okay I need to be honest with you guys trying to record this portion of the chapter has always been difficult so if you notice I've not been uploading as much and this sub chapter uh particularly complection of skeletal muscle is because um it is a very difficult part of the chapter for me to teach uh not only because of the subject matter because it's quite a complicated subject matter personally in my opinion and there are a lot of things that I have to try to simplify and still make it relevant for exam questions because the problem with trying to teach confection of skeletal muscles is either from what I notice is um either teachers usually over complicated to a point where it becomes too esoteric to a point where it's lost in the students um or it becomes to oversimplify which perhaps can be an insult to the students intelligence so trying to balance the two between making it too complicated and too simplistic has always been a problem for me especially when it comes to this so I'm going to try my best to teach this one uh to the best of my abilities now when we talk about contraction of skeletal muscles so you will notice that I'll have to repeat a lot of things over and over over again like even though this is a video format and you can rewind the video I want to try to make it as organic as possible so I hope I hope it so if if I keep repeating a point over and over again like a broken record just understand that you know I'm trying to keep it as natural as possible now if you remember skeletal muscles have to contract so that they can move specific parts of your body so I'm throwing out a relaxed muscle over here you can see that and the orange parts are the tendons you don't have to know that in detail but in reality what exactly happens to a muscle when it contracts we know if some of my students who they into physical education and physical fitness they would know that when a muscle contact what actually happens is it bulges okay it bulges as you can see over there at the bottom but the point is I also want you to notice the length what has happened to the overall length of the muscle the overall length of the muscle has reduced the relaxed muscle is longer and the confed muscle is shorter so the point that I'm just trying to make over here is if a muscle needs to conact the overall length has to become shorter so you're like good I get that part but also remember that inside the muscle which the muscle fibers which are the citium of the cells they contained these things called myofibril as I'm showing it to you right now okay the myofibril I'm just going to draw out one myofibril so this is the myofibril of the Rel relaxed muscle I'm just taking the myofibril out and the reason why we just focusing on the myofibril is because you know the myofibril is the part that does the contraction for the muscles remember I told you the myofibril has areas which are darker and lighter we talked about the reason why they are darker and lighter lighter because of actin filaments darker because of myosin filaments correct so the light area the dark area all that's fine and then we also had the Z lines which are found in the light area and the M lines which are found in the dark area and of course the distance between the two Z lines are referred to as something called as the saroma okay I didn't mention that as well now so the question is how does the myofibril of the contracted muscle look like so will there be any difference because the if you see the contracted muscle it's shorter right so will there be any changes in the myofibril and they answer is yes right off the bat you notice something rather odd you notice that oh there are some dark areas and light areas that's fine but there's also some much darker areas in uh in the myof fibro so what exactly is going on over there where did that very dark area comes from and also look at the overall length of the myofibril the overall length of the myofibril I know it's just a section of it but the length of the myofibril has also become shorter so the question is how did did that exactly happen so what I'm going to do is I'm just going to take out one saroma section saroma is just basically the distance between one zet line and the next zet line right so I've taken that one out and I'm just going to enlarge the image to explain to you what's going on in the myofibril so you can see here beautiful here I'm complimenting my own F uh but you get my point um the you can see the light areas and the dark area that's fine dark is because of myosin light is because of the thin actin filaments all good but in the myofiber of the contractant muscle we notice that the dark areas the dark areas start to appear so what's the reason why the dark areas actually start to appear let's go through it the light areas as I'm pointing to you over there the light areas only contain actin filaments and the dark areas as I've mentioned again repeating myself the dark areas just contain myosin only but the much darker areas if you can see that I'm not too sure if you can see that through the video but the much darker area appear because of the overlap between actin filaments and myosin filaments that's why that area starts to become much darker and another thing I also want you to notice is look at the distance of the ZET line or the Sakia distance in the relaxed muscle that's the distance of the saroma but in the contracted muscle you notice that the saroma distance has reduced so the filaments have actually moved okay so the question here is how do the filaments move we need to talk about that in the next video but to summarize in this case here as you can see okay so what I'm going to do is if you're not exactly sure I'm going to draw out the the same structures again you have the myofibril just a small section of the myofibril right here it's relaxed you can see a dark area and you can see a light area now I'm I'm just going to do a S not I say animation but what I'm just going to do is is a very simple P where I'm just going to show you how the thin filaments move so look at the thin filaments they're moving inwards towards the that that part of the filaments moving towards the uh right side and then the actin and myosin filament overlap which creates an even darker area okay and I'm also going to do it for the other side of the uh actin filaments so you see that part moves to the right and and then that part also overlaps with the myosin filament to create an even darker area so this is what happens during the contraction of the muscle so that's why that so when the when the sucoma distance reduces the distance shortens right and when the when every part of the myofibril does this what happens to the overall length of the muscle the overall length of the muscle becomes short that's what happens so when it's confecting the Sakia distance reduces between the two Z lines and when it's relaxing the distance increases again so this is what I want you to understand about this another very important thing is sometimes in the exam they may ask you about something called zones so what we have done is we have split the zones in we have split the myofibril into different zones as well so what do I mean by these zones is as follows we have the three zones which are known as the a Zone H Zone and I Zone the a zone is usually the place where it can be the darkest and the a Zone usually contains myosin filaments it contains the entire length of the myosin filaments and it may also contain overlapped actins I need you to be careful with this part right here it contains myosin filaments always and it may contain over lapping actin filaments the H Zone which is the second darkest area and it contains only myosin filaments that's what I want you to understand and the eyesone is the lightest area because it contains only actin filaments so I'm showing you the myofibril a section of a myofibril right here I'm going to switch it okay relax at the top confecting at the bottom now where is the azone the the aone I told you it must contain the entire length of the myosin filament and it may also contain any overlapping actin filament so that part is the a zone right there for the relaxed muscle because at this point it's just containing only the entire length of myosin filament but at the bottom for the contacting muscle notice like I said based on definition it contains the entire length of the myosin filaments and it will also contain overlapping actin filaments that is the azone as well so either when the muscle is relaxing or Contracting the azone does not change the a Zone's length always remains constant you need to understand that it's a little bit confusing I know but you need to understand that part let's move on to the H zone for the relaxed muscle by Definition h Zone only contains myosin filament so during the muscles relaxed state in the myofibril the H zone for the relax muscle will also be the same almost the same length as the a Zone because it only contains myosin filaments but in the confected muscle notice the H Zone has become smaller because lesser parts of the myofibril contains only myosin so that section is the H Zone as the muscle conact the H Zone will become shorter that's what we have to understand and of course eyesone for the relax muscle musle acting filaments only the light areas only that's the length of the eyesone and when it contracts the eyesone also decreases as well because certain parts of it will overlap with the myosin filament so if we just take the actin filaments only the length has decreased so please know your zones in detail as well a Zone will always remain constant no matter what whether it's Contracting or relaxing some students will be like but you said that the a Zone has to be the darkest so shouldn't it include only the very dark area or the darkest region it should but for some reason we have decided to say that the a Zone remains constant okay Just Go With It uh for the purpose of your exam it's a little bit confusing I know but I hope you understand that part