so in the previous video we talked about how during the confection what exactly happens is uh in the myofibril the filaments will sort of move where the actin filaments and the myosin filaments slide along each other and overlap along each other causing the entire length of the saroma to reduce and as the sarcoma reduces if this process happens throughout the entire length of the myofibril the overall length of the muscle also decreases which causes the muscles to contract so that's what we know so um in this video what we're going to do is we're going to talk about something called as the sliding filament theory the sliding filament theory attempts to explain how the myosin filaments and actin filaments move closer together the sliding filament theory explains that the myosin head pulls the actin filaments close together making the saroma distance shorter so in a way what I'm doing here is I'm drawing out the mline and also the myosin filaments how do you know this is the myosin filament because it's thick and it also contains the myosin heads you can see that over there and what I'm going to do is I'm going to throw out the actin filaments and how do we know those are the actin filaments because they are thin and the in filaments are supported by the ZET lines and of course the distance between the two zet lines is the saroma or the zomia is just the band or the dis between the two zet lines that's it it's just the distance of the dis between the two zet Lines Just you can Define it in either ways that's fine now point is I want you to notice something important here right now right off the bat you notice that the myosin hit and the actin filaments are not tght touching each other at least not yet the sliding filament theory says that for muscle confection to happen the myosin headit and the actin filaments must first join to each other or they must bond to each other or they must form a cross link with each other there are many ways you can Define it either either ways are fine so I'm just going to say that the myosin headit attaches to the actin filament first how it attaches don't care yet we will talk about that later we are simplifying this so the first thing is the myosin heads must attach to the actin filament or you can also say in the exam cross link between the actin filament and myosin heads so far so good now what happens then is the myosin head will tilt backwards and when the myosin head tilts backwards because it was attached to the actin filament what happens it pulls the ACT AC in filament and as it pulls the actin filament the entire length of the thin filament together with the Z line gets pulled together I'm going to show it to you again this is before they got attached then they attached to each other and then the pole happened before the pole after the pole before the pole after the pole this is fun okay now so then what happens is that the end of it no the first myosin heads will then actually let go of the actin filament and they will start the process again and keep pulling the filament as you can see the thin filaments are continuing to move in this case and as the Thin filaments continue to move what actually happens is the distance between the two zet lines become even shorter there you go and then now you notice that the other myosin hits can also attach to the actin filaments those myosin HS will then help to pull the actin filament making it easier for contraction to happen this is called the sliding filament theory so the sliding filament theory as an introduction by the way explains that number one what actually happens is you have the actin filaments the myosin hits attached to the actin filaments and pull them and tilt and pull tilt and pull tilt and pull tilt and pull until it causes the distance of the ZET line to become shorter or the saroma distance too decrease this is a generalized explanation of the sliding filament theory this at least I always tell my students if you find this topic difficult at least for the purpose of the exam you can't get the full marks let's say you are too weak in this particular topic you must at least remember this part okay because I think this part is quite straightforward this part just says that the myosin head must attach to the actin filament the myosin head must tilt backwards and pull the actin filament and as this process repeats over and over again the saroma distance decreases and when the saroma distance decreases that is muscle contraction this is the minimum that you have to all know when you're describing the actions of the actin and myosin of course we have to go deeper okay because usually when they ask you to describe sliding filament theory it's about 6 to 7even marks so we are going to talk about exactly how the process happens because there are a few questions that we have to explain earlier the myosin heads and the actin filaments were not touching each other right what causes them to bond to each other what causes the crossling to happen what causes the myosin head to tilt backwards what causes the first myosin head to let go and reattach to the actin filaments and keep pulling it again so we need to talk about all these processes in detail as well so what we're going to do here is we're going to look at the sliding filament theory in detail okay because we need to explain how that happens I am going to take out just one section of the actin filament and also uh one section of the myosin filament so as you can see here this is just a part that is focusing on what happens within the one actin filament and the one myosin filament because we want to see it in detail right as a reminder the actin filament uh is this thin filament that contains the actin proteins the small black circular things it has those orange y shaped structures which are called the troponine which excess calcium ion receptors and it has those purple things if you if you're color bline it's okay but those purple things are just uh going covering sort of covering the actin filament and um and those are not the actual Colors by the way so don't worry you don't have to say it's purple um and those uh purple color things which are covering the actin filaments are known as tropomyosin and I did mention that the tropomyosin is to prevent the myosin hit from attaching to the actin filament then we also have the myosin HS which are located on the thick myosin filament and myosin hits face away from the mline and they also the head is also an enzyme which is atps atps which just means they can h nalize or break down ATP okay we've introduced all this in the previous video now let's get into it so just as a reminder sliding filament theory is what happens when the myosin head must first attach to the actin filament and it must pull the actin filament so how does that happen so in this case over here what we I'm just doing is I'm just simplifying the structure and I'm just zooming in even further and I'm focusing on one of the actin filament and one of the just a small small part of the myosin filament and just focusing on one of the heads now right off the bat immediately you notice that before the myosin head does anything one ATP molecule has to be attached to the myosin head if there is no ATP on the myosin head this entire process will not happen because remember we are talking about muscle contraction and ATP is required for muscle contraction at the very start one ATP molecule must be attached to the my in head before anything begins okay so what happens first the purple color the thick purple color part which is covering the actin filament is known as tropomyosin I did say that the tropomyosin is to prevent the myosin head from attaching to the actin filament and look are the actin filaments and the myosin head touching each other no they are not there's a small little Gap there yes but they are not directly touching each other yet the first thing that needs to happen is calcium ions will have to bind to something called troponin now calcium ions will actually come from something called the sarcoplasmic reticulum I'll talk about the SR or sarcoplasmic reticulum later for now we are just focusing on the sliding filament theory so calcium ions have to initiate the beginning of the muscle contraction so as the calcium ion binds to the troponin because the troponin is the receptor it causes the tropo iosin shape to change so see earlier the shape of the tropomyosin was covering the actin filament now the shape changes a little bit and the that change of the shape is significant because it exposes the actin filament to the myosin head as I've highlighted over there and because the head is now exposed to the actin filament what happens is a cross link is able to form between the myosin head and the actin filament or you can also say in the exm the myosin head is now able to attach to the actin filament now after they have attached to the actin filament I think I've talked about this earlier while ATP is still attached to the myosin head the myosin head is now able to tilt backwards and as it tilts backwards towards the mline or it just moves backwards it will also pull the actin filament because like I said the actin filament was attached to it it will pull the actin filament and this is known as the power stroke so let's go back to this structure over here as it pulls the actin filament look at it what happens the distance between the two Z lines become shorter so the saroma distance reduces that is what's happening the actin is now overlapping on the myosin so let's go back to this as you can see the actin is now also overlapping on the myosin over here so you see after it pulls the actin filament it has to kind of go back to its original position and pull it back again like rowing a boat so what happens is the myosin head will then hydroly the ATP into ADP and phosphate and it will as it hydes the ATP into ADP and phosphate it will detach you can also see it's no longer attached to the actin filament it will release the actin filament and then it releases the ADP and phosphate as well okay it releases it into the pyop plasm or the cytoplasm and that about it so all you just have to mention in the exam is the myosin hit hydrolyzes the ATP into ADP and phosphate and releases it this also causes the myosin head to detach from the actin filament so that's good that's fine but as you can see over here right now it's no longer touching the actin filament and that's not good because you want it to you know reattach to the actin filament so what causes it to reattach to the actin filament so that it can continue to pull the actin filament well the answer is very simple in the muscle in the muscle fiber we have a lot of uh mitochondria and the mitochondrian will generate ATP and the ATP as you can see here it goes towards the myosin head and reattaches to the myosin head and as it reattaches to the myosin head look at what happens to the shape of the myosin head it's able to you know stand up again and it's able to reattach to the actin filament and start the process a new new ATP attach reattaches to myosin head allowing a new cross link with the actin filament and the process repeats over and over and over again until it reaches the maximum of the contraction or if the muscle runs out of ATP this is referred to as the sliding filament theory so I've summarized it for you over here in this diagram and when you're writing out your notes it will it will actually be helpful if you um you know uh write out the caption as to what happens for each of the picture so for number one calcium ions bind to the troponin on the actin filament number two tropomyosin changes the shape and exposes the actin filament to the myosin head number three the myosin head attaches to the actin filament or you can also say cross links happen between the myosin head and the actin filament process number four the myosin filament will tilt backwards pulling the actin filament in the process as well and this causes the distance of the saroma to decrease process number five the myosin head which also acts as an enzyme which is atps Will hydroly and release the ADP and phosphate and this also causes the myosin head to detach from the actin filament see it lets go of the actin filament and process number six ATP reattaches to the myos H which causes it to also form a new cross link with the actin filament and start pulling the actin filament again then the process repeats itself and as the process repeats itself over and over and over again the distance the actin starts to overlap more with the myosin the thin filaments and the thick filaments are sliding along each other that's why it's called the sliding filament theory and that is why the sarcoma distance decreases and of course as the saroma distance decreases the overall length of the muscle also decrease and that is muscle contraction right there woo that is a lot but sometimes some students will be quite um honest and they'll tell me teacher this is too difficult for me to explain I can't memorize the whole thing now if you can't memorize the whole thing um a cop out or a sort of easy way out you will not be able to get the entire if sliding filament questions come out and if they ask you to explain sliding filament theory usually it's about six marks if you can't get all the six marks it's okay let's try to get at least three to four marks so the three to four marks that you should get is as follows at least you must remember that the calcium ions will attach to the troponin at least mention that and it causes the myosin head to bind to the actin filament if you want to skip the tropomyosin you will lose a mark there if you do not talk about it but it's okay so you can talk about how the calcium ion initiates the whole process at least referring at least a reference to the calcium ion is important the calcium ion binds to the phonin and it causes the myosin head to directly bind to the actin filament okay just mention that you'll at least be able to get one Mark over here and then you just have to mention that the myosin hit tilts backwards pulling the actin filament and number four the coma distance decreases at least if you can mention this you will not be able to get six marks but you'll at least be able to get three marks if you are lucky okay but if you want the entire six marks if you are asked to explain sliding filament theory in the exam then you have to explain process number one to process number six as pictured over here okay so let's do this again as we can see over here I'm drawing out the thick myosin filaments and the thin actin filaments so what happens first remember calcium ion has to bind to the troponin and as the calcium ion binds to the troponin what happens then is the Thom myosin changes shape exposing the actin filament this causes the myosin hit to attached to the actin filament then the myosin headit with ATP attached to it will pull or tilt backwards and it will pull the actin filaments as well and the distance of the s coma will decrease and of course the myosin head hydes ATP and releases the ADP and phosphine and it also causes it to detach from the actin filament and then the new ATP molecule comes in and the process starts again so I'm just writing it out over here there we go so I'm just highlighting it so look at the saroma at the at the beginning as I'm highlighting in yellow over there at the top the suoma distance was that long right the between the two Z lines that was the distance uh it was that long but as the myosin head pulls the actin filament look at the distance of the saroma it has reduced so please mention that in the XM too that will give you a point and of course uh this and this is the entirety of the sliding filament theory okay and of course uh some students will ask does it just stop here of course it it doesn't stop here because you still have those other myosin heads and those other myosin hits will also join in and pull the actin filament until the muscles cannot contract anymore there is a limit to how much your muscles can contract anyway so I hope you understand this part of the sliding filament theory