we're going to discuss the myofibrils structure in detail so what we're looking at in this picture is a muscle cell muscle cells are going to be covered by the sarcolemma within the muscle cell we find lots of myofibrils those kind of look like the straws the myofibrils are going to be covered by the sr which interacts with the t tubules now when we look inside of a myofibril what we're going to see are sarcomeres so we're just going to talk about the structure of the sarcomeres in myofibrils so myofibrils are composed of numerous sarcomeres basically they are the repeating functional unit of the myofibril and this is where the action of contraction takes place now sarcomeres are going to run from z disk to z disk they get their name because they look like a zigzag line in the middle of a sarcomere we have an area called the m line the m line helps us secure filaments in place now the m line is a structural area that we'll talk about in a little bit now also in the sarcomere you're going to see thin filaments which are composed of actin they kind of look like mardi gras beads the thin filaments are going to run from the z disc towards the m line in areas that we have just thin filaments we call this area the eye bands so i bands are the areas that contain just thin filaments which are primarily made out of actin we also have thick filaments thick filaments are going to be made out of myosin and the thick filament's entire length that area is going to be called the a band so from one end of a thick filament to the other end of the thick filament is known as the a band now what we'll see is that right here we have an area of just thick filaments it's not the entire length of it it's just the area that is only thick filaments and we call that area the h zone so the entire length of the thick filaments is the a band but just this area in the middle is called the h zone and that's composed of only thick filaments and if you look at it it kind of looks like an h here's one leg of the h the other leg and then the m is the going across the middle but we can see that we have thick filaments and thin filaments in the same area so this little box is known as the zone of overlap so one sarcomere will begin and end at the z disc the thin filaments which are primarily made out of actin look like beads and they run from the z disc towards the m line in the middle of the sarcomere we have the m line which helps secure our thick filaments in place the thick filaments are going to be secured by the m line they're going to move towards the z disc the area that is just thin filaments is known as the i band the area that is the entire length of the thick filaments is the a band the area that's just thick filament is the h zone and then the area where we have both thick and thin is the zone of overlap now think about thick and thin thick is going to be larger so it stains darker thin is going to be lighter so it stains lighter that's where we get the striations that we see in muscle cells skeletal muscle cells so you can see that the eye band is only thin filaments so it's lighter the a band is the entire length of thick filaments and within the a band we have the zone of overlap which is going to be the darkest area the h zone is only thick filament so we can still see that it's still darker than the i band now you also notice that this is a transmission electron microscope which is a super powerful microscope and if you look at it we are basically magnifying this 22 thousand times you can just imagine how tiny these areas are now what we're going to do is we're going to actually talk about what is actually found in the thin filaments and the thick filaments what is that m line made out of what is this little thing that looks like a slinky so there's a lot of other proteins present that we haven't quite discussed yet as usual you always want to kind of look at the interactions and animations provided by the publisher because they're really short tutorials that kind of put everything together in the big picture so within a myofibril we're going to find lots and lots of filaments we talk about the filaments in groups we have the contractile filaments the regulatory filaments and the structural filaments or we can just call them the proteins contractile proteins regulatory proteins and structural proteins we're going to start off with the structural proteins there are five major structural proteins nebulon dystrophin myomesin titan and alpha actin i'm going to talk about all of them but i only have pictures of few of them so we'll start off with looking at the ones we have pictures of right here the slinky is titan what we can see about titan is it gets its name for its large size and we can see that structurally it helps secure the thick filaments to the z disc so the thick filaments actually do not touch the z disc directly but there is some secure structural support by titan now the other thing about titan is even the structural protein it also helps with function think about that childhood toy that everybody had that could stretch and recoil and climb downstairs it was called the slinky well doesn't titan kind of look like a slinky and if you think about it titan has the same functions as the slinky it can stretch and recoil so it kind of looks like springs so titan helps with elasticity as well as extensibility so extensibility to stretch elasticity to bounce back okay the next one is myomesin that right here is what helps form the m line it kind of looks the chain links myomesin is found in the m line and it helps secure thick filaments together so it secures adjacent thick filaments in place i always think of myomessin as don't be messing with my myosin so it helps keep those thick filaments which are myosin in place next we have dystrophin so right here is a myofibril here is dystrophin and then there right here in the blue line is your sarcolemma what would you say dystrophin is doing myofibril sarcolemma in between them is dystrophin if you said it's helping secure the myofibril in place you're correct dystrophin helps create tension between the sarcolemma and the myofibril so when the myofibril moves the sarcolemma will move with it when it gets so when it contracts and it comes shortens sarcolemma comes in when it relaxes and lengthens sarcolemma goes out so it transmits tension between the myofibrils and the sarcolemma and the last one is going to be nebulin that i have a picture of okay if you can look right here nebulin is basically running along the length of the thin filament so it looks just like a string what nebulin does is it helps regulate the length of actin it's kind of like a ruler for actin so actin runs a long nebulin and that helps control actin's length so it can't get stretched out and not too much tension is going to be created because of the nebulin now the last one that we had a picture of i'm sorry that we don't have a picture of is alpha actin alpha actin is going to help secure that thin filaments to the z disk so we don't have a picture of it but it's basically a structural protein that secures thin filaments to the z disk so nebulin dystrophin alpha actin myomesin and titan are the structural proteins okay contractile proteins think about the name contractile what do you think these proteins are involved in if you said contraction you're correct these proteins are involved in shortening with force the contraction the movement of the muscle cells there are two contractile proteins myosin and actin so we'll talk about myosin first okay myosin is known as the thick filament and if you look at myosin it looks like a double-headed golf club so it's lots of myosin molecules bundled together and we can see that the tails will face each other at the m line and the heads will face out towards the z desk but notice they do not touch the z disc now what makes myosin molecules unique is that on the myosin head we're going to have a molecule called atp ace it's an enzyme this enzyme atp ace is going to be the site where atp binds to the myosin head so right there this is just a pictorial area is where atp ace is located and what this enzyme does atp ace is it helps us break down atp into adp phosphate and energy this process is called atp hydrolysis which just means to break down atp using water now why do we need to break down atp atp is energy when we cut off that last phosphate all this energy is released so atp carries the energy and when we need to use energy we break it down to adp a phosphate molecule and then energy that energy is going to be stored in myosins head so we break down the atp molecules to create a dp phosphate and energy that energy is going to be stored in the myosin head because the myosin's head has the ability to swivel back and forth so if you take your fist and then you bend your wrist and then straighten the back out and then bend it and straighten it and bend it that's basically how the myosin head moves we call it swiveling or flexing for the head to swivel or flex it has to have energy the energy comes from breaking down atp through atp hydrolysis so because of this what myosin is able to do is it eventually will actually attach to the thin filaments and when the head swivels it pulls the thin filaments on both sides towards the m line so when we talk about the contractile proteins myosin is the actual protein that does the work for the contractile proteins the other contractile protein is actin and that's the one that looks like mardi gras beads now actin is part of the thin filaments the reason we can't say thin filaments are just made up of actin if you look in this picture you'll notice that there's this brown string there's these blue areas and then we also have nebulin running through there that's not in this picture so thin filaments are made up of multiple proteins not just actin but when we look at actin we'll notice that it is attached to the z disc by alpha actin and then it runs us towards the m line but doesn't touch the m line you also notice that it overlaps thick filaments and we call that the zone of overlap now why is actin important well it's the other contractile protein that myosin is eventually going to want to bind to so if you look on actin you'll see that we have these little black dots these little black dots are called myosin binding sites so what do you think is going to bind to actin the myosin so myosins will attach to actin on the myosin binding sites now there's one slight problem you'll notice that in this picture right here those myosin binding sites are covered if they're covered myosin cannot bind to actin so we have our thick filaments made up of myosin the tails are in the middle heads face the z disc the myosin molecules have the ability to break down atp and store the energy in the head so the head can swivel when that head swivels as long as it's attached to actin and it swivels in it will pull actin towards the m line where does myosin attach to actin on the myosin binding sites problem is that they're covered so what covers these myosin binding sites on actin well those will be the regulatory proteins as you can see there's two regulatory proteins troponin and tropomyosin so let's talk about each of these okay what we're seeing here is the thin filament so the thin filament has actin and on actin we have the myosin binding sites but they're covered what are they covered by myosin binding sites are covered by the regulatory protein tropomyosin tropomyosin covers the myosin binding sites meaning that it's basically regulating whether or not myosin can bind to actin but tropomyosin has is going to be controlled by the other regulatory protein troponin troponin needs to change shape so it can actually pull tropomyosin off of the myosin binding sites now for troponin to change shape and pull tropomyosin off of the binding sites troponin needs what i call it's key are it's lover troponin's lover is calcium now where did we talk about calcium before with troponin well the calcium is coming from the sr so when that calcium gets released from the sr into the sarcoplasm it's going to find its lover troponin when it binds to troponin troponin changes shape once your opponent changes shape it's going to move tropomyosin when tropomyosin is moved those myosin binding sites are open so what we see in this picture is that the regulatory proteins are considered to be turned off because they're covering the myosin binding sites which means actin and myosin cannot link so myosin can never move actin towards the m line now what would turn these regulator regulatory proteins on calcium so for it to be turned on calcium will bind to troponin troponin changes shape pulls tropomyosin with it when tropomyosin's moved the myosin binding sites are open are exposed why is this because the calcium ions were released where do those calcium ions come from if you said the sr you're correct now once calcium attaches to troponin it moves pulls tropomyosin with it those binding sites are open so myosin can now attach to actin now before myosin attaches to actin that head needs to be stored with energy so what does myosin do to atp if you said breaks it down you're right and when it breaks it down energy is stored in the head which allows myosin's head to swivel when the head swivels it pulls actin towards the m line this is going to cause the shortening of the sarcomere so just a quick review we have our two regulatory proteins troponin and tropomyosin they're going to be found located on the thin filament they're going to be around actin tropomyosin is going to cover the myosin binding site troponin is going to control tropomyosin for tropomyosin to move troponin needs to attach to its lover which is calcium if calcium is not available everything's covered up and we consider this the off position if calcium is available it binds to troponin causes it to change shape this pulls tropomyosin with it exposing the binding sites so myosin can now attach to actin before myosin attaches to actin it needs to break down atp and it does this because it has atpase the enzyme breaking down atp into adp phosphate and energy that energy is going to be stored in the myosin head so myosin's head can flex