 Welcome back to the lecture series on bioelectricity in NPTEL. So, today we will be initiating the 16th lecture and this whole series of 42 lectures of 40 lectures. So, in the last lecture we talked about the like the pretty much the tail piece of the stretch reflex arc. I told you that like the signal from the sensory neuron gets carried from the muscle spindle to the sensory neuron and in the sensory neuron it is split up into 2 into the interneuron and within the interneuron actually it gets split up into 2 to the gamma motor neuron and also motor neuron. The gamma motor neuron are the ones which tells the spindle to come back to its original shape and alpha motor neurons are the one which tells the extra fusel or the other the big chunk of our muscle to come back to its original shape. So, in the last lecture where I ended was this there are some central problems which kind of frustrated the scientist fairly early during say 1940s, 50s with the action potentials which are being known. The logic was very straight forward logic is something is muscle is a three dimensional structure. So, think of a structure let me let me draw it is formally welcoming on the lecture 16th and so say for example, you are. So, imagine this is the muscle these are the muscle layers which consist of lot of my tubes and all those things my fibers and all. Now, the neurons which are coming through they are coming like this. So, these are the motor terminals or the neuromuscular junction and these are the synaptic buttons and everything. So, this is the zone where basically the acetyl choline is being secreted out. So, now what was the fundamental question which was being asked is. So, at this stage action potential travels down. So, if I have to represent it like this action potential travelling down travelling down here out here there is acetyl choline coming out and then it generates another series of action potential in the muscle. So, these are the skeletal I am just putting SK skeletal muscle for MAP action potential and this one is the MN for the motor neuron action potential and out here what you see is essentially the a chemical synapse. Now, the question which was raised is how from the surface the electrical signal travels all the way down into the muscle and how this brings about a contraction in the muscle as of now we talked about high electrical signal followed by a chemical signal followed by another electrical signal fine. But what we have an answer is how these electrical signals chemical signals than electrical signal leads to the mechanical force generation by the muscle and how from the surface it travels down all the way because theoretical calculation showed that given the time window. So, for example, if you look at this picture given the time window at t 1 when this is reaching here and if I call this as time t 2 when the chemical synaptic event is taking place and considering t 3 as the action potential of where t stands for time. So, if you follow these times by the time the muscle the mechanical motion of the muscle takes place mechanical motion of the muscle by the time the mechanical motion of the muscle is taking place it is being calculated that if this impulse from here to reach all the way down it will take more time by the theoretical calculation. Then how this event is happening so fast so subtle in a very nice way. So, what are the catch in this game and that is where started ultra structural study of the muscle and which eventually clarifies lot of our understanding. So, today we will be trying to answer those questions which are known in this respect that how the signal from the surface of the muscle reaches all the way down. So, in order to understand this we need to understand two concepts so and mind it these were all figured out by electrophysiological recording. So, example you have a electrode out here you have electrode out here if I call this as e 1 and this is e 2. So, the stimulus which is generated here will lead to an electrical signal on the electrode 2 and that made us all the theoretical calculation to figure out that there must be something else what is that something else. In order to understand that something else we have to go into the ultra structure or the real fine structural details of the muscle. So, coming back so we will be talking about now the structural details skeletal muscle. So, I have already discussed within very brief that how the skeletal muscles are being formed. So, the individual cells coming close to each other they align they form myotubes and these myotubes super coiled with each other to form myofibers these myofibers eventually super coiled with each other form the muscle fiber. So, what we will be talking about the ultra structure of individual myotubes. So, if you look through a cross section of a skeletal muscle you will see lot of striations something like this this is a skeletal muscle cross section you are looking at you will see something like this these are the striations this you can see in any regular ordinary microscope and do not need any something like this and of course, the nuclei are sitting in between out here this is a classic striated skeletal muscle and one more observation which you people will see you see it let me you will see this muscle is something like this muscle structure is bit of a like this gorge like a structures and of course, on that you have the striations all over the place these are the striations and here you have the nuclei sitting this is how the structure overall looks like. So, now we will get into the molecular details of this structure. So, when the when the development takes place. So, when you talk about individual myocytes coming close like this these are the individual myocytes what I am drawing now and they divide just to get a recap that works they are dividing for the nucleus in all of them here they are dividing like this and. So, then these ones align to form the what we have already discussed are the myotubes fine. Now, when this myotubes formation is taking place what is reorienting is that what I have not talked to you people is this there are two sets of proteins which are present on them and I am putting the two proteins in two different color picking up one as very thin filaments like this these are called actin filaments they are lot lot of such actin filaments and they are thinner as compared to another set of thick filaments like this these are those thin filaments which form pretty much the cytoskeleton and architecture of the muscle and then you have the thicker filament which are extra. So, you see there are thicker filaments which I am drawing now in turquoise blue color. So, muscle contains thick and thin filaments and these thin filaments are called thin filaments these are called actin whereas, thick filaments are called myosin. Broadly these are the two major proteins both actin and myosin are specific fibrous protein and they are kind of you know very thing long strands of proteins fine. So, now when the myotubes getting formed these actin and myosins are realigning themselves they realign in a specific passion and their realignment essentially is something like this we will come to the ultra structure after this as if they are inter digitated among each other and it is this realignment of this actin and myosin filaments which leads to what you see in the previous slide this striated architecture. Now, coming back it is the variation of the myosin which pretty much decides what kind of muscle it will be whether it will be a skeletal muscle whether it will be a cardiac muscle whether it will be a smooth muscle or it will be a muscle spindle or something. So, these myosin has a lot of sub types myosin heavy chain myosin light chain myosin extra light chain likewise and they all vary in their molecular weight of the proteins what are making these myosin chains. So, myosin chains are one of the rate limiting factor in describing the phenotype or the muscle type in our body with this background now what we will do we will go into this picture and we will have a microscopic view of this how really this looks like what is the exact pattern. So, there is a pattern so pattern now we are going to talk about the pattern. So, we have already decided the color code since we have decided the color code. So, these are arranged if you under the microscope it was observed the thin filaments and the thick filaments are arranged in a very interesting way they are arranged something like this and I am following the same color code what I have followed just I just do it wrong actually I should have drawn it in such a way that hold on give me one minute let me finish this drawing and then I will get back to you. So, these red ones what I have drawn so now you are looking it under a very ultra high microscope with very high magnification these red ones are the actin filaments what we decided in the previous slide if you look at it we represent these thin filaments as actin. So, now where are the myosin standing now we will introduce the myosin we will change the color code now. So, the myosins are sitting like this now these are thicker lines and these thicker lines have myosin sitting like this and mind it this is exactly the same thing what I was trying to draw out here. So, this is this arrangement what I am drawing now if you look at very carefully these myosins have something like this as if there is something edging out like this from their surface and arrow kind of things which are popping out from there. So, these are the thick filaments and so let me just finish this drawing and then I will explain some of the finer details which are as of now I have introduced. So, this is the initial structure this is the smallest unit of something called sarcomere and I am coming to that now now I will try to introduce the specific terminology where they are. So, overall if you look at this picture carefully you will see there are zones where both the proteins actin and myosin has overlapped fine yet there are zones where they are not overlapping there are zones where there is only myosin and there are zones where it is only actin you could see that I mean if you look at this picture coming back to the slide. So, if you look at this part out here where I am circling this is only myosin this is the zone where there is only actin this is the zone which is the overlap zone of actin and myosin fine. So, now this whole unit is termed as from here 0.1 to 0.2 this whole unit is called sarcomere. First terminology which we have to remember sarcomere next these lines are called z lines z lines z lines the central line is called m line and this zone which is devoid of where you do not see any actin overlapping this is called h zone. So, we have introduced z lines m lines h zone and if you look at it carefully there is one more introduction I will do that is out here there are certain connecting structure like this there is a name for these specific proteins which are kind of you can call them like a spring kind of connectivity these are called titan the color just using titan. So, these are coiling kind of protein as if like you have a spring which pulls something this is the basic architecture of a cross section of a muscle. Now, we will go to some more finer details. So, now coming back to the structure so there are something which is called dark band and light bands. So, if you look at it this is the zone where there is the maximum if you if you look at this zone where both actin and myosin are overlapping these are in a contrast picture if you see it you will see those are darker as compared to next section which is much more lighter. So, if you now go back and see this picture you will see there are kind of you know something like something like you will see a dark and light bands and those dark and light bands something like this. So, if you look at the dark and light bands now you can understand from where those light and dark bands are arising they are arising because at points there are two proteins which are overlapping with each other at points there are only single protein which is. So, in the light falls where you have two proteins overlapping over one another they show a darker contrast as compared to the zone where there is a single protein light is passing through. So, that is why you see light and dark band now coming back to the further molecular details of this actin filaments how they really look like. So, now we will be talking about the ultra structure of this actin filaments. So, to the next light so now if you look at these actin filaments. So, they are not that simple as they look like they are in a finer details they are something like this those straight thin lines what I have drawn they are super coiled structures of proteins like this and within that they have bunch of molecules of let me get slightly more there are bunch of molecules and we will name them they have physiological and anatomical significance. So, they have something called active site what are those. So, these balls what you see. So, this is an this is an actin filament or the thin filament. So, whenever I talk about thin filament it is with respect to the myosin there is something there are protein called tropomyosin. There are specific proteins which are embedded out here like this which are called troponin they have something called these are called G actin filaments. So, these are the troponin molecules this is basically where troponin and tropomyosin are on top of each other it is something like that if I have to get a further ultra structure of this it will look like this. These are the tropomyosin molecule troponin molecules are on top of this like this on all of them something like this all the green dots you are seeing those are the troponin on top of tropomyosin. So, this is one piece of information which you have to remember because we will be coming to this all this connectivity and what exactly happens. Next thing we will be talking about the ultra structure of the second component which is this component we have not talked about this yet. So, talking about this component this component if I again recap the z line. So, the z line looks like this on the z line you will see super coiled structures and the super coiled structures have something like this kind of things what I have drawn before what these structures are exactly and mind it this is the m line which is the from both sides you could see at different level they have. So, these are basically called if I get in further details what I have drawn here that extension. So, it is essentially that extension is nothing but there is something called myosin tail which is here which is here also and you have something called hinge kind of situation here on which something can move like this and then can come back to its original position and this is called myosin head and this head what you see can almost like this it can move like this. So, essentially this head can take a situation like this also in other word it can move like this. So, now going back recapping what we have talked about as of now if you see this picture. So, essentially what is happening this head can move in both directions and like this it can move like this, but if it moves like this something else will happen will be coming to that this is where we are getting into the sliding filament theory. Now we are entering into the little trickier parts of the game there is something called sliding filament. So, before I get into the sliding filament theory what I will do is I will kind of take you back here to give you some idea before I really explain this theory in details. So, what essentially is happening is this on the rails of these actin and myosin imagine something like this is an actin on the top and this hand this hand is an actin and underneath the other myosin like this. So, myosin can move its head like this. So, when it moves it head like this it they slide on top of each other and the sliding motion as if there is a small ball like a structure which is the myosin head something like this muscle contraction is nothing, but the sliding motion of two proteins which are parallel to each other, but one of them has a small ball like structure which we call them technically as myosin head which is moving like this something like this motion this is my myosin one which I am moving my hand this thing how that is being done where we get the energy water what regulates it and that is where lies introducing the fact or what was as of now accepted is called sliding filament theory sliding because they are sliding on each other and it is the filamentous proteins these are filament actin myosin moving now. So, the filaments are sliding over one another and that is where it got say got its name sliding filament theory. So, now we will be talking about the sliding filament theory that will give you an idea how the muscle exactly contracts and then we have to talk about the tail piece, but how that electrical signal actually lead to this contraction process. So, we are first of all we are in ultra structure just to give you a recap what all we have finished. So, we talked about the problem we define the problem that how the electrical signal reaches here then we talked about the ultra structure in overall ultra structure and from there we talked about the light and dark bands and then we talked about how these filaments are getting arranged and then I have given you the overall site architecture of the of the muscle where you have showing the sarcomere out here this is the sarcomere out here you have the m lines which are the myosin actually the myosin which is vertical out there you have the h zones you have the z lines and we talked about the titan protein because this structure is very important and the single unit from one z line to the other next z line is called the sarcomere these functional unit and understanding is very essential and then we talked about the presence of some specific protein within this filamentous structure of actin just tropomyosin and troponin and then we talked about the presence of myosin and myosin and myosin head which is a moving head up to this we have talked about and then at this point I introduce you to the sliding filament theory what really sliding filament is all about ok. So, at this stage what I will do I will close in here will start from the sliding filament in the next class and then we will move on to the where the electrical signal actually influences the sliding motion before I do so what I wish you people please go online on a Google just give a search about the ultra structure of a skeletal muscle giving search what you will essentially see is you will there are much better pictures which I cannot really draw on a slide like this you will see much beautiful picture of this whole structure very nice way and I expect you people really to go through that because that will help you to appreciate this structure in finer detail once I will be entering into the sliding filament theory this will be a big help if you take the time and go through it very carefully ok. So, I will close in here we will come back on sliding filament and about the sarcoplasmic reticulum and the function of the sarcomere and that is what will be our closing in the first simple circuit of the brain which has been discovered by different bioelectrical phenomena by different multi macroscopic phenomena and likewise. Thank you.