so in the previous video we talked about the structure of the skeletal muscles we looked at the internal parts and I just want to focus a little bit about this thing called the myofibril and the myofibril has this kind of pattern right of dark and Light which I've mentioned before now in reality the myofibril is something that encompasses the entire skeletal muscle withth and length and if I were to remove the new nucleus and the mitochondria and the T tubu the myofibrils can actually cover the entire breadth of the cell as I'm showing you in this diagram here so the nucleus mitochondria are all still present I've just not included them in this particular diagram as you can see so um like I said earlier the myofibril forms a pattern of dark and light areas and if I were to show you a transmission electron micrograph of a skeletal muscle you you can see that that is taken using a temm transmission electron microscope and you can see the pattern of the myofibril as well now what exactly is this myofibril the myofibril is actually the contractile units which means to say that this is the part of the skeletal muscle that does the actual contraction how it does that we will look at that in the next video this is just looking at the myofibril and the myofibril as you can see here it has those patterns of dark and Light so that part where I'm circling forms the dark area I hope you can see that based on the actual diagram as well and I'm also circling the light area to compare it yeah of course it's not aligned very beautifully in my diagram and the actual picture of the skeletal muscle fiber but I hope you can appreciate that even in the transmission electron micros micrograph you can see the patterns of dark and light that's the first important thing you have to see that now the question over here is as follows why EXA ly as some parts of the myofibril dark and some parts of it light now the reason for it is okay I'm going to take out one small part of the myofibril just cut it open you can see dark and light and what I'm going to do is I'm going to draw it out in a more I'm going to magnify it and just draw another simplified approach of it now you can see that the dark areas and the light areas actually have those lines and those lines are called the protein filaments that's that is why people associate proteins to muscles uh because a lot of times students will say oh we need proteins to build muscles yeah I mean you need proteins for a lot of things but muscles require a lot of proteins as well because the myofibril contain these things called the protein filament in reality the reason why some parts of the myofibril is dark and some parts of the myofibril is light is because of the thickness of the protein filament so when the protein filaments are thicker that causes the area to actually they look darker and when it when that part of the myopel is thinner it causes the that area to be lighter so if a question asks pertaining to the myoi why are some parts dark and some parts light you just have to say that the darker parts have thick protein filaments known as myosin and the light area of the myofibril has thin protein filaments called actin and you need to remember the names of the filaments okay actin filaments are the thin filaments myosin filaments are the thick filaments so if I were to just look at the picture again so look at the part of the actual temem of the skeletal muscle fiber so that part that part is dark so the dark areas over there have myosin filaments which are the thicker filaments and the lighter areas have actin filaments which are the thin filaments that is why the pattern of dark and light emerges in the transmission electron micrograph so this is the first thing that we have to understand about the pattern of the myofibril the other things to know about the myofibril are as follows we have these things called the ZET line and the mline the Z line adjust this uh perpendicular structure that runs along the actin filaments and it holds the actin filament the mline is just a structure that holds the myosin filament in place that's basic basically it another important thing to also know is the distance between the two zet lines okay between one Z line and the other Z line that is known as the saroma so as you can see in this diagram over here if you can see in this diagram the lighter areas which I've circled in green those two are the light areas and that line running perpendicular to it is known as the Z line right that one's also the Z line over there but in the dark area you can see a line running perpendicular to the whole thing that is the m line there we go and of course the distance between the two Z lines are known as the salomia now the reason why we have to talk about this is because when we study the contraction of muscles further in detail in the next video uh the Z lines will be more important to be explained