hi everyone dr mike here in this video we're going to talk about everybody's favorite physiological concept when it comes to skeletal muscle which is the sliding filament theory but before we begin we need to talk about some really important terms that we need to wrap our head around before we can move on first thing is what you're going to find is when we start talking about skeletal muscle there is some important prefixes that you need to understand these prefixes include that of sarco and that of myo now importantly what you're going to find is sarco is greek for muscle myo is latin for muscle and they seem to be used as a prefix so in front of words when we start talking about skeletal muscle in various aspects so for example there's a term called the sarcoplasmic reticulum that is simply the endoplasmic reticulum for skeletal muscle we have the sarcolemma that is simply the plasma membrane of skeletal muscle right we've got this sarcoplasm that's the cytoplasm of skeletal muscle now when we look at myo we've got a myocyte that's a muscle cell we've got a myofibril which is what myocytes are made out of and we've got myofilaments which are the contractile proteins that allow for muscle to contract so just so you know these two prefixes pretty much mean the same thing etymologically so their origin is just slightly different all right sarco is greek myo is latin now that we've done that let's talk a little bit about some anatomy so what you can find here is we have a whole skeletal muscle right here this is the bicep right sitting right here importantly we take this whole muscle and we transplant it right there what you'll find is that whole muscle is comprised of multiple bundles that we call fascicles so let's just pull one of those fascicles out we're going to find that these fascicles are made up of multiple muscle fibers and we pull one of those out now importantly when we talk about muscle cells aka myocytes we're talking about a muscle fiber so here's the thing this muscle fiber is the muscle cell so a muscle fiber is also known as a myocyte and also known as a muscle cell right there and you can see as a muscle cell if you remember back on the three different types of muscle of the body and you talk about some of the differences you know that skeletal muscle is cylindrical as you can see here it's striated it's got those stripes and it has multiple nuclei which you can see here by those purple dots now interestingly the muscle fiber is made up of more components these components are called myofibrils and i'm pulling one of those myofibrils out and you can see it's the myofibrils that have those stripes also known as striations and the myofibrils are made up of myofilaments and these are the contractile proteins that you can see here now the myofilaments the contractile proteins are actin and myosin if we label those this one here is actin and this one here is myosin and importantly myosin is the thick contractile protein actin is the thin contractile protein and we'll get there in a sec so overall when skeletal muscle contracts what's happening is these proteins are shortening which shortens the myofibril which shortens the muscle fiber and importantly muscle fibers will run where 98 percent of muscle fibers run the full length of the whole muscle so that means when this muscle fiber shortens the fascicle shortens the whole muscle shortens and because skeletal muscles cross at least one joint usually you've got movement of the skeleton and that's the whole purpose of skeletal muscle now let's move back here to the myofibril a couple of things you're going to see these different banding patterns and you can see this under the microscope now these different bands or regions have different names all right so for example we've got the a band here we've got the i band which is actually sitting around about here now here's the thing an a band is the darker band right it comprises interestingly if we have a look at this a band of regions in which the myofilaments the myosin and actin which you can see here so in green that's the myosin in blue that's the actin you can see that there's parts in which they overlap these parts that they overlap is going to be where you find dark bands these dark bands here of the a-band now areas in which they don't overlap so these areas here which is going to be these areas here right now you can see z-disc z-disc is this line here why is it called zdisk because it looks like a z right so there's the z disk right there so z disk is corresponding to that area there so this white band here is corresponding to this band here where there's no overlap between those two protein filaments that's also known as the eye band all right where it's lighter where it's darker the a band that's where we've got all this overlap now you can see in the middle here this is called the m line and i like to call the m line because you've got those three lines and if you just to connect them up it looks like the letter letter m so that's the m line simply the center point of this the reason why i've highlighted one z disk to another z disc is because when these proteins bind to each other and contract or shorten which is going to be the focus of this lecture we're going to get there in one second this is the smallest unit of contraction which we call the sarcomere the sarcomere is the smallest unit of contraction all right let's now have a look at this importantly we need to know how does this thing bind and contract so first thing is we need to look at that muscle fiber and you need to remember from one of my previous lectures the neuromuscular junction that you're going to have a motor neuron a lower motor neuron that's coming down and it's going to be speaking to this muscle fiber it doesn't bind to it there's a little gap so it goes from neuron gap muscle amazon is the neuromuscular junction so watch my neuromuscular junction video if you don't understand it i'll give you a very quick run through here this is a lower motor neuron lower motor neuron and the signal of the low mode neurons coming from the spinal cord and had previously spoken to the upper motor neuron which originates at the brain at the motor cortex sending electrical signal down now once it gets to the end it releases a neurotransmitter called acetylcholine an acetylcholine will bind to acetylcholine receptors on that muscle fiber now the sarcolemma what was the sarcolemma again that was the plasma membrane right of the muscle fiber once acetylcholine binds to acetylcholine receptors it opens up sodium channels and sodium rushes in now remember back on action potentials if you haven't watched my action potential video watch that action potential sodium rushes in now when you get sodium rushing in it brings its positive charge with it because we all know that sodium has a positive charge right so now when you've got let's just say that this is the muscle cell and you've got sodium sitting outside and now sodium is rushed inside it's taken that positive charge with it and it depolarizes the membrane it doesn't go deep within the cell it just depolarizes this membrane right here so you get this depolarization of the sarcolemma but importantly what we need to realize here is the polarization just of the membrane isn't good enough because if you have a look we want muscle to contract whole muscle to contract muscle isn't just the membrane you can see it's made up of all these individual thousands in actual fact myofibrils so we need depolarization to go deep within the muscle fiber itself so what you're going to find is that there are these tubules these tunnels that run all the way through this muscle fiber they're called t tubules and the action potential doesn't just go across the sarcolemma it runs down these tubules and goes deep deep deep within that muscle fiber importantly now once the depolarization has happened deep all this is the neuromuscular junction and in that video i'll go into great detail once it goes in it releases calcium now importantly you've got depolarization going down the t tubules it reaches something called the sarcoplasmic reticulum what did i say that was that's the endoplasmic reticulum for muscle fibers the sarcoplasmic reticulum you need to remember remember is it just pulls or stores calcium and whenever calcium is released muscle contraction will occur so we're getting there right so we've got all this depolarization happening and you've got the sarcoplasmic reticulum sitting deep within the muscle tissue it's holding on to all this calcium once the depolarization is released calcium is released all right stay there keep that in your mind what we've just run through is an actual potential has just gone down the lower motor neuron released acetylcholine acetylcholine binds to acetylcholine receptors allows for sodium to move in to the sarcolemma depolarizing it it needs to go deep within the muscle so it goes down t tubules and depolarizes deep within that muscle fiber once it depolarizes deep within the muscle fiber it tells the sarcoplasmic reticulum to release all that stored calcium and now calcium is floating freely throughout that muscle fiber let's go back here we've now got the myosin which is that green thick chain and the actin which is that blue chain we've also got this orange thing here this springy thing this is a very big protein molecule called titan it's actually one of the biggest protein molecules and it's a spring and it holds that myosin into the z-disc so that when contraction occurs when contraction occurs it can spring back all right so importantly something that you need to be aware of is when we look at the myosin it's got these little arms with these little heads on it like golf clubs so it's got these arms with these heads on it and these heads want to bind to binding pockets on this actin but here's the thing they can't and the reason why they can't bind to these binding pockets on actin is that there is a bike chain that's wrapped around this actin that will not let the mice and heads bind to that pocket so if i were to just draw that up a little bit better over here make it a little bit bigger right let's draw up the actin let's draw up the myosin okay the myosin head wants to bind to binding pockets in that actin and the thing is it can't and the reason why it can't is because there is a bike chain that's wrapped around that acton now this bike chain think about it if the acton is a bike and you want to hop on that bike and take it for a ride and there's a bike chain wrapped around it that bike chain is useless without a padlock so there's also going to be a padlock on here so you need to open that padlock so once you open the padlock the bike chain falls away if the bike chain falls away the bike is available the actin is available for the mice and heads to bind now here's the thing this bike chain is called tropomyosin that lock is called troponin we need a key for that lock once we have a key for that lock the whole thing opens up and the actin is available for the mice and heads to bind to what's the key well where did we stop in this process of the neuromuscular junction we stopped at the point where calcium was released from the sarcoplasmic reticulum calcium is the key now i've got all this calcium floating around once calcium comes along calcium ions specifically it is the key it will bind to this padlock the troponin open that padlock up and the tropomyosin falls away if the tropomyosin falls away these myosin heads can bind to their binding pockets and we can start that sliding filament mechanism so first aspect here is the first thing we need is that calcium binding now that calcium binding releases troponin and tropo myosin complex so now we have free actin for the that heads the bond but they still can't bind yet this is the thing we need step two to occur what's step two well think about it anytime you want some activity some action to occur we need what atp so we know that there's huge amounts of mitochondria floating within muscle fibers and the mitochondria through oxidative phosphorylation and the electron transport chain produce huge amounts of atp so we've got atp floating around now atp needs to come along and this is what happens with atp let's zoom in now on one of these mice and heads so i've got one of these myosin heads here and i might just draw up some actin like that the myosin head needs to first into position right then it needs to bind to the actin so let's get this down myosin head needs to into position and then it needs to bind to the actin then it needs to perform a power stroke and move the actin when it does that and it moves the actin it shortens that whole sarcomere from zed disc to z-disc the whole thing shortens so it does that then it needs to disassociate back into position bind power stroke release back into position bind power stroke release and this is muscle contraction muscle shortening how does atp play a role here this is what happens the myosin head is just sitting here atp comes along and binds atp will then disassociate into adp and phosphate remember atp is adenosine triphosphate there's three so if you take one off it becomes adenosine diphosphate adp and a single phosphate when you release the phosphate from atp this is the energy that we have to be able to do things so that atp this is the first step will then disassociate and form adp and phosphate and that phosphate and adp stay on the myosin but when that happens it allows that energy less for the head to and bind so now that's cocked into position and is now bound to the actin but we still have the atp and phosphate there right we've still got the a so adp and phosphate now what happens is the adp and phosphate will be released and when they release that head will go back into position but when it goes back into position what do you think happens to that actin molecule it slides and that's known as the power stroke so that power stroke adp and phosphate are released and the head moves now the thing is the head myosin head is stuck to that actin molecule until another atp molecule comes along when another atp molecule comes along and binds the atp binding releases the head and we're back at the start then what happens let's talk it through right it's really important let's talk it through myosin head actin molecule we need the head to and bind atp binds to the myosin head atp then disassociates the adp and phosphate cox the myosin head and binds the adp and phosphate jump off the head performs a power stroke but it's stuck atp comes along and binds releases the myosin head atp disassociates the adp and phosphate right the head and binds adp and phosphate dissociate power stroke atp oh the myosin head stuck we need more atp disassociate atp goes to adp and phosphate cox binds adp and phosphate dissociate power stroke it's stuck atp comes and the whole thing repeats this is the sliding filament mechanism that occurs so then the second thing that happens is we need atp to bind then it turns to or disassociates into adp and phosphate this equals the and bind then adp and phosphate let's just say leave that results in power stroke atp binds and the whole thing happens again release and then we're back at the start again all right so this is what's happening in this sliding filament mechanism when this whole thing these myosin heads bind to the actin and they perform this power stroke they're pulling it inward so these two actin molecules here they come closer and closer and closer towards that m line to the middle and that is the shortening of the whole sarcomere right the smallest contractile subunit of the myofibril and the whole muscle shortens that shortens that shortens that shortens that shortens the whole thing shortens and you've got skeletal muscle contraction that is the sliding filament mechanism