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Current time:0:00Total duration:16:32

Anatomy of a skeletal muscle fiber

Video transcript

I think we have a respectable sense of how muscles contract on the molecular level let's take a step back now and just understand how muscles look at least structurally or how they relate to things that we normally associate with muscles so let me draw let me say I draw a flexing bicep right here so that's someone flexing their bicep that's their elbow and let's say that's their hand right there so this is their bicep and it's flexing so this is their bicep I think we've all seen diagrams of what muscles look at least on kind of a macro level and it's connected to bones at either end so let me draw the bones I'm not going to detail where so let's connect it to the bones at either end by tendons so this right here so right here would be some bone right there would be another bone that it's connected to and then this is tendons which connects the bones to the muscles so this right here is tendon tendon we have the general sense connected to bones when it contracts it moves some part of our skeletal system so we're actually focused on skeletal muscles skeletal the other types are smooth muscles and cardiac muscles cardiac muscles are those as you can imagine in our heart and smooth muscles are these are more involuntary slow moving muscles and things like our digestive tract and I'll do a video on that in the future but most of the time when people say muscles we associate them with skeletal muscles that move our skeletal system around allow us to run and lift and and talk and do it and bite things so this is what we normally associate let's dig in a little bit deeper here so if I were to take a cross-section of this of this bicep right there if I were to take a cross-section of that muscle right there so let me do it big so if this is the cross section so the bicep or that muscle I'll stop saying biceps because I want to be general so I'm going to take a cross section of it so this is the cross section is where I've taken my cut and then it looks something like this this is the inside of this muscle over here now I set back here we had our tendon back here we had our tendon and then there's actually a covering and it's it's there's no strict demarcation or dividing line between the tendon and the covering around this muscle but that covering is called is called the epimysium FP my CM and it's really just connective tissue that covers the muscle kind of protects it reduces friction between the muscle and the surrounding bone and other tissue that might be in this person's arm right there and then within within this muscle you have connective tissue on the inside let me do it another color I'll do an orange this orange tissue right here and I'll make this orange tissue and then it's it divides off little I guess we call them fibers of some sort so we have this orange tissue right there this is called the Paramecium and that's also just connective tissue inside of the actual muscle peri my cm and then each of these things that the Paramecium is dividing off so each of these things that the Paramecium let me say let's say if we were to take one of these things and allow it to go a little bit further so if we were to take this thing right here let this what this Paramecium is dividing off and if we were to pull it out actually let me do this one right here if we were to pulled this one out just like that so you have the Paramecium surrounding it right this is all Paramecium and it's just a fancy word for connective tissue if para might seem and there's other stuff in there you could have nerves and you could have capillaries all sorts of stuff because you have to get blood and neuro neuronal signals to your muscles of entry so it's not just connective tissue it's other things that have to be able to eventually get to your muscle cells so each of these each of these I guess you call it sub fibers but these are pretty big sub fibers of the muscle this is called a fascicle this is a fascicle right here so that right there is a fascicle that's a fascicle and then in the connective tissue inside of the fascicle is called the endomysium endomysium so let me draw the endomysium right here so that once again more connective tissues has capillaries in it it has nerve has nerves in a all of the things that have to eventually come in contact with muscles with muscle cells we're inside of a single muscle so let me draw let me draw the endomysium so all this green connective tissue is endomysium endo my cm and each of these things that are in the endomysium are an actual muscle cell this is an actual muscle cell so I'll do it in purple so this thing right here I can pull it out a little bit let me pull this thing out a little bit just like that so if I pull this out this is an actual this is an actual muscle cell this is what we wanted to get to but we're going to go even within the muscle cell to see understand how all the myosin and the actin filaments fit in into that muscle cell so this right here is a muscle cell or a myofiber myofiber the two prefixes you'll see a lot when dealing with muscles you're going to see Myo which you could imagine refers to muscle and you're also going to see the word Sarco like sarcolemma or sarcoplasmic reticulum so you're also going to see the prefix Sarco and that's flesh so you know sarcophagus or you could think of other things that start with Sarco so Sarco is flesh but you know when muscle is flesh and Myo is muscle so this is myofiber this is an actual muscle cell and so let's zoom in on that actual muscle cell so let me actually draw it really a lot bigger here so an actual muscle cell it's called a myofiber it's called a fiber because it's it's it's longer than it is wide and they come in various rather myofiber like this so this is our my muscle cell right here and I'll take a cross-section of the muscle cell as well so this is my myofiber myofiber and these could be you know relatively short several hundred micrometers or it could be quite long or at least quite long by cellular standards we're talking several centimeters so several centimeters I think I'm going to sell that's quite a long selling because it's so long it actually has to have multiple nucleuses and actually to draw the nucleus is let me do a better job drawing the myofiber I'm going to make little lumps in the outside membrane where the nucleus is can fit on this myofiber remember this is just one of these individual muscle cells and they're really long so they have multiple nucleuses let me take its cross section because we're going to go inside of this muscle cell so I said it is MOU it's multinucleated so so if this is if we kind of imagined its membrane being transparent then we there'd be one nucleus over here another nucleus over here another nucleus over here another nucleus over there and the reason why it's multinucleated is so that over large distances you don't have to wait for proteins to get all the way from this nucleus all the way over to this part of the muscle cell you can actually have the DNA information close to where it needs to be so its multinucleated I read one I think it was 30 or so nucleuses per millimeter of muscle tissue is what the average is I don't know if that's actually the case but the nucleuses are kind of right under the membrane right under the membrane of the muscle cell and you remember what that's called from the last video that write the membrane of the muscle cells the sarcolemma sarcolemma sarcolemma or sarcolemma however you want to call it these are the nucleuses the nucleus is and then if you go even within if you take the cross section of that there are even tubes within that there are tubes within that called myofibrils so here I could take there's a bunch of tube inside of the actual cell let me pull one of them out so I pulled out one of these tubes this is a myofibril my o5 real and if you were to look at this under a light microscope you'll see it has little striations on it you'll see it has little striations oh the striations would look something like that like that like that then there'll be little thin ones like that like that and this is inside of these myofibrils is where we'll find our myosin and actin filaments so let's zoom in over here on this myofibril we'll just keep zooming until we get to the molecular level so this myofibril which is remember it's inside of the muscle cell inside of the myofiber the myofiber is a muscle cell myofibril is a you can view it as a tube inside of the muscle cell and that's these are the things that are actually doing the contraction so if I were to zoom in on a myofibril you're going to see it it's going to look something like that and it's going to have those bands in it so the bands are going to look something like this you're going to have these little short bands like that like that then you're going to have you're going to have wider bands like that like this little dark thing let me trying my best to draw them relatively neatly and there could be a little line right there then there's the same thing repeats over here so each of these units of repetition each of these units of repetition is called a sarcomere this is a sarcomere sarcomere and these units of repetition go from one this is called a Z line Zeeland to another Z line and all of this this terminology comes out of when people just looked at on a microscope and they saw these lines they started attaching names to it just so you have the other type terminology we'll talk about how this relates to the myosin in the act in a second this right here is the a band a band and then this distance right here or these parts right here these are called the I bands I bands and we'll talk about really in a few seconds how that relates to the mechanisms we tour the the units that we talked or the molecules that we talked about in the last video so if you were to zoom in here if you were to go into this into these myofibrils if you were to take a cross section of these myofibrils what you'll find is if you were to cut it up maybe slice it this well it's hard to if you were to slice it parallel to the actual screen that you're looking at if you you're going to see something like this so this is going to be your z band which is really just so that's your Z band this is your next Z band so I'm zooming in on one sarcomere now this is another Z band then you have your actin filaments you have your actin filaments now we're getting to that molecular level that I talked about so you'll have your actin filaments I'll draw them like that I'll just draw a couple you have your actin filaments that look like that let me label it so these are the actin actin filaments these are the actin filaments and then in between the actin filaments you have your myosin you have your myosin filaments let me draw my sins let me draw it in this color where the myosin filaments had those two heads on them right they each have two heads and the two heads like that that crawl along that crawl along the actin filaments I'm just drawing a couple of them and then they're attached to the middle just like that and we'll talk about in a second what happens when the muscle actually contracts and I could draw it again over here so it has many more heads than what I'm drawing but this just gives you an idea of what's happening so these are the myosin fill these are the myosin I guess proteins and this they all intertwined like we saw in the previous video and then there'll be another one over here I don't have to draw it in detail so you can see immediately that the a band corresponds to where we have our myosin so this is our a band here a band and there is there is an overlap right you they do overlap each other even in the resting state but the I band is where you only have actin filaments no myosin so this is the I band I band right there and then the myosin filaments are held in place by Titan which is kind of you can kind of imagine as a springy protein by Titan now I want to do a different color than that let me do it in so that right there so that's the myosin is held in place by Titan that right there is Titans it's attached to the Z band by Titan so what happens so we have all of these when a neuron when a neuron excites so let me draw an end point of a neuron right here the end point of an axon of a neuron right there it's a motor neuron it's telling this guy to contract you have the action potential the action potential travels across travels along the membrane really in all directions and then it eventually if we look at it from this view there you have those little T you have those transverse or t tubules they essentially go into the cell or and and continue to to propagate the action potential those trigger the sarcoplasmic reticulum to release calcium the calcium attaches to the troponin to the troponin that's attached to these actin filaments that moves the Tropo myosin out of the way and then the crawling can occur the myosin can start using ATP to crawl along these actin filaments and so as you can magic as they crawl along they're going to put the they're going let me switch colors they're going to their power stroke is going to push they're going to push the you could either view it as the actin filaments in that way or you can say that the myosin is going to want to move in that direction but you're pulling on both sides of a rope right so the myosin is going to stay in one place and the actin filaments are going to be pulled together the actin filaments are going to be pulled together and that's essentially how the muscle is contracting so we've hopefully in this video connected the big picture from the flex muscle all the way over here to exactly what's happening at the molecular level that we learned in the last few videos and you can imagine when this happens to all of the myofibrils inside of the muscle right because the sarcoplasmic reticulum is releasing calcium generally into the into the cytoplasm of which is also called Maya plasm because we're dealing with muscle cells the cytoplasm of this muscle cell the calcium floods all of these myofibrils it's able to it's able to attach to all of the troponin or at least a lot of the troponin it's on top of these actin filaments and then the whole muscle contracts and then when that's done each muscle each muscle fiber myofiber or each muscle cell will not have that much contracting power but when you couple it with all of them that are around it if you just have one actually working or a few of them you'll just have a twitch but if you have all of them contracting together then that's actually going to create the force to actually do some work or actually pull your bones together or lift some weight so hopefully you found that mildly useful
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