so in this video we're gonna talk about how muscle fibers contract and I we need to talk about the micro anatomy of skeletal muscle cells before we can really talk about like their contraction mechanism and we know that skeletal muscle fibers are long cylindrical cells that contain multiple nuclei because these cells can be up two feet in length we have special terms to describe some of the components of these cells like the sarcolemma is the muscle fiber plasma membrane the sarcoplasm is the muscle fiber cytoplasm and skeletal muscles actually have a lot of glycans and these are involved with glycogen storage that way these cells can you know have a source of energy during you know peak activity and myoglobin is used for excess co2 storage that way skeletal muscles that rely heavily on oxygen can continue to you know produce contraction force even during peak activity now skeletal muscles have modified organelles like Maya fibrils are these long protein cylinders that fill most of the muscle cell cytoplasm and sarcoplasmic reticulum is the modified endoplasmic reticulum of muscle cells whose function is to store calcium and then T tubules are essentially tubes of sarcolemma or muscle fiber plasma membrane that traverse deep into the cell and their purpose is to basically transmit electrical impulses within the cell now myofibrils are these densely packed rod like elements of protein and essentially these can come to cell can contain thousands of these and they account for about 80 percent of the muscle cell volume now myofibrils are the are the proteins that give these muscle cells their striation appearance as well as contain what's called a sarcomere which is the functional unit of muscle contraction now sarcomeres are made of myofilaments we have thick and thin Maya filaments and we'll talk about the molecular composition of these myofilaments as well now this is showing the muscle cell or a fiber remember the specialized plasma membrane of the muscle cells called the sarcolemma and deep within this you're gonna find all of these protein cylinders you're called the myofibrils now myofibrils are made of repeating segments of sarcomeres so sarcomere would be about from here to here repeating across and the sarcomeres are made of thick and thin myofilaments which we'll talk about the anatomy of here pretty soon now striations the stripes that are formed by repeating dark and light bands along the along the length these myofibrils so we see that if this is one skeletal muscle cell that contains thousands of miles you know that are the repeating segments of dark and light areas are basically what make the striations on this muscle cell and it's essentially all the sarcomeres that are lined up in repeating in a way that allows these muscle cells to you know produce a tremendous amount of contraction force now the sarcomere is the smallest contractile unit or functional unit of a muscle fiber or cell it contains a bands with half of an eye band at each end and these are actually found between what are called z-disks now individual sarcomeres align end to end along the myofibril like boxcars of a chain that way when one sarcomere contracts is going to pull on its surrounding sarcomeres in a reality really is just sarcomeres all along a length of this myofibril that contract around the same time ultimately allowing the whole myofibril itself to shorten which makes the muscle cell shortened so if we can zoom in on this myofibril remember this is actually the protein cylinder that would fill your muscle cell and your each muscle cell or fiber contains thousands of these myofibrils and we're just looking at a piece of the myofibril now the sarcomere is from z-disk here to the z disk on this side so the distance between these z discs is actually the sarcomere and the sarcomeres are basically made of thick filaments shown in red as well as thin filaments that are shown in blue and these thick and thin filaments actually overlap and they interact with each other in a way that allows for this whole sarcomere to kind of slide and pull against itself so that the Zetas get pulled closer towards the M line here so the whole sarcomere shortens but because these are made of repeating segments of sarcomeres the whole myofibril also shortens so the Iban is actually found here between two different sarcomeres it's basically the space between the thick filaments on this side versus the thick it's on this side and the Aban is basically just the distance of one thick filament from end to end the H zone is the basically the space here in the middle the sarcomere where the thin filaments you know don't quite touch and there's a space between the thin filaments at either side and the reason why we define these zones is that you know we can look at these in terms of how the sarcomere changes before and after contraction and this is not in a contracted state this isn't actually more in a relaxed State now the mile filaments we have acted in myosin mass filaments the actin mile filament is also called a thin filament that was shown in blue in that picture the myosin mile filament is the thick filament that's the one that was shown in red in that picture now the actin myofilaments extend across the eye band as well as part of the a band and these are actually anchored to the z disks so that the act and bio filaments are the things that are being pulled on which allows easy disks to get pulled closer together within the sarcomere and the myosin filaments are the thick ones which extend the length of the a band and they're connected at the em line which is the very center of the sarcomere so what happens is the myosin mial filaments have little elements that actually pull an actin and that allows the whole sarcomere to shorten now sarcomere in cross-section is more of a hexagonal arrangement of one thick filament surrounded by many thin filaments like up to six of these and we're just looking at one angle here of the actual sarcomere we're in cross-section you'd see that that's actually little more complex and densely packed so here's our thick filament also called Amaya myosin filament and here's our thin filament also called the actin filament now the myosin filament has all these little extensions here which are called the mouse and heads and these are basically enzymes that use the chemical energy in ATP to produce a ratcheting kind of motion to grab on the thin filament here and it basically pulls the film thin filament closer towards the M line now the myosin filaments have heavy chains that intertwine to form a tail that's just the main tube of this protein chain and the light chains form this myosin globular head group now the head group is actually what uses the chemical energy in ATP to form what's called Brij and these cross bridges are basically when the myosin head forms a bond or association with the act in her thin filament now myosins are offset from each other results in a staggered array at different points along the filament now the thin filaments are composed of a fibrous protein called actin axons of polypeptide that's made of kidney-shaped g actin globular subunits and these actually have the active sites for the mouse and hence the bind and g actin subunits linked together to form long fibrous f-actin or filamentous actin and to F actin strands twist together to form the entire thin filament so in reality we're saying that that F actin is made of you know lots of G actin that are all stuck together like beads in a chain where if the G actin is the bead the f-actin would be the chain in two chains or f-actin filamentous strands wrap around each other to form the whole thin filament now we also find other proteins associated here like triple myosin and troponin and these are regulatory proteins that are bound to actin and they play a role with you know preventing or allowing muscle contraction to occur you know when necessary so if you look at the thick filament here we see that our we have our myosin heads and our myosin heads are the light chain or globular portion of the myosin protein filament and these are the ones actually use the chemical energy ATP to move back and forth and they produce this ratcheting motion that allows them to grab on to a nearby thin filament and basically pull like little hands so imagine these are all like little hands that are pulling on our thin filament now they're staggered in their position which allows many different positions of mash and heads to pull on the six different thin filaments that would surround this one you know thick filament so on the thin filament here we have our actin filaments and essentially member it's made of two coiled F actin strands and these f-actin or filamentous actin strands are made of g actin subunits that's what all these are now g actin has the active binding site here for the myosin to attach to these little divots here are places where the myosin heads could attach or form an Associated bond between the thick and thin filaments in fact if you go back here you can see that your mouse and heads you know could fit pretty well within these active sites however if you look here there's actually proteins that are blocking the active sites on this thin filament so we have this long kind of gold colored or you know yellow colored protein here which is basically called tropomyosin then the function of triple myosin is actually to block the active site on actin during muscular rest that way your muscles aren't always contracted and then troponin is a nearby protein that's associated with tropomyosin and troponin calcium-sensitive now the function of troponin is to bind calcium which causes a change in the conformational state of this protein that allows for tropomyosin to move out of the way to expose the active sites here that way the myosin heads could potentially bind but if the muscles relaxed and there's not calcium present troponin and tropomyosin relaxed and they basically blocked the active sites so looking at you know an electron micrograph picture here you can see this is the thick filaments with all your mouse and heads sticking off and then here's our thin filament which is what they're attached to basically you're looking at muscle in its crossbridge you know where these two are connected to each other in their contracted state under an electron micrograph now the molecular composition of the myofilaments can also include things like elastic filaments or dystrophin the elastic filament here is a protein called Titan and Titan is what gives muscle tissue its elasticity because it holds the thick filaments in place and it helps to recoil after stretch now Titan also resists excessive stretching so plays a very important role in stability of your circles now dystrophin links the thin filament proteins to the sarcolemma remember sarcolemma is the muscle cell plasma membrane and if dystrophin is you know mutated or non-functional as he find like in most muscular distrophy we see that the muscle cell plasma membrane or sarcolemma is a little weaker and it's more likely the cells can rupture and guy now other proteins like nebula in my abusin see proteins bind filaments and sarcomeres together and they maintain proper alignment of the sarcomere so they're very important proteins here now the sarcoplasmic reticulum is also their specialized feature of skeletal muscle cells and you find these in a little bit and smooth and cardiac muscle as well it's basically a network of smooth endoplasmic reticulum tubules that surround each myofibrils now most of these run longitudinally and they have these terminal cisterns that form perpendicular cross channels at this AAI band junction and the whole purpose of this you guys the 4sr is to store calcium so sarcoplasmic reticulum serves as a storage side of calcium within muscle cells and it releases calcium when necessary and calcium is a stimulus for muscles to contract the t tubules are also highly associated with sarcoplasmic reticulum and essentially what these are and as they sound like our tube which is a protrusion of sarcolemma or membrane deep into the cell interior and what they do is that increase the muscle fiber surface area now they're continuous with the extra souther space and they allow for electrical nerve transmissions to reach deep into the interior of each muscle fiber these t tubules penetrate the cell's interior at this a Iban junction and they form what's called a triode which is basically the place where t tubules meet the terminal cisterna of sarcoplasmic reticulum now this is significant because it's actually the electrical impulses are currents that go down the t tubules eventually getting the terminal cisternae of your sarcoplasmic reticulum to trigger the release of calcium into the muscle cell which allows for these muscle cells to begin a contraction now the tree out here is important because these contain integral membrane proteins that protrude into the inner membrane space and your SR is nearby which allows for these integral proteins to control the opening of calcium channels in the SR so that once electrical currents travel deep into these t tubules it can trigger the opening of these SR cysts turns that way calcium can leak out of the sarcoplasmic reticulum into the sarcoplasm and then bathed the myofibril and in calcium now this is actually all stimulated by an electrical impulse that travels down that T to Buell and thus causes a change in the shape of these SR proteins which cause calcium to rush in the cytoplasm so looking at a picture here in blue is the sarcoplasmic reticulum and these things that are kind of grayish in appearance are the t tubules you see that t tubules dive down deep into the muscle cell and they're associated with your longitudinal SR now it's important to note that is when electrical impulses or currents travel through these tubules all along the way they're triggering they're triggering the sarcoplasmic reticulum to release calcium and then calcium is basically going to get washed into this myofibril protein tube which allows for its association with you know troponin to get the muscles of the contracts so contraction is the activation of cross bridges to generate force and shortening occurs when tension is generated by these cross bridges on thin filaments and the contraction ends when these cross bridges become inactive now we call this the sliding filament model of contraction and it states that during contraction thin filaments slide past the thick filaments causing actin emmaus and overlap more then either the thick nor thin filaments change in length but they just overlap more due to a change in their position now when the nervous system stimulates the muscle fibers these myosin heads are allowed to bind to actin which forms a cross bridge and it causes a sliding motion or contraction due to the ratcheting movement of these mouths and heads now cross bridge attachments form and break several times and each time they pull the thin filaments a little closer towards the center of the sarcomere in a ratcheting action now this causes shortening of the whole muscle fiber which then would ultimately cost shortening of the whole muscle and these z-discs are pulled closer towards the M line remember z-disks here on the side and they actually are pulled towards the center of the sarcomere the Ibans walls are shortened and the H zone has disappeared when the when the thin filaments overlap a bands also move closer to each other so as we're sitting here in the sarcomere from z-disc to z-discs you know here's our a band which is the thick filament with our myosin heads when mouse and heads bind to actin they have a ratcheting motion which pulls on these these thin filaments towards the M line of the center of the sarcomere and it's actually gonna pull our z-disks closer together causing this whole sarcomere to shorten up November what holds your thick filament in place the mass mouse and film it in place is this large elastic protein called Titan and it serves as a large molecular spring now this is actually showing the sarcomere in a contracted state where you know the myosin heads have pulled on thin filaments towards the M line our H zone has disappeared because of the thin film and several apt and we can see that our Z discs are now closer together which means that the entire circa mirror is a little shorter than it was before