If you're seeing this message, it means we're having trouble loading external resources on our website.

如果你被网页过滤器挡住,请确保域名*.kastatic.org*.kasandbox.org 没有被阻止.



理解骨骼肌细胞的结构.. Raja Narayan 创建




now why don't we take a step back and take a look at how muscles work on more of a macro level so why don't we start out by just drawing a little muscle right here let's say this is just the bicep and on either side there's a little bit of tendon it's kind of attached to it right here so that's some tendon so I'll label that right there and that's just some tendon that's on either side of our muscle our bicep right here and the tendon attaches our muscle over to some bone so there's a little bone right there there's a little bone right there label these guys so that's one bone and and then here's another one so if it's our bicep it's anchored onto our humerus this is our bicep right so I'll just draw it in the arm here this guy sort of like flexing right there coming down to the elbow there and if we want a better idea of what's going on here in this bicep this muscle why don't we just take a cut so let me just cut right about there we'll get a cross-sectional look of what's going on in our muscle so here's a little cut we just did right about here our muscle just kind of hanging out in the middle sort of that same shape that comes down about here and we still got our tendon of course that's anchoring our muscle over to our bone don't forget the tendon that's still right here and the tendon is just the type of connective tissue and this is somewhat continuous with the connective tissue that covers the outermost layer of our muscle right here so I'm just drawing that in this outer layer of muscle we've got here is called the epimysium the EPI mise 'i'm however you want to call it and that sort of continuous with the tendon and it's supposed to help protect our muscle here so it doesn't shear against the bone or all the other things that are in the compartment of our arm if we're talking about the bicep but as you know this can apply to the muscles in our leg or in our jaw when we're chewing anything that we control so in addition to this connective tissue layer there's another connective tissue layer that's sort of on the inside right here underneath the epimysium and so this connective tissue layer I'll just draw sort of as a circle or as a sheath that's kind of sitting around here this guy is called the para my cm the peri my cm and this Paramecium covers subunits of muscle that sit right here and there's a bunch of them and they all have their own name so I'll I'll take this one right here and just kind of draw it out a little more so we can take a closer look inside so this little dude right here this muscle subunit that's covered by this Paramecium that I'm shading in right here this is called a fascicle it's got two names actually so it can be called a fascicle fast kick 'l CI c le it's also known as a fasciculus a fasciculus so it depends on whether you're talking about fascicles or a single fasciculus however whatever term you want to use and then within the fascicle within each of the FASTA khals there's another connective tissue layer this is called the endomysium the endomysium now this covers individual muscle cells so finally we've reached the individual muscle cells I'll draw one of these dudes coming out right here this is an individual muscle cell that's covered by the endomysium and so the muscle cell that I'm writing out over here it has a special name as well so we can call it a muscle cell but we can also call it a Myo Myo meaning muscle fiber so this is shaped like a fiber because it is longer than it is wide and again this endomysium just like the Paramecium contains nerves and blood vessels that can help conduct neuronal signals and blood towards the individual myofiber and the connective tissue that sits around here okay so now that we've gotten to the muscle cell why don't we just scroll down a little bit and just kind of just focus in on this guy now well we might be tempted to draw the muscle cell just kind of like that fiber that I just drew over there like this sort of rectangle remember in fact that it's sort of shaped a little differently like a pipe that has a couple of bumps outside do you guys remember why do we have bumps on the outside of our muscle cells or the outside of our myofibers I think I heard one of you said because there are nuclei that sit on the outskirts of our muscle cells and that is absolutely correct this is a single nucleus that I'm drawing right here here's one nucleus and this is sort of the storage unit of DNA that can help us replicate or make more of our myofibers or our muscle cells and they sit on the outskirts of our myofiber towards sort of the edge of our plasma membrane this plasma membrane has a special name in muscle it's called the sarcolemma sarcolemma we've got a couple of important prefixes that we sort of mentioned here remember I mentioned Mayo kind of from above my o just means muscle just keep that in the back of your mind and then Sarco whenever you see Sarco that refers to just flesh and we often see this in the context of muscle because the covering of our muscle cell right here that membrane we call it the sarcolemma the cytoplasm right here that's within the muscle cell we call that the sarcoplasm the sarcoplasm and as we get further in we're also going to talk about the sarcomere and so if we're talking about our myofiber right here let me just make it look a little more tubular so the myofiber itself this muscle cell has a bunch of smaller units within it - and these smaller units are where we have our main contractions occurring so I'm gonna draw one of these guys out right here and this is just called the Myo like muscle fiber rill not fiber but myofiber rill now we've come to where we're storing our myosin and our actin that's sitting inside here this is where the actual contraction will occur so if we look at our muscle cells under a microscope we'll that they've got these striations on them these bands because remember another name for skeletal muscle is striated skeletal muscle so they have these lines that are here that you'll see under a microscope so if we blow that up let me just get some space down here to talk about it so if I were to draw just kind of a blown-up version of it right here we'd have our striation line right there I'll draw another one right here and right here just put it in this line and this box right here and we have all these bands that we would see under the microscope right so we have the striations that are on the sides and there are these bands that are kind of going across this unit right here that we're looking at now what are these striations right here well we talked about these before Sal mentioned these are the Z lines so the Z lines are these striations we see under the microscope and so I've drawn three of them here for you guys Z lines I don't just connect that one back here and remember the space between two Z lines going from here all the way to here that's the sarcomere the sarcomere and this is our most basic unit of contraction this is where we're going to have our actin and our myosin fibers interact and have us flex that are most macro level we'll get back to in a second and there are different parts of the sarcomere right there's the part that's designated the a band that's in the middle there's this other part up here called the eye band the eye band all right so let's focus in on a single sarcomere right here a single sarcomere so I'm drawing the outskirts of our sarcomere of course so that's going to be our z line that's hanging out on either side I've drawn two of them here anchored to our z line is going to be our actin filaments here are the actin filaments that we've heard about before I'll just label this this is our actin filament remember sitting inside is going to be our myosin and our myosin filament remember it's got two heads and its associated right here with the actin it wants to kind of pull on the actin and just crawl along it crawl along the actin and I'll draw one here as well it's mice and heads two myosin heads right there and they're attached top and bottom like that and they just want to walk across all right so I want to make sure that I draw that here too all right and you get the picture down here and so anchoring our myosin filaments in the sarcomere is going to be Titan will just draw the Titan here it's not attached to the ends of the bison but you can kind of see that it's holding it in place from somewhere deeper in right there so that's our art Titan sorry Titan and again this is our myosin this guy myosin filaments with the two heads that come up and at this point we can appreciate some of the band's we talked about over here the part that's both myosin and actin is called the a band that's the a band that we drew on the left side over here and the part that's only actin actin that doesn't involve any of the myosin that's this point right here and it continues on into the other sarcomere this is the I band the I band the way I think about it I kind of looks like a 1 right so it's got one of our two major filaments and then a is the alternative one the other one that's got myosin and actin in it so that's the a band and the AI band all right so now when you recall so there's this axon fiber that's going to come in and release a message an action potential that's going to come here and depolarize our sarcolemma and it's gonna sort of spread everywhere it's not just gonna go in one direction and one of the things that we have in our sarcolemma RT tubules that can allow the depolarization or this action potential to go deep within our muscle cell or our myofiber to cause the sarcoplasmic reticulum to release calcium calcium then as you remember goes on to bind troponin that's sitting on our actin will then tell tropomyosin get the heck out of the way and then our myosin our myosin filaments that we drew right here can go ahead and use ATP to sort of walk along our actin filaments and so they would sort of walk along this way and they would you know relatively to the actin filament standstill they would be crawling this way but wouldn't really do any of the moving they'd be anchored down it's the actin filaments actually that move the actin filaments are going to be moving closer in to the center and that effectively causes our eye band to get smaller the eye band is going to get smaller when we have our sarcomere contract and because the a band involves however far the myosin spreads the a band does not change only the eye band changes here as we effectively bring the two z lines closer to each other and shorten the length of our sarcomere and that's what's happening on our most micro molecular level right here with the sarcomere contracting and all of that began with this axon fiber spreading the signal so I hope you can appreciate just kind of going from the top right here when we're contracting our skeletal muscle and we go through all these smaller layers what's happening at this molecular level right here what's allowing us to contract to flex our arm or kick a ball or do something of that nature and I hope you found all of this to be somewhat useful to you