Structure of a Skeletal Muscle - Online Tutor, Practice Problems & Exam Prep

Skeletal muscle structure is organized from the whole muscle down to individual muscle fibers. Each muscle fiber, surrounded by endomysium, contains myofibrils, which are bundles of myofilaments (actin and myosin) responsible for contraction. The sarcolemma, a specialized cell membrane, facilitates electrochemical signals, while T tubules and the sarcoplasmic reticulum manage calcium ion release, essential for muscle contraction. The repeating unit of contraction is the sarcomere, contributing to the striated appearance of skeletal muscle.

Organization of Muscle Tissue

We're now going to talk about the structure of a skeletal muscle, and we really want to think about the organization of the skeletal muscle tissue. Here, we're going to discuss that organization starting at the whole muscle and getting down to the individual muscle cell or the muscle fiber. Later on, we will delve further into the individual proteins responsible for contraction. But for now, we are discussing it from the muscle down to the muscle cell or muscle fiber.

Before we proceed, let's orient ourselves to the picture here. We see a bone, that's the humerus, and you see a tendon coming off the bone, pulling this muscle away. We see this sort of whole body of the muscle that we then sort of cut off in cross-section. In this cross-section, you can see these groupings within these bundles, and we've cut that off, but we've pulled one of those things out. And then we do the same thing; we cut it off in cross-section again, and pull one of those things out. We are hinting at even more levels of organization down below. You're very likely to see a picture like this in your textbook, in a lecture, maybe even on the test. Practice organizing your thoughts around it and think about what these different levels are. So what's in here? Well, there are muscle fibers. Muscle fibers are the individual muscle cells, and really the bulk of the muscle is going to be those muscle fibers. But also running out all throughout are nerves. In our image here, we have these yellow nerves cut off in cross-section. These nerves need to run all through the muscle because they need to connect to every individual muscle fiber. The nerves of the nervous system are what tell the muscle fiber when to contract. Also needing to run through there though are the blood vessels, and we see those cut off in cross-section here in blue and red, and we also see two of them pulled out. Again, these blood vessels need to get down to every muscle fiber because those fibers use a lot of oxygen as they burn through ATP as they contract. Finally, we have connective tissue. There are going to be several different layers of connective tissue that you are going to need to know about in this diagram. So every time we've talked about connective tissue, we've put it in a gray box just like you see here.

Let's talk about these bundles. The first thing that we're going to talk about, the smallest level for this diagram, is going to be the muscle fiber. A muscle fiber, we've said, is just another name for a muscle cell. These muscle fibers are going to be really long, multinucleated cells, and you can just sort of think about it. Basically, one muscle fiber is basically the length of a muscle cell. Importantly, in a skeletal muscle, a muscle fiber never stops, and then another one starts sort of in series with each other. The muscle fiber runs the entire length of the contraction. We've pulled one muscle fiber out here, and you can see it. You can see these multiple nuclei along the edge there. Remember, multiple nuclei; they're formed from smaller cells, and you need lots of nuclei because they're really big cells that need all these nuclei to support their functions. To surround the muscle fiber, we are going to have a layer of connective tissue that we are going to call endomysium. Endomysium is going to surround every individual muscle fiber. Here, it's really important just to remember your prefixes. 'Endo' means within. So at the deepest level, surrounding every individual muscle fiber, we have the endomysium. As we go up, we can see muscle fibers in cross-section all bundled together. Running in between each of these muscle fibers, all this white connective tissue here, that's going to be that endomysium. Endomysium surrounds the individual muscle fibers.

But you can see we have a bundle of muscle fibers together that are organized, and that is what we are going to call a fascicle. A fascicle is just a bundle of muscle fibers. When we say a bundle, we're talking somewhere about dozens of muscle fibers together. This fascicle is organized, again, by being surrounded by connective tissue. It has the endomysium within the fascicle, and surrounding the fascicle is the perimysium. That prefix 'peri' just means to surround something. What does the perimysium surround? It surrounds the fascicle. If we look up a level, we can see up here each one of these things is a fascicle, and you can see each one is surrounded by this connective tissue. So this connective tissue that you see at this level is the perimysium.

Now, this level that we're looking at is the whole muscle. This muscle is really a bundle of fascicles. Again, we're talking about a few to several dozen fascicles, usually to make up a muscle. What holds this whole muscle together? Well, around the outside of this muscle, we have another layer of connective tissue that's surrounding the entire thing. That, we are going to call the epimysium. That prefix 'epi' means on the surface of something, around the outside. So, the epimysium surrounds the outside of the entire muscle. This connective tissue is going to be really important for contraction. Each muscle fiber is connected to the endomysium that surrounds it, and that endomysium, just like the muscle fibers, running all the way down through the muscle, and the same thing with the perimysium and the epimysium, and it all reaches the end of the muscle, and it's sort of bound together, and that's the connection. The connective tissue is the connection that connects the muscle to the bone. Now if it comes together like you see here in sort of like a rope, that is what we are going to call a tendon. A rope-like connection, where all those connective tissues are bundled together like a rope, that's a tendon. Some muscles, however, have more flat connections, where the muscle is flat, and it comes in more like a sheet-like connection. That sheet-like connection, we call an aponeurosis or the plural, aponeuroses. You can think like your abdominal muscles. They don't come together to a point to connect to your bones like a rope. They come in flat with a sheet at the end. Now aponeurosis, you're probably familiar with tendons, but aponeurosis, you may need to know for this unit. You're definitely going to want to know it later on when we learn the individual muscles, because we're going to learn what bones they connect to and how they connect, and we're going to be using it a lot there.

Alright. So that's the structure of a muscle from the muscle fiber up to the whole muscle. Later on, we're going to go from the muscle fiber down. But first, like always, we have example and practice problems to follow. I'll see you there.

Structure of a Skeletal Muscle Example 1

Our example tells us that marbling in beef is the presence of fat in connective tissue layers within the meat. This intramuscular fat makes the meat more tender and flavorful. Below are 2 cuts of meat displaying differences in marbling. On the left is a typical steak, as you might find at the grocery store. On the right is Kobe beef, a delicacy from Japan known for its extreme marbling. In the picture on the right, we see that there's fat dispersed throughout the meat, appearing as clumps and specks of fat throughout the steak.

Considering that marbling is present in connective tissue, we aim to predict in which structures of the muscle the excess fat found in Kobe beef might be located. First, we recall our connective tissue layers: the epimysium, the perimysium, and the endomysium. The epimysium surrounds the outside of the entire muscle, but as marbling occurs within the meat, the epimysium, although possibly fattier, is unlikely to contribute directly to the marbling.

Next is the perimysium, which surrounds individual fascicles. This can be observed in the meat from the grocery store, where lines of connective tissue and fat run through the muscle. This same feature, more pronounced, is likely what contributes to the visible fat in Kobe beef. Therefore, much of the marbling probably consists of fat within the perimysium.

Finally, the endomysium surrounds each individual muscle fiber within the fascicle. While muscle fibers are microscopic and thus not visible to the naked eye, it is plausible that some of the specks of fat seen in the Kobe beef are within the endomysium.

In summary, the perimysium surrounds the fascicles and the endomysium surrounds the individual muscle fibers. These are the connective tissue layers within which we expect to find the excess fat contributing to the delicious taste of steak tartar or Kobe beef.

See you in the next video.

What layer of connective tissue surrounds and separates muscle fibers?

• A. Epimysium.
• B. Perimysium.
• C. Endomysium.
• D. Fascicle.

A fascicle is a bundle of:

• A. Epimysium.
• B. Muscle Fibers.
• C. Myofilaments.
• D. Muscles.

The Muscle Fiber

To continue talking about the structure of a skeletal muscle, we're now going to talk about the muscle fiber, and from here, we're going to go down to some of the individual proteins that are going to be involved. And I just want to ground ourselves first though. What we're saying here is that for a muscle fiber to contract, the tissue must shorten. And that's really sort of our fundamental question is how does a muscle fiber shorten? How does the muscle actually get shorter? So to answer that, we're going to start by looking at this picture here, and we have a picture of a muscle fiber, at least a small section of it. And remember, the muscle fiber, that's just another name for the skeletal muscle cell. That's these giant cells, these big cylinders, extremely long cells, basically the same length as a muscle. And as we look here, we're going to see that it's sort of we see a section of it and we see it sort of cut away in cross-section in pieces, and we can see some of the structures inside, and some of those are pulled out so that we can see what's going on in there just like we did previously with the whole muscle. Now remember, this muscle fiber is going to be surrounded by the endomysium, that smallest layer of connective tissue that surrounds and separates each individual muscle fiber. So to start, we'll just note that's sort of drawn in down here, but we're sort of pulling it away so that we can see what's inside. And the first thing that we know well, it's also cut away, but this would be wrapping the whole thing, would be this sort of translucent section here that you see, and that's just going to be the cell membrane. But the cell membrane of a muscle fiber is highly specialized, so we give it its own special name. We're going to call it the sarcolemma. And sarco is just a prefix from Greek that means flesh, so it's used sometimes to refer to muscles. And lemma is also from Greek. It means a covering or a wrapping or a sheath around something. So we're going to say that the sarcolemma is just this plasma membrane of the muscle fiber that is wrapping the myofibrils. And the myofibrils, we'll get into more detail in a second, but these are these long, rod-like organelles that are within the sarcolemma that make up the muscle fiber. And so here, we can think about this sort of repeating structure of bundles that we started talking about previously. We had the muscle was a bundle of fascicles. The fascicle was a bundle of muscle fibers. And now the muscle fiber is going to be a bundle of these myofibrils. Alright. But before we talk about the myofibrils, I want to talk about the membrane a little bit more. So the sarcolemma is highly specialized to sort of send an electrochemical signal telling the muscle fiber to contract. But you also need to get that electrochemical signal down deep within the muscle fiber, so that the whole cell is contracting together. To do that, we have the T tubules. And the T tubules, you can see here in pink, they're sort of coming down here and they are surrounding like a little ring each around these myofibrils. Now these are connected to, and really they're kind of an extension of the sarcolemma. So we're going to say that the T tubules are these membrane canals, or you could say tunnels or passageways, that are extensions of the sarcolemma and reach down deep within and surround each one of these myofibrils in a little ring. So as the electrochemical signal comes down the sarcolemma, it's going to then dive down in these T tubules and get deep within the muscle fiber. Now, what they're actually trying to stimulate is this next membrane system that we're going to talk about, and that's going to be the sarcoplasmic reticulum that we're writing down here in blue. Now, the sarcoplasmic reticulum, you might notice that sounds a lot like endoplasmic reticulum. There's a reason for that. The sarcoplasmic reticulum is the highly specialized endoplasmic reticulum of muscle cells. And the sarcoplasmic reticulum, you can see it surrounds these myofibrils. You can see it's really here in close connection with these T tubules, and then it kind of spreads out in this sort of convoluted connected tunnel tube-like system of membrane, and you can see it sort cut off in cross-section all along here, and it's surrounding and separating these myofibrils. Its job is to store and then release calcium ions. The signal for the muscle to actually contract down, like, at the molecular level is the release of calcium ions from the sarcoplasmic reticulum into the myofibrils. Now we're going to get into that in a lot more detail going forward. But for now, let's now focus on the myofibril. So the myofibril, we've already said, is this myofibrils. These are these long rod-shaped organelles, and you can sort of see a bunch in cross-section here. We have a few here, and then we've pulled one out so that you can really see it there. The myofibril is really just sort of this organizing structure, and what it's organizing, is going to be the myofilaments. And you can see again, we've pulled out the myofilaments there. And if you look here at these in the cross-section, it might be a little hard to see because they're very small, but the cross-sections you might also notice have these really regular organized structures within them and that's those myofilaments cut off in cross-section. The myofilaments, that's what's actually going to be responsible for getting this muscle to contract. So the myofilaments, these are the proteins the actin and the myosin. Alright? There's going to be other proteins going on in here, but actin and myosin, these are the major players that are actually going to get this muscle to get shorter. This, though, is going to be the first structure that isn't really long. Remember, our muscle fiber is basically the same length as the muscle. The muscle the myofibrils the same length as the muscle fiber. The myofilaments are going to be organized in these repeating structures, and you can kinda see it in this myofibril that we pulled out. You have sort of a darker section that has more purple in it, and then a lighter section, a darker section more purple in it, a lighter section that pattern sort of continues in these filaments that we pulled out. That repeated structure is going to be the sarcomere, And the sarcomere, we are going to say, is the contractile unit. We're sometimes going to call this the fundamental unit of muscle contraction. What actually gets shorter in the muscle? The sarcomere gets shorter. And the muscle fiber gets shorter because you can see this repeated structure is repeated over and over again a lot. One myofibril has 1,000 or tens of thousands of these in succession. I did a quick back of the envelope calculation. If you have a 10 centimeter long muscle, which is about, I don't know, yay big, it's going to have something like 40,000 sarcomeres, end to end in every single one of these myofibrils. That repeated structure, repeating over and over again, these actin and myosin filaments, the way they're organized, is going to create the striated appearance that you see when you look at a skeletal muscle. From that striated appearance, when you look at the cells, it looks striped. You're seeing the repeated structure of these proteins, the actin and myosin. Now, this is that structure. We're not going to worry about it too much detail here. We're going to talk about it in a lot more detail going forward. For now, we have, like always, example and practice problems to follow, and then I look forward to getting to the step-by-step process of how these signals are passed and how these proteins actually work to make the muscle contract. See you there.

Structure of a Skeletal Muscle Example 2

Our example wants us to arrange the following from superficial to deep by placing the correct letter in each blank. We have myofilaments, epimysium, perimysium, endomysium, and sarcolemma. When I think of a muscle and the structure of a muscle, what's most superficial, what surrounds the entire muscle, is going to be a layer of connective tissue. We call that layer of connective tissue that surrounds the entire muscle the epimysium. So I'm going to put 'b' first. Remember, 'epi' means on the surface or on the outside of something. The epimysium is the connective tissue layer that surrounds the entire muscle.

Within the epimysium, we're going to have these bundles of fascicles. Looking down at the fascicles, well, it's two arrows down. There's a space between it. What surrounds the fascicle? We have another connective tissue layer, and that's going to be the perimysium. So 'c' perimysium, the connective tissue layer that surrounds the fascicle, is going to come next.

Well, the fascicle is a bundle. It’s a bundle of muscle fibers, but I don't see 'muscle fibers' on my list here. So when I look I think what's associated with the muscle fiber, what I see next is the sarcolemma. The sarcolemma is going to be the cell membrane of the muscle fiber. However, there's another connective tissue layer that comes first. The connective tissue layer that surrounds each muscle fiber and, therefore, surrounds the sarcolemma is going to be the endomysium. Remember, the prefix 'endo' means within. It's deep within the fascicle. So, the endomysium surrounds the sarcolemma. The sarcolemma is the cell membrane of the muscle fiber, so 'e' comes next.

And now we've made it to the myofibrils. Myofibrils, remember, are those long rod-like organelles that make up the majority of the inside of a muscle fiber. Well, what are these myofibrils made of? They're made of the contractile proteins. We're going to call these the myofilaments. We only had one option left, but it was the right answer.

Alright. So the myofilaments, the proteins that are actually going to do the contracting, deep, deep within the muscle. To recap, superficially, we have the epimysium that surrounds the entire muscle. The perimysium is within the epimysium, and it surrounds the fascicle. The fascicle is going to be a bundle of muscle fibers, but each muscle fiber is surrounded by the endomysium. The endomysium surrounds the cell membrane of the muscle fiber, which is the sarcolemma. Within the sarcolemma, we have the rod-like organelles called myofibrils, and the proteins that make those up are going to be the myofilaments. That's our answer. More problems to follow. I'll see you there.

The plasma membrane of a muscle fiber is called the ____________ and it wraps bundles of ____________. 

• A. Sarcolemma: myofibrils.
• B. Sarcoplasmic reticulum: myofilaments.
• C. Sarcomere: myofilaments.
• D. Sarcoplasmic reticulum: myofibrils.

True or False: if false, chose the answer that corrects the statement.

The smallest contractile unit of a muscle is the muscle cell.

• A. True.
• B. False, the smallest contractile unit is the fascicle.
• C. False, the smallest contractile unit is the myofibril.
• D. False, the smallest contractile unit is the sarcomere.

Here’s what students ask on this topic:

What is the structure of a skeletal muscle from the whole muscle down to the individual muscle fiber?

The structure of a skeletal muscle is organized hierarchically. Starting from the whole muscle, it is composed of bundles called fascicles. Each fascicle contains multiple muscle fibers (or muscle cells), which are long, multinucleated cells. Each muscle fiber is surrounded by a layer of connective tissue called the endomysium. Fascicles are surrounded by another layer of connective tissue called the perimysium. The entire muscle is encased in a connective tissue layer known as the epimysium. This organization allows for efficient contraction and force transmission from the muscle to the bone via tendons or aponeuroses.

What are the roles of the sarcolemma, T tubules, and sarcoplasmic reticulum in muscle contraction?

The sarcolemma is the specialized cell membrane of a muscle fiber that conducts electrochemical signals. T tubules are extensions of the sarcolemma that penetrate into the muscle fiber, ensuring that the signal reaches deep within the cell. The sarcoplasmic reticulum, a specialized form of the endoplasmic reticulum, surrounds the myofibrils and stores calcium ions. When a signal is received, the sarcoplasmic reticulum releases calcium ions into the myofibrils, triggering muscle contraction by interacting with the myofilaments (actin and myosin).

What is a sarcomere and why is it important in muscle contraction?

A sarcomere is the fundamental unit of muscle contraction within a myofibril. It is composed of repeating segments of actin and myosin filaments. The arrangement of these filaments gives skeletal muscle its striated appearance. During muscle contraction, the sarcomere shortens as the actin and myosin filaments slide past each other, a process known as the sliding filament theory. This shortening of sarcomeres in series leads to the overall shortening of the muscle fiber and thus muscle contraction.

What is the difference between a tendon and an aponeurosis?

A tendon is a rope-like structure that connects muscle to bone, formed by the bundling of connective tissues (endomysium, perimysium, and epimysium) at the end of a muscle. An aponeurosis, on the other hand, is a flat, sheet-like connective tissue that also connects muscle to bone or other muscles. While tendons are typically associated with muscles that exert force in a specific direction, aponeuroses are found in muscles that require a broad attachment area, such as the abdominal muscles.

What are myofibrils and myofilaments, and how do they contribute to muscle contraction?

Myofibrils are long, rod-like organelles within muscle fibers, composed of repeating units called sarcomeres. Myofilaments are the protein filaments within sarcomeres, primarily actin (thin filaments) and myosin (thick filaments). During muscle contraction, myosin heads bind to actin filaments and pull them inward, causing the sarcomere to shorten. This process, repeated across thousands of sarcomeres in a myofibril, leads to the overall shortening of the muscle fiber and thus muscle contraction.