 Welcome back to the NPTEL lecture series on bioelectricity. So, as of now we have finished five lectures three of the introductory section and two for the animal electricity. So, talking about the animal electricity as of now we talked about the structure of the membrane, the positions of the membrane proteins which can be an ion channel could be other receptors and other functional proteins on the membrane. We talked about the position of the glycoproteins and from there we talked about the Nernst equation where basically we described how charges which are or ionic charges which are present on either side of the membrane inside and outside how there is a balance being established between the chemical gradient and the electrical gradient. There should be a electrical neutrality as well as there should be a concentration dependent neutrality. So, what is the governing dynamics for those kind of situations? So, what I will do from here is this we will be moving to the ion channels and action potentials. So, initially I thought that I will be starting in the ion channels and then I will come to the action potential, but historically if you look at it there is the discovery of action potential which eventually paves way for the discovery of the ion channels. When action potential was discovered there was absolutely no idea about the presence of ion channels. This was back in 1940s during the time of Hodgkin and Huxley. They came up with the Hodgkin-Huxley formalism for which they were awarded the Nobel prize. At that time there was hardly any idea because protein chemistry was just kind of starting. People were not really very clear about the membrane structure and what Hodgkin and Huxley precisely mentioned in their seminal manuscript is that these are basically some kind of pores through which ions are moving inside the cell or exiting from the cell and assuming that those are pores they did the whole formalism which is famously known as Hodgkin-Huxley formalism. After that the ion channels were discovered 1970s and 80s saw a tremendous progress in the field by the discovery of patch clamp electrophysiology and that credit goes to Irwin Nihar and Burt Sackman the two German physiologist biophysicist who discovered a technique by which you could study the dynamics or flow of ions through a ion channel or through an ion channel or through a pore and that of course for that discovery they were awarded the Nobel prize and soon after that there was a huge rush to understanding ion channels and after that comes the molecular biology techniques were really coming in the forefront with the discovery of PCR and other molecular biology techniques and sequencing NUMA it was Suishama NUMA who awarded the first sequencing of the different channels of this I think it was the sodium channel and likewise and then came 1997 there was a time when the first channel structure was deciphered that was by Roderick McKinnon and in between there was one more structure which was discovered before that but that was not purely a channel that is a bacterial dopsin membrane protein. So, if you look at the history of the ion channels and action potentials and bioelectrical phenomena across animal kingdom you will see there is always a gap and then there is a quantum jump and then there is a gap and there is a quantum jump and the field has progressively developed for the last century 1900 it started with or even much before that it started with intracellular extracellular recording electrodes people had absolutely no idea about ion channels or anything or even membrane as a matter of fact then came as I was telling you 1940s 1930s and 19 I should say 30s and 40s and Hodgkin-Huxley formalism then with the discovery of action potentials then came the discovery of patch clamp and now the whole thrust area is on understanding membrane protein structures especially the ion channel structures and some very very tough job. So, once we will be talking about the techniques of measurements I will tell you why it is so tough and till date we hardly have hardly have one maybe for which we have at least one ion strom resolution or slightly more than one strom resolution image. So, most of these ion channels are still people are hunting trying to hunt down the exact structure at the is in crystallography techniques using cryo em techniques cry electron microscopy technique and several other techniques. So, what I will do instead of moving to the ion channels first I will give you a feel about action potential I will just go by the historical perspective. So, action potential is observed in all the excitable cells of the body and what are the excitable cells of the body and why we call them excitable cell first. So, the excitable cells of the body differs from other cell types in terms of the presence of high proportion of voltage gated sodium potassium channels and it is the shear number which differs them from the other cell type which do not have that and it is developmentally programmed that some specific cell types in our body have this interesting structural features and they have a lot more or significantly higher number of voltage gated sodium potassium calcium channels and that makes them very I should say susceptible or that equip them to respond to any kind of signal from outside. So, what are those cells which are called excitable cells of the body. So, there are three kind of excitable cells in our body the nerve cells, the muscle cells or muscle tissue and the neuro endocrine cells. Nerve cells constitute the whole nervous system pretty much all the neurons in the nervous system their functional hallmark is that they are electrically active having saved this let me highlight one point which many a time people miss we see there is currently a lot of progress in developing neurons from skin cells developing neurons from stem cells. So, let me point out here neuron can only be called a neuron when it is functionally active in terms of its expression of voltage gated sodium potassium and other eye channels and it should be able to functionally shoot an action potential. If it is not an excitable electrically active cell in spite of the fact it expresses all different kinds of marker or proteins it cannot be called as an excitable cell or a neuron or any other excitable cell. So, this is something I wish to very pinpoint because many people get confused that oh there are neuronal marker that means it is a neuron essentially unless they are functionally a neuron you really cannot call them a neuron. It is just another cell which has expression of some of the characteristics of a neuron, but you cannot call them a neuron. So, nervous system the major excitable tissue of the body muscular system muscle system which includes your skeletal muscle the bulk of your body which is electrically excitable you have cardiac which is electrically excitable all your life the very moment it loses its electrical excitability most likely you are in deep trouble. Third the smooth muscle which lines your GI tract these are the three muscle component excitable muscle component of the body which has the ability to shoot an action potential. Then there is another specialized group of cells up in the deeper ridges of the brain those are called neuro endocrine cells. These neuro endocrine cells are the ones which has dual function they have a endocrine function as well as a neuronal function. So, these cells excuse me essentially receives neuronal signal and shoot an action potential and thereby secret hormones or pro hormones. So, these cells are also excitable cells. So, these all these three cells or all these three tissues share the common hallmark of shooting action potentials and more specialized are the ones which are muscle they have the ability to translate that electrical signal into mechanical signal using excitation contraction coupling apparatus will come to all those things which is a hallmark of cardiac cell cardiac tissue and skeletal cell. Whereas all the different kind of neurons which includes a motor neuron, sensory neurons, high brain neurons they have different pattern of action potentials and that is what distinguishes them from one to another. There is another supporting cell within the nervous system called glial cells though they are part of the nervous system, but they do not shoot an action potential. They do have some percentage of voltage gated sodium channels, but not in high number they shoot something like action potential, but it never overshoots you. So, the only cells in the nervous system and nervous tissue which shoots action potential are the neurons. This is another thing which need to be understood very clearly. So, enumerating them in the slide. So, we are into lecture 6 now. So, the action potentials, so the excitable component nerves and here mostly the neurons which are involved in it. Then you have your neuro endocrine cells and then you have the muscle and within the muscle you have skeletal cardiac and you have smooth. Within cardiac if I had to do another classification on that within the cardiac tissue there are again two kind of cells the pacemaker cell and so let us see the examples of these excitable tissues. So, you have the neurons. The neurons you have motor neuron. I am just putting n for neurons. Motor neurons, sensory neurons, you have hippocampal, cortical and here it is worth mentioning that these action potential and pattern of action potential should by different kind of cells and the kind of computation they do is function of the morphology too. So, if you look at the morphology of these different neurons as such. So, talking about the motor neuron these are the largest neuron in our body and they are the one which are form the earliest among all of them and if you look at the morphology it is something like this there huge cell body like this and there is a long, long, long process the axonal process here you have the nucleus and these are the dendritic processes. So, these are the dendrites dendritic process axon is the nucleus and this axon could be either insulated or nonic. If it is insulated it is called myelinated. So, the insulation is just like you see these cables on which there is a plastic covering almost exactly similar to that you have something like this. So, there is a slight discontinuity you will observe out here you see out here out here these are called nodes of Ranvier and these are the myelination sheet and these myelination sheets is either they are myelinated by oligo dendrocytes or they are myelinated by Schwann cells these are the two myelinating cells and these are the ones which I was trying to tell you these are the supporting cells or the glial cells apart from it there is one two more glial cells out there which is the astrocytes. So, these are the myelination ones which are involved in myelination and apart from it there are supporting cells which are kind of you know present in the proximity of these neurons likewise all over the place and which are called astrocytes. So, here it is what mentioning that these these ones these ones these ones are the family of glial cells this is the family of glial cells which are present these are supporting cells and essentially what you see this myelination which I am just for your understanding putting another word here is called insulation the idea is that if simultaneously there is another neuron which is moving like this say for example in blue I am showing another neuron unless these two cables another neuron another axon moving and that is how exactly in the real life it is there like that and unless there is there is myelination or there is separation by these by these insulators there will be a enormous short circuit taking place out here. So, this is a short circuiting what I am trying to show in red so without insulation there will be a short circuit these are some of the basic understanding which is very essential to understand how these are propagated yet there are. So, this is what you talked about a motor neuron yet there are cells in the sensory neuron which are unmyelinated they have no wrapping the way I showed you the wrapping out here that is partly related to the fact that there are certain neurons where you want the signal to be lost and those neurons do not have any myelination. So, this is another structural feature which helps them in their electrical conduction and many of these cells looks like this some of these sensory neurons are like this they could be unipolar they could be bipolar. So, here is axon they have a long then right sensory neurons yet there are cells which has very dense dendritic arbor something like this cell is a smaller than this dendritic arbor something like this of course it has an axon here is the axon here is the dendritic tree dendritic tree and here is the cell body and a cell body out here. So, these are some of the cells which are called these are the cells which are present in the higher centers of the brain and called purkinje or their hallmark is there is enormous amount of dendritic arborization along it and the way it works is this the electrical signal first of all reaches the dendrites from the dendrite there is an summation of the electrical signal then it travels along the axon and communicated to the next to the dendrites of the next neuron and likewise the information flows in one direction and we will talk about why the information cannot flow in the reverse direction there are some reasons that information is if we go back once again. So, the information transfer is pretty much like this like this. So, the information are coming to the different dendrites likewise and this information is travelling like this why is it so and here it is once again. So, what helps in this is again to do with the ion channels how it is happening and there is a spatial and temporal summation. So, what I meant by that this is something which at this stage itself we need to highlight. So, for example, let me draw let me draw a pyramidal neuron. So, I have to this Purkinje cells a Purkinje neurons and this is a sensory eye drew I have written it sorry. Now, I drew a hippocampal neuron which is mostly the cell body is more like a pyramidal shape something like this these are the dendritic triggies which is and here is the axon which is moving along. So, these are the classic pyramidal neuron of hippocampus or the center for learning and memory. So, what I meant by spatial and temporal summation. So, within the central nervous system each one of these neurons receives at one point of time they are connected physically connected with 10,000 other connections. So, even if at one point of time they receive 100 signals or 200 signals which they do it is enormous amount of calculations. So, what I meant by this 10,000. So, it is something like this. So, imagine these are the connections these are the different sources from different other sources the axons are forming synapse. So, like this forming synapses on it these are this pink one are the other neurons which are making connection on the body of it from different other sources they are making connections. So, this is an imaginary drawing I wanted to show you. So, imagine at one point of time there are such 10,000 physical connections on one neuron and this is kind of on the lower side there are 10,000 connections means there are 10,000 synapses I will be introducing synapse. So, imagine that the at one point it can receive 10,000 such signals with slight delay of time or at the same spot. So, at this zone the amount of calculation it needs to do is enormous and the summation of this calculation what you see is transmitted along this. So, what essentially is happening out here is this at this zone there is an two sets of things which are happening there is a spatial and there is a temporal computation spatial and temporal computation. So, all the different signals which are reaching like this they may be at time t 1 time t 2 time t 3 time t 4 time t 1 time t 1 time t 2 time t 5 time t 10 likewise. With little stuff you can see staggering. So, the mammoth is just imagine and if this is just the temporal thing I am drawing. So, here comes the time axis and here comes the signal. So, here is a signal reaching followed by another signal coming followed by another signal coming followed by another signal coming and summation another signal which has already come there and the signal before this likewise. So, there is a continuous addition of these signals taking place and these hatched zones where you are seeing the signal one is kind of you know on the previous one. These are the zones where the part of that signal is being added to the previous signal and simultaneously if this is with respect to the time there is on the space say for example, if this is the cell body one is added from here one is added from here one is added from here one is added from here at the same time say for example. So, overall what is happening there is an summation at time function and space function and the output what you see is your the one which is carried by the just trying to show you in this picture carried by the axon this is the output and this is the level of complexity with which a signal is being process which even the most simple chip cannot think of it is probably the final frontier of human endeavor in terms of understanding the genetic in terms of understanding the neural code and all these codes all these coding lies in that seminal discovery the last in the middle half way through the last century by Hodgkin and Huxley the action potential it is those which coded. So, if I have to understand this say for example, I show you and say for example and apple like this this is an apple and I show you say for example, mango. So, what makes an apple and apple and what makes an mango and mango and your eyes are looking at it. So, these are the eyes what distinguishes. So, the store information which is stored has a specific pattern whereas, the information which is stored has a different pattern and these pattern from here is are getting stored in your brain as what you call something mango something in apple and something in mango. So, in other word what I am trying to highlight if I know this signal and if I know this signal then technically I can feed that signal to make any blind person whose eyes are non-functional to see an apple when this person is actually not seeing an apple or mango when this person is actually not seeing a mango same way by the same token comes the other side of the story which is say for example, hearing. So, you are hearing say for example, you are hearing something like Beethoven or you are hearing something like oriental music in a different trains are coming and here a different trains are coming. So, technically if these signals are put in these signals are put in the visual area which is process the visualist information you should be able to see the music. This is something that imagine and all these things are happening by that simple most ordinary event which you call the action potentials. All the crux lies in action potential and the next frontier where we are heading is basically the visual area and the neural code. So, with this background what I needed to kind of highlight to you people I will move on. So, I one more thing I just forgot here which I realize because I told you in the beginning I will show you that. So, regarding the cardiac cells I have forgotten I will come back I have told you that there are two different classifications as I will start the cardiac I will tell you what are the different cell type. Of course we have highlighted about the neurons, but I have not highlighted about the what are the different cell types in the cardiac cells which shoot action potentials. There is something which I have missed as of now, but I will come back to that. So, in the light of this what I have taught you just now one second. So, understanding action potential is the most critical thing unless we understand this basic phenomena it is really tough to appreciate what are the electrical events which are taking place. So, now let us try to understand action potential. So, for example, now here you have a cell sitting like this and this is the axon and here are the dendritic trees dendritic arborization and dendritic spines and all that that is the nucleus. Now, I told you that these cells are rich in voltage gated voltage gated I will just abbreviate it as V G voltage gated sodium channels and voltage gated potassium channels. Interestingly they are voltage gated. So, I will represent these voltage gated sodium channels as in red they will be represented by red color and voltage gated potassium channels will be represented by green color. So, interestingly it has been observed that voltage gated sodium channels are concentrated at a specific parts initially when the neuron is forming they are scattered all over the place. Then they migrate at a specific spots and it has been observed for the cells which are myelinated for example, a cell of this kind which is this is a myelinated cells likewise. So, voltage gated sodium channel population is fairly high in this zones fairly high here again fairly high here these are the nodes where they are high and rest of the places they are scattered around like this. So, this anatomical understanding is very essential to appreciate if you look at these zones they are fairly fairly dense. Whereas the voltage gated potassium channels follows very similar trend they are also scattered around all over the place. They are fairly high in the cell body and one thing I will try to highlight these cells depending on the situation they aggregate and then they again dispersed this is the zone which is called your axon hillock. So, now having drawn this so we know outside the cell in this milieu your sodium concentration is fairly high around 150 millimolar whereas the potassium is fairly low which is around 10 millimolar. So, these things are known to us even actually it is less than 10 millimolar it is around I think 4 millimolar and again from book to book it varies 145 millimolar whereas inside we know sodium is 12 millimolar and your potassium is 155 millimolar we do it in black so that you can read it properly this is your sodium this is your potassium. So, looking at the sheer number you can understand that if there is sodium which is 12 inside and which is 155 outside then most of the likelihood that sodium from outside will try to rush inside the cell, but this is being continuously prevented this is not being allowed whereas if you look at the potassium which is around 144 or 155 inside and 4 outside. So, on all likelihood potassium will try to move from inside to outside. So, let us do something let us do the calculations based on the nurse equation if you guys remember I was telling you that you know based on that you can. So, let us do the simple calculation this is the extracellular this is the intracellular and the ratio iron outside work upon iron inside and we will just for do for two ions sodium and potassium. The sodium outside is 145 and inside it is around 12 and this is all in millimolar this is in millimolar whereas potassium is 4 and outside it is 155 and if you see the ratio of these two. So, this will be coming approximately 12 whereas potassium will be coming around 0.026 and if you calculate the equilibrium potential. Equilibrium potential in millivolt the values will come for sodium it will be positive minus 67 millivolt whereas for potassium it is minus 98 millivolt. Now, this minus 98 millivolt says you something what it says if you see this diagram the diagram before out here if you put an electrode outside the cell and inside. So, for example, I have a electrode like this and I have another electrode. So, I have the measuring device out here something like this and I have another electrode outside and I am trying to measure the voltage. So, if you if I measure the voltage with the inside with respect to outside will see it is approximately 70 to 90 millivolt. So, inside the cell it is more negative as compared to outside and the membrane potential is between minus 70 and minus 90 depending on the cell type. So, if you look at this value in the light of this you will see that value of potassium equilibrium potential is very close to cells resting membrane potential which is around minus 970 to minus 90 millivolt. Why is it so the first question this is so because potassium is fairly leakage and as a matter of fact across the membrane if we draw something like this if this is the situation of the potassium and this is say for example, outside and this is the semi permeable membrane and this is inside the cell then you will see potassium is kind of you know always a potassium is there is a slight degree of diffusion which takes place outside the cell and of course, the entry is because of the conduction. So, always there is potassium which diffuses out whereas in the case of sodium if you see if I again draw the same thing for sodium this is inside the cell and this is outside the cell if you look at it sodium could never get an entry neither it leaks out nor it moves in. So, sodium and that is why if you look at it these two value if you compare once again yeah. So, if you compare these two value minus 98 and these they are fairly close they are close because this reason that sodium diffuses out it is much more leaky actually as compared to sorry potassium is sorry I am I beg your pardon potassium is leaky whereas sodium is very tightly it cannot really pass through there is hardly any way that sodium can really you know pass through unless otherwise there is there are specific channels which opens up and allows the entry of sodium inside the cell. So, this is the hallmark of beginning of understanding action potential. So, essentially what is needed is that somewhere or other in order to create a disturbance in this structure if I go back in order to create a disturbance in this structure you have to have some way by which these channels what you see out here these channels have to be opened without opening these channels you cannot create any kind of movement in them they will remain static. This is the very basic fundamental understanding all these background is needed to understand action potential we do not understand this and the rest will make sense. So, what I will do I will close in here and in the next class we will see when these channel opens what it leads to and what are the series of event which takes place and we will explain the whole action potential in that process. Then we will move on to the propagation and we will come back to the structure of the ion channels and how those structures are helping in this kind of conduction processes. Thanks a lot.