we essentially did today is we introduce ourselves to the structure of the membrane out here from there we moved on to talk about the characteristics of the membrane and then we talked about the different transport phenomena which are involved across the membrane. Then we talk introduce ourselves to the Nernst equilibrium and here in the Nernst equilibrium I try to help you to visualize the situation how the Nernst equilibrium is being maintained and from there we moved on to the formalizing the Nernst equation and then we introduce you to the ion channels. So, I will close in here and in the next class we will go in depth with the different ion channels and immediately followed by the action potential. Thanks a lot.