 Welcome back to the lecture series on animal physiology in NPTEL section. So, today what we will be discussing is in the neuromuscular junction, how the energy is changed from electrical energy to mechanical energy. So, we talked about the neuromuscular circuit. So, here something is very interesting what you have to appreciate first, before I draw the whole circuit and you know how the energy transduction is taking place. So, what exactly is happening is that say for example, there is a signal a sensory signal which is being sent by the sensory neurons to the spinal cord or may be to the higher centers of the brain. From there through the ventral route a motor neuron is bringing the signal to the target organ asking it you know you move your hand like this or you walk like this you know likewise x y z this could be any signal. So, once the signal comes so signal is being transmitted as an electrical signal electrical impulse action potential train of action potentials ionic electricity is coming. But now that ionic electricity eventually the end result what we see is that that ionic electricity leads to the contraction and a movement of a muscle how it does so. So, in other word what is happening is that ionic electricity which is being transmitted by a by the nervous system is translated into some form of mechanical energy via a chemical route. So, here some molecules do that whole transduction what we call talk about the transduction procedure. So, there is a transduction which is taking place electrical impulses through a chemical route translate into a mechanical energy and that mechanical energy in the form of end output what you see is the contraction of a muscle. So, if you go back and you think about what is happening in the stretch reflex arc. So, in the stretch reflex arc what is what is happening. So, there is a stretch in your muscle. So, here is your muscle and there is a stretch in your muscle. So, there are stretch sensors all over it what we call as the interfusal fiber or muscle spindle. So, they sense a mechanical change in them a change in length and then those sensors which are sitting here they change this see the change a mechanical change and they translate that mechanical energy into electrical energy first transduction then that electrical impulse in the form of ionic waves ionic current waves travel all the way from here all the way to the spinal cord fine after coming to the spinal cord then there is a computation process taking place that computation may be supported by the higher motor neuron which are present in the motor cortex of the brain or may be a very local computation which is taking place at the spinal cord level especially in the borderline zone of the dorsal horn where the information enters and the ventral horn and through the inter neurons in between and those motor neurons which are sitting there then brings back the message and all this all along this it is only the electrical impulses which are getting transmitted ionic electrical impulses. And then once it reaches the muscle at the neuromuscular junction it transmits this electrical impulse to the muscle and what the end result we see the there is a mechanical movement in the muscle accompanied by an action potential. So, how it does so now we will redraw the circuit and after that there are three phases here first phase is what I will do I will redraw the circuit neuromuscular junction how that what I have just narrated you second thing is that we will look at the ultra structure of the muscle because that will help you people to appreciate that how the ionic current electricity is translated into a mechanical force via certain chemical routes. And third we will club all these events together and we will appreciate this very wonderfully evolved phenomena. So, let us start with the section 5 and lecture 4 section 5 the nervous system actually we lose almost a part of the muscular system also nervous system lecture 4. So, let us look at the circuit how it looks like. So, here is the circuit element this is the part of the spinal cord and here you have the in the blue zone in between is the ventral horn V H and this is dorsal horn or the one which are carrying the sensory information V H. So, motor neuron sitting here this is the neuromuscular junction here is the target tissue which is a muscle and will and here is the sensory neuron whose cell body is sitting slightly outside the dorsal horn and here is the ascending pathway and this is the local circuit where dorsal horn is synapsing on the ventral horn dorsal horn neuron is synapsing on the ventral horn neuron and going by the arrow of information. So, this is the arrow of information which is moving in the black from here it is reaching this third level of computation and it is coming back like this. So, today our objective will be to look at the cross section of this muscle in depth in detail because this is essential to understand this. Finally bit of a further detail of this zone these are the two zones what we will be dealing today. So, one of the pertinent question which was asked sometime in 1960s was this the question was like this. So, say for example, we have muscle like this it is a thick muscle fine or may be some other part of the body where a thick muscles. So, the neurons are coming all along like this and sending the signals how the signal reaches all the way to the depth because neuron is not really going all the way to the integrity detail why it kind of cover two three layers down why the contraction how that whole transmission taking place. So, if I have to draw it will be something like this say for example, here is a muscle here is your target muscle and here is a motor neuron which is coming and on the surface it is out here for the near vascular junction this is the cell body of the motor neuron here is your motor neuron here is your target muscle. So, the governing question which made people to explore this is that how come the signal reaches all the way down because these are very small ionic electric current and it has to travel all the way down. So, it may sound very simple, but it is actually a very big problem. So, what promotes this kind of transmission. So, there it all started. So, going back to the previous slide. So, now what we will do I told you that there are two levels what I am going to discuss. So, this is the level one l one I am putting it level one and this is level two these are the two levels. So, first level is the one which I am now defining that how this signal reaches all the way down. So, this is just I am showing like this. So, the arrow is pointing towards like towards on the left hand side. So, here so how this signal is moving. So, in order to understand this problem you have to look in depth to the structure of the muscle. So, how the muscle structure looks. So, if I had to draw the muscle structure before that and I am not drawing at this stage the ultra structure ultra structure is much more different. So, the structure of the muscle is something like this. So, if you look at muscle structure in the cross section it is more like this. So, you see there are gorge kind of structures. Gorgeous are like you know on two sides there are if think of it like if you have mountains like this and there is a small narrow cliff narrow gap in between that is called a gorge. So, it is like a gorge like structures and muscles are like that. So, coming back to the structure of the muscle is more like this. This structural feature has a big significance that how the muscle conducts all these pieces of information. So, this is your skeletal muscle. Now, I told you. So, this is in blue what I am showing is wherever the neuromuscular junctions are getting formed. For example, a neurons processes are likewise fine. So, these are the zones of these are the hot zones where you have all the neuromuscular junctions are getting formed and these are where the neurotransmitters are getting released right in the in the blue blue button like structure what I am drawing now this is where now here comes electrical impulse coming fine it reaches here. So, once it reaches here what it does it leads to the these are the neurotransmitters let us in our situation we are talking about acetylcholine. So, acetylcholine secretion leads to the opening of ion channels out here the cation channels. So, these cation channels leads to the entry of the sodium inside the muscle fine after this it is clear to all of you. So, there is lot of cation cation which are entering entry of the cation channel what it does it leads to an action potential in the muscle. So, if the muscle was sitting like this this leads to and rise in the so if it is sitting at minus 70 millivolt this will lead to an action potential in a muscle like this. So, now the muscle is experiencing. So, if you have a electrode out here in a close proximity somewhere out here if you have a say for example, I place an electrode like this with respect to the and here you have a change in voltage. So, you will be able to see a change in voltage like this. Now, this electrical signal which is generated by the muscle because of the neuromuscular junction has a role to play. So, now what is happening this electrical stimulus from the neuronal side now transmitted into the muscle. So, now this electrical impulse is moving on the muscle surface and then maybe it will travel like this and you know likewise it will travel. So, while it is travelling there is something very interesting happens. So, the membrane is getting polarized, depolarized, polarized, depolarized, polarized, depolarized as this signal is moving all through on till it dampens down. But while it is doing so it is a local phenomena taking place at these junctures because these are very specialized structure. So, what I will do I will highlight this part on this section. So, out here within the muscle I told you there are some specific organs called sarcoplasmic reticulum they are all over present mostly present like this out in this. These are called sarcoplasmic reticulum S R sarcoplasmic reticulum. The sarcoplasmic reticulum are the store house of calcium they have lot of calcium in them and it is very very tightly regulated. So, what happens when a impulse reaches here. So, for example, this impulse reaches here on the surface as well as it is kind of not very clear on the surface of the sarcoplasmic reticulum whose membrane becomes continuous with the membrane of the muscle. So, if I have to kind of again look at the finer details. So, it is something like that if this is the membrane of the sarcoplasmic reticulum then this is the muscle membrane they are almost kind of on top of each other. Out here it is still not very clear how it is like kind of arrange. There is a series of voltage sensors sitting here and at two level at this at this let me use different color for you people to understand. So, the one which I am now filling with green are the second level of voltage sensors. One of them are on the sarcoplasmic reticulum side the other one is pretty much at the border of the sarcoplasmic reticulum and the muscle membrane likewise whichever. Now, one of these sensors again it is not clear till date one of these sensors changes a voltage change on the membrane voltage change in the membrane and as soon as it changes sensors the voltage change it is kind of a hinge joint it is if I have to highlight this in much more in depth it is something like this. This one is a sensor and this one as soon as it as soon as this senses it has a hook like structure like this which is attached to the other sensor. So, as soon as this sensor this one which is in the red this one senses that there is a change in voltage this sensor transmits a signal to this one and this one raises up. And what exactly happens in that process is that this sensor which is sensing in the completely red color. So, now there is a bit of a description what you have to kind of listen to me and process it slowly. So, say for example, I showed you the intersection zone of it. So, imagine this is one sensor this is another sensor my two hands like this. So, on my right hand side this one this one is the one which is sensing the voltage. So, there is a change in voltage what it does and this one is attached to the sarcoplasmic reticulum because this is at the border. Once this senses this ask it to open up like this and as soon as this one opens from sarcoplasmic reticulum I told you there is huge amount of calcium the calcium goes out. So, what essentially happens at this stage is this if this receptor the one which I was showing on with my right hand side. If this one gets activated this one leads opens up a door out here which opens up lift the cap and then what you see that essentially immediate out here is a situation like this. And the series of molecules which are all over the place for a very transient period of time and these molecules are calcium. What calcium does and before I say what calcium does let us talk about the nomenclature of these different receptors. So, here what you have is now if I kind of show you in terms of in a much more bigger detail it is this is one which is opening up on top of the sarcoplasmic reticulum another gate is out here which is in close proximity with it which is the voltage sensor and. So, put the names now here is the voltage sensor and here is calcium channel opener. So, this is sensing a voltage which ask the calcium channels to open. So, here is the voltage sensor and this is asking it come on you have to open up and as soon as this opens up the next thing what we see along the along this whole milieu of the muscle is there is lot of calcium which is coming out from the sarcoplasmic reticulum. Out of this it has been discovered that these this complex consists of two different proteins which makes this voltage sensor and the calcium channel opener. One of them is called ryanodyne receptor r y a n o d y n a ryanodyne d y n a ryanodyne receptor sometime in short it is called r y r receptor. The second receptor is called dihydro pyridine receptor d y p r sorry d h p i think that is how you pronounce it these two receptors. So, where is the problem is that what is not clear is that which one is which one. So, they remain as a complex like this something like a complex like this if this one is the ryanodyne receptor than this one is the dihydro pyridine receptor. So, what is not clear in this complex of the molecular details yet to mankind is which one is sensing voltage and which one is opening up the calcium channel. So, there is one which is actually opening up or there are two situation here some people believe that one of them is a voltage sensor other one is the calcium channel opener fine it acts like a hinge like this fine. There is another set of belief which says that both of them are doing both the jobs together. So, there is nothing like you know I will do this and I will do this nothing of that sort they do it together. So, this part is still a very hotly debated topic specially this dihydro pyridine receptor and ryanodyne receptor has been extensively very extensively studied in the cardiac system especially in the cardiac excitability because one thing people have to realize these are very programmable channels or programmable structures because it not only sense out this calcium it has a mechanism by which the sarcoplasmic reticulum pulls back the calcium it is just like you know I take a bucket of water drop the water in a room and immediately within no time within a delta t time the water has flown I have a mop I just mop it out back to the bucket from the bucket where I. So, they have a very tightly regulated mechanism. So, it is not like that I drop a bucket of water and then let the water kind of you know dries up and it does not work like that biology is very well controlled system it throws the bucket immediately in no time pretty much in no time by the time it has done its job it is put back because if it remain for a slightly excess moment that will change the way our muscle twitch the way signals are getting transmitted that will be completely compromised. So, this is something what you people have to appreciate that it is not that simple it is all happening at a very very narrow zone of time within few picosecond femtosecond nanosecond or the max at a microsecond level these things are all getting over these molecular reactions are very very well coordinated and very fast. So, as of now it is not really clear out of this rhinodyne receptor or dihydroperidine receptor which one is really doing the sensing job and which one is really opening up the channel this is kind of still are usually debated topic in the field of neuromuscular junction muscle biology cardiac physiology you know now an expert in question comes there is a secretion of calcium and then what we end result what we see that there is a muscle contraction, but then what calcium does how come calcium is essential for the muscle contraction to take place. So, as of now we are only talking about there is a voltage sensing mechanism by which there is the all of us like at that particular moment of time the whole muscle milieu is all there is a if I have to plot the calcium wave it will look like this that in this diagram yeah something. So, what I will see essentially is that as soon as the signal reaches. So, as soon as I have some way to you know plot it out here the first signal is the neuromuscular junction signal. So, this is the neuromuscular junction and the next thing very soon I see that there is a wave of calcium. So, if I have the y axis telling calcium. So, I will see a wave of calcium a calcium wave going up and this is a electrical signal fine. So, this is what this calcium wave does what this calcium spike and I am showing spike because I told you that calcium does not remain there forever as soon as it is being beat even faster than that it is being pulled back by the sarcoplasmic reticulum you cannot afford to have calcium sitting there and loitering around. So, then what this spike actually leads to in order to understand and how this spike has some role to play in mechanical electrical to mechanical transduction that is what we are going to discuss in the part 2 of this which is essentially this part L 2 L 1 we have understood now this is what is happening. So, that is how signal when it reaches here through those gorge like structure they reach all the way down and then the electrical signals are reaching all the way down to the muscle. Now, we will be talking about how that is bringing about. So, this is how it is happening. So, it is reaching all the way the muscle is receiving all the electrical signal all throughout like this. Now, we will talk about the structure of the muscle. So, as of now what we have learnt about is you have you have individual muscle cells I have already discussed this part these muscle cells are coming close to each other these muscle cells are coming close to each other and forming the myotubes these are the nucleus these are the nucleus these are the nucleus. So, these are the myocytes and these are the myotubes what I essentially did not mention. So, and eventually it forms myofibers and everything, but I essentially did not mention at that point of time is that this fibers. So, these individual muscle cells have a criss cross of two different kind of proteins I will come to that what are those. So, all of them have like this they are aligned like this in individual cells at different orientation like this. So, when the muscle is attaching and forming a myotubes muscle cells. So, these proteins align themselves like this let me tell you this is not a random I will come to that these are the protein fibers which are constituting your muscle. So, what are these proteins there are two proteins which are making this muscle structure here the green one I represent as actin filament and the blue one I represent as myosin filament here I wish to highlight something this myosin so actin fairly is well conserved is myosin filaments have different variations different variants there is skeletal muscle myosin is different as compared to myosin of cardiac muscle they are different myosin of smooth muscles are different myosin of intrafusal fibers are different intrafusal fibers. So, there are different variants of myosin what does that mean that means in terms of functionality they have different force generating abilities and that essentially means force generating abilities and now if you look at it so intrafusal fiber needs different kind of force whereas skeletal muscle needs different kind of force cardiac muscles need different kind of force smooth muscles needs again another different kind of force. So, how this is being regulated this is being regulated by this different kind of myosin fiber, but is it random. So, if during 1960s there is enormous amount of research which was done it was figured out that this is not random this is very very well organized. So, how this is getting organized so I have to do these are the pictures what you see in the books and they are kind of tough to figure out. So, I will draw a simple picture for you people which will kind of clarify your doubts that how the muscle how the cross section of the muscle. So, this is what we are doing we are going through a cross section of a myotube or myofiber especially let us think of simple unit as a myotubes. So, let me see what is the color coating I am using actin is in green fine I will keep actin as green. Now, bigger marker fine now actins are arranged. So, these are the green actins. So, now we are looking at the real cross section of a muscle. Now, on the other side like this, like this, like this, like this, like this fine. Now, we have another component which is a blue component which is our myosin am I am I right fine myosin component which is in blue. So, what I was trying to draw here you see all these random patches out here what you see here they are not actually random. Now, I am looking through the microscopic structure what you are seeing now. So, here you have the myosin filaments. So, the myosin filaments are not that simple as you think these filaments have a head like structure like this. We will come to the ultra structural details of those likewise, likewise we have this kind of a structure. Then yeah like this, like this these are myosin heads we will talk about what those head means likewise, likewise, likewise. Then you have them like this, like this, like this, like this. These are the structures which you will see in text book, but they are bit complex structure that is I am just taking the additional step to ensure that you people understand the whole biomechanics involved here. So, this is how the basic structure looks like. So, within this structure this is small this unit is called a sarcomere, this unit is called a sarcomere, sarcomere. Be very careful because that is why I am drawing it in real time. So, that you people understand this this is called z disc and these are the terminologies which I cannot help you just have to people have to kind of remember or you know appreciate it and this zone is called the cross bridge zone cross bridge zone and top of this particular part this part which is under a microscope these means. So, for example, I draw a draw a line like this draw a line like this where you only see under a microscope the actin bands you are not seeing the once again. So, this is called from here to here is called I band this is a lighter band then from here to here what is being called is called the A band I band A band and let me give the color coding this is your myosin this is your actin. So, we finish all the color coding this is very essential. So, now what I will do I will come back to this drawing I will come back to this drawing before I come back to this drawing I will kind of give you an idea the ultra structure because I do not want to make it very crowded let us talk about the ultra structure. So, whatever color codes again myosin let us talk about the myosin. So, when I was drawing the myosin it looks like I was drawing it like this I told you that there is a myosin head here. So, what I will do now I will draw the it is like this. So, this head has the ability to move like this it can take a shape like like this essentially it can move like this. So, this actin sorry this myosin head has a zone which is called where with me it is a complex drawing. So, this is called actin binding site and there is just underneath it there is something called a small zone out here this is called ATP binding site the energy molecule ATP binding site fine. And there are some other nitty gritty details which are not really essential for you people these are called light myosin you do not need to know this, but since if you come across in a book kind of do not get confused. And out here you have the heavy myosin these are different variants of myosin sorry myosin and this is the tail of the myosin which is dangling out there. Now, what we will do we will draw the actin simultaneously that will help you to kind of appreciate how the actins look like. So, here is the actin filament which is in green situation yes acting is in green fine. So, actin has something called this actin is something like if you have to take a sectional view it will be look like this it will be it will be more like spiral kind of structure of different proteins likewise it looks like this the way I am drawing it different protein. So, if I represent the proteins like sorry like this different proteins sitting out there now if you look at this structure the way it looks like is within actin you have something called a troponin there are proteins out here which are called. So, you can represent them troponin in accept troponin and tropomyosin and underneath that you have the calcium binding site this is very important because I told you that it is the calcium which leads to the mechanical energy transaction from electrical to mechanical energy transaction. So, calcium binding site is somewhere out here. So, these are kind of imagine this is the calcium binding site and on top of that is the troponin. So, what exactly happens this situation let us go back to the basic drawing again. So, what exactly is happening is this whenever. So, look at this drawing. So, whenever under a normal condition the contact between under normal condition they are not in touch fine there is no touch you look at this arrow there is a gap out here it physically they are not in touch they are they are kind of close to each other but they are not really bounded by anything they are loose they can move like this this is not a problem. But as soon as the situation comes like this calcium comes in now this is the part what you have to understand. So, now there is a calcium influx because of it. So, the calcium comes and bind to the actins now this is the calcium binding in red I am showing as soon as the calcium binds there it removes the blockage. So, the blockage is being created by. So, the binding between actin and myosin cannot take place because it is the troponin and tropomyosin which blocks that. So, for example, if my cell phone is here and if this is the myosin myosin filament then here is the actin which is my this hand which is an actin actin has this tropomyon in tropomyosin all this cell phone. So, it is like this fine. So, now the calcium comes as soon as the calcium comes what happens. So, my mouth is on calcium now. So, in other word what happens the calcium comes calcium removes this blockage like this it is exactly something like that happens at the molecular level the calcium comes and it removes the troponin tropomyosin blockage. So, as soon as that blockage is removed the next thing what happens at this zone if you look at in depth at this zone what is happening at this zone now the myosin head is in direct contact with the here is the actin and here is the myosin head is in direct contact with the actin. Now if you go back to the next thing. So, here it has a ATP so actin binding site. So, this actin binding site is now active. So, this is in touch with the myosin fine now at this stage there is a ATP molecule which binds here this ATP molecules helps to give it up once again. So, this ATP molecule back as soon as this is. So, now think of let this one bigger now this is in contact and there is an ATP molecule which comes and binds here let me represent ATP with some other color. So, that the confusion is not there and this ATP molecules leads to a motion like this and this motion imagine now this motion is taking place in all of them like this essentially these one can move other direction this direction. So, all of a sudden it gives something called a power stroke this is called a power stroke when this power stroke is nothing but your which leads to muscle contraction. Now if you think of it what exactly happened in this whole process. So, I told you that initially there is this nerves. So, let me go back to the slide. So, that will help you to under appreciate now what happened first slide. So, here the electrical impulse came and this is transmitted through the rhino this is transmitted through the rhino dine and sarcoplasm through the rhino dine and dihydropenidine receptors and then there is a rise in calcium and I showed you that calcium spike which took place that calcium spike leads to a motion in the muscle and that was essentially done because this calcium came because of the rise in calcium this calcium which got rose up here you see this calcium this calcium essentially went here and bind to the actin molecules and after binding to that and it has already I showed you it has an actin calcium binding domain out here within the actin molecule it binds to the actin molecules and remove the blockage between the actin and the myosin if this is the actin this is myosin there is a blockage because of some of the troponin tropomyosin as soon as calcium comes here it binds to it and then this get and it has a power stroke using ATP and that is how the muscle contraction takes place. So, this is essentially is a mechanical electrical to mechanical energy transduction process which is very very fundamental all across the living system and this is what I wish you people to appreciate. So, I will close in here I hope you people have understood this concept if you go through any picture online or book now everything will make sense to you. Thank you.