Skeletal Muscle Anatomy: Study with Video Lessons, Practice Problems & Examples

Skeletal muscle anatomy is centered around sarcomeres, the fundamental units of muscle contraction. Each sarcomere consists of thick myosin and thin actin filaments, organized into distinct regions: the H zone (only myosin), I bands (only actin), A band (overlap of both), M line (center anchor for myosin), and Z discs (boundaries of sarcomeres). Understanding these structures is crucial for grasping how muscle contractions occur, as they facilitate the interaction between myosin and actin during contraction, leading to muscle movement.

Skeletal Muscle Anatomy

In this video, we're going to begin talking about skeletal muscle anatomy. So, muscles are accomplished by molecular motor protein interactions and so the motor protein myosin is responsible for pulling on thin actin microfilaments during a muscle contraction. And muscle contractions in skeletal muscle tissue are one of the best understood mobility systems. And so for that reason, we're going to talk about the muscle contraction in skeletal muscles later in our course. But before we can talk about muscle contractions and skeletal muscles, we first need to understand skeletal muscle anatomy. And so in our next video, we'll talk more about skeletal muscle anatomy. So I'll see you guys in that video.

Skeletal Muscle Anatomy

In this video, we're going to talk about skeletal muscle anatomy. And so notice down below, we have an image showing you skeletal muscle structures and it ties together nicely with the text that we have up above and so we're going to start with the far left structure that we have highlighted here which are referred to as sarcomeres. So sarcomeres are complexes of thick myosin filaments and thin actin microfilaments that are in special arrangements. And so later in our course, in our next lesson video, we'll be able to talk more about the structure of the sarcomere and these special arrangements that these, thick and thin filaments are in. But for now, if we take a look down below at our image, we can see that these brackets that we see here in here are labeling the sarcomeres and so, this one bracket is labeling 1 sarcomere and this one is labeling another sarcomere and so there's actually another sarcomere here and here and here and so these repeating units of sarcomeres, are referred to as myofibrils. And so, these myofibrils again are repeating units of sarcomeres. And so if we take a look at our image down below, we can see that, this entire thing right here would be referred to as the myofibril. And notice that, here we have another myofibril. Here's another one. Here's another one. And here's another one. And so together, all of these myofibrils, which you'll notice, is that they are surrounded by what's known as a sarcoplasmic reticulum, which is a membranous structure. And so down below, notice that this yellow structure that we see here that's being labeled as the sarcoplasmic reticulum is, actually going to be very, very important when it comes to a muscle contraction. And so later in our course, when we're talking about muscle contractions, we are going to talk up, we are going to revisit this sarcoplasmic reticulum, which is going to be important for releasing calcium. But again, we'll talk more about the sarcoplasmic reticulum and the calcium that it releases later in our course when we're talking about the muscle contraction. But for now, we can see that the sarcoplasmic reticulum is, the yellow membranous structure that we see here. Now, what you'll notice is that these myofibrils that we see here are all packed inside of what's known as a muscle fiber and so a muscle fiber is just a long, multinucleated cell and so, multinucleated just means that it has multiple nuclei. So, instead of having just one nucleus like most cells, these muscle fibers have multiple nuclei, multiple nucleuses if you will. And so, what you'll see is that, here what we have is, the muscle fiber. And so you can see that this is all one really really long cell and so it is quite long and it it can actually run the length of the entire muscle. So that is quite long for 1 individual cell. And again, they are multinucleated. So you can see that each of these blue structures that we see here represent nuclei and they're going to repeat and, be within, this muscle fiber. Now, again, you can see that within the muscle fiber, the myofibrils are going to be packed inside and extend the length of the entire fiber. Now, notice that, this, entire muscle fiber that we see here is leading up to what's known as a muscle fascicle and so, This muscle fascicle, all it really is, what we can see here, it's just a bundle of muscle fibers. So it's just a bundle of individual cells, a bunch of, these long multinucleated cells packed together. And so, what you'll notice is that a bunch of these, here here we only have one muscle fascicle. But you'll notice here's another muscle fascicle. All of these are muscle fascicles and so a bunch of muscle fascicles are going to make up the skeletal muscle. And so the skeletal muscle are gonna be the voluntary muscles, the ones that we actually have control over, such as our biceps for instance, here. And, they are going to have a striated or striped appearance and, the striped appearance that you can see throughout here where you have a dark band followed by a light band, followed by dark band. This is the striated striped appearance and really, it all comes down to the structure of the sarcomere that gives the skeletal muscle this striated striped appearance. And again, the skeletal muscle is going to consist of muscle fascicles together. And so, you can see here we have the human body and you can see the skeletal muscle that's scattered throughout and so we can label this as the skeletal muscle. And so really this completes our lesson on skeletal muscle anatomy. And again, in our next lesson video, we'll be able to talk more about the structure of these sarcomeres and, later we'll be able to talk about the muscle skeletal muscle contraction. So I'll see you guys in our next video.

Match the following parts of the muscle with the correct description:

a) Sarcomere. ______: voluntary muscles (i.e. triceps) that consist of muscle fascicles.

b) Myofibrils. ______: repeating units of sarcomeres.

c) Fascicles. ______: multinucleated cells that consist of a bundle of myofibrils.

d) Sarcoplasmic Reticulum. ______: membrane-bound structure found within muscle cells.

e) Skeletal Muscle. ______: complex of thin actin and thick myosin filaments.

f) Muscle Fiber. ______: a bundle of skeletal muscle fibers.

Alright. So here we have a practice problem that will help you guys better memorize the skeletal muscle anatomy and so this problem wants us to match the following parts of the muscle that we see over here, with the correct description that we see over here. And so this first description that we have here says, voluntary muscles, for example, our triceps that consist of muscle fascicles. And so recall from our previous lesson videos that these voluntary muscles are going to be our skeletal muscle and so we can go ahead and label this blank here with option E and then cross it off from our list. Now, the next one says, repeating units of sarcomeres and of course, from our previous lesson video, we know that the units of sarcomeres are going to be the myofibrils. And so the myofibrils are these repeating units of sarcomere. So we can put B here for myofibrils and cross it off. Now the next blank here says, multinucleated cells that consist of a bundle of myofibrils. And so, if myofibrils are these repeating units of sarcomeres, well, a bunch of myofibrils are going to be found inside of these long multinucleated cells that are muscle fibers. And so muscle fibers, F here, is going to be the multinucleated cell. Now, the membrane-bound structure found within muscle cells is, of course, going to be the sarcoplasmic reticulum which is really surrounding these myofibrils inside of the muscle fiber. And so the sarcoplasmic reticulum is D, so, we can put that here for this membrane structure. And again, the sarcoplasmic reticulum is going to be important when we talk about muscle contraction because it's associated with releasing calcium but again, we'll talk more about this idea later in our course when we talk about muscle contraction. So, the next blank here says, complex of thin actin and thick myosin filaments. And of course, this is going to be the sarcomere. So we can go ahead and label that as A. And in our next lesson video, we'll talk a lot more details about the structure of the sarcomere and then, of course, last but not least, what we have is a bundle of skeletal muscle fibers and of course, this is going to be the fascicles, the muscle fascicles. So, we can go ahead and place that in as C. And so, this is the answer to this practice problem and if you fill this out a few times using the image up above, you should be good with understanding the skeletal muscle anatomy that you should be familiar with and so that concludes this practice problem and I'll see you guys in our next video.

Skeletal Muscle Anatomy

In this video, we're going to focus on sarcomeres. So, sarcomeres are important because a muscle contraction is actually the result of a sarcomere contraction. And so later in our course, we're going to talk about sarcomere and muscle contractions, but before we can do that, we need to understand the structure of a sarcomere. And so a single sarcomere actually has several different regions and components and we're going to talk about all of those regions and components down below here in our text. Now, notice that the text is actually color coordinated to match the regions and components down below in our image. And so notice up above here, what we have is the same exact visual representation of the sarcomere that we had in our last lesson video. And so really what we're doing here is we're zooming in on this specific region of the sarcomere and we're blowing it up and visually representing it in this representation that we have here. And so if we go up to our text, we can focus on the first region here, which is right here and this is going to be the H zone. And so what helps me remember what the H zone is is I know that the letter H is quite a thick letter. It extends a lot horizontally from side to side. And so because the letter H is such a thick letter, the H zone is only going to contain thick myosin filaments, and it will have no overlaps with thin actin microfilaments at all. And so if we take a look down below at the image of our sarcomere, notice that the thick myosin filaments are in red right here. And of course, the thin actin microfilaments are in green at these positions on either side. And so, of course, the H zone, because H is such a thick letter, that reminds us that the H zone is only going to contain thick myosin filaments. And so notice that this region here highlighted with a blue background, contains only thick myosin filaments and no overlap with any of the thin actin microfilaments. And so, because that's the case, that makes it the H zone. And so we can label this region, down below indicated by this bracket right here as the H zone.

And so, now, we can move on to the next region and so, the next region is going to be the I bands and so because the letter I, what helps me remember what the I Bands are is that because the letter I is such a thin letter, that reminds me, that the since I is such a thin letter, that the I band is only going to contain thin actin microfilaments and it will have no overlaps with thick myosin filaments at all. And so if we take a look down below at our image, notice that the region that only contains thin actin microfilaments. So the thin letter I, reminds us that this is going to be the I band. So from this region to this region over here and, over here from this region to this region over here. Now, the next region that we have is referred to as the A band and so what helps me remember what the A band is is that the letter A is actually mostly a thick letter but towards the top, it's actually quite thin. And so because the letter A is mostly thick but has a little bit of thin, this means that it's going to contain all of the thick myosin filaments, and some actin filaments as well. And so if we take a look down below at our image, the A band, again, because A is mostly a thick letter and, something. It's going to contain the entire length of the thick myosin filaments. But notice that there's also going to be some overlap with thin actin microfilaments as well. And so the A band is really just going to be this entire region that we see here, really just defined by the length of the thick myosin filaments. And so you can see that this yellowish background that we have, that extends from here to here indicated by this bracket down below is going to represent our A band.

So, now, we can move on to the next component and so, the next component that we have is the M disc or the M line for that matter. And so, sometimes they'll be referred to as the M discs, other times they'll be referred to as the 'M' line. But what helps me remember what the 'M' disc or 'M' line is, is that the letter 'M' is for the middle. And so this is going to be a vertical anchor to myosin in the middle of the entire sarcomere in terms of being in the middle of the H zone and the A band. And so, down below, what you can see is that the M disc is this disc shaped structure that we see here in the center. So, we can go ahead and label it as the M disc and notice that the M disc is in the middle of the entire sarcomere. Present in the middle of the H zone and the middle of the A band. And it's also a vertical anchor to the thick myosin filament. So, you can think M Disc for anchoring the myosin filament. So then last but not least, what we have are the Z discs. Also known as the Z lines. And so what helps me remember what the Z discs or the Z lines are is that because Z is the last letter of the alphabet, the Z disc is going to mark the end of a sarcomere. But because these sarcomeres are repeating units, the end of a sarcomere also marks the beginning or the start of another sarcomere. And so Z discs mark both the start and the end of a sarcomere. And really, what you can see down below is that these purple structures that we see on either end, these purple discs represent the Z discs. And so one single sarcomere actually has 2 Z discs on either end. And a sarcomere can be defined from the region from Z disc to Z disc. And so, the Z discs are important because they are vertical anchors to the actin, thin microfilaments. And the Z discs are going to be centered on the I band. And so what you'll notice is that, here what we have is the I band and this is one sarcomere here. But, if we had a neighboring sarcomere, there would be another I band over here. And so, the Z disc here would be centered on the I band. And the same goes for over here. There would be another I band here. And so, the Z disc would be centered between the I band. Now, again, the Z discs are vertical anchors to the actin. So the actin microfilaments that we see here are anchored to the Z discs. And so that's important to keep in mind. And so really this is the structure of a sarcomere and we'll be able to get some practice utilizing the concepts that we've learned here in our next video. So I'll see you guys there.

Correctly label each part of the sarcomere in the electron micrograph below:

So this practice problem wants us to correctly label each part of the sarcomere in the electron micrograph down below. Over here, we have the different parts of the sarcomere. The best way to familiarize yourself with the sarcomere is to get practice labeling different parts of the sarcomere on different images of the sarcomere. What we need to realize here in this image of our sarcomere is that this line over here, as well as this line over here, represent the Z discs. The Z disc is represented by letter F. This line over here, indicated by this bracket right here. We can indicate with another letter F is the Z disc. From Z disc to Z disc, indicated by this bracket up here and this line is going to indicate the length of an entire sarcomere. For that reason, we can use letter C to label this blank up above. Then, cross off this one and also cross off F. If, from this line to this line represents the sarcomere, then smack dab in the middle here, this line represents the M line. Called M for the middle. You can think M for the middle. We can go ahead and label this blank right here as letter B, for the M line. What we can see here is that this light-colored region indicated by this line over here, is going to represent the thin actin microfilaments. We can go ahead and label this arrow right here as letter E to represent the thin actin microfilaments and of course, that means that this right here, labeling the darker region, is going to represent the thick myosin filaments. So, we can label it as letter D, to represent the thick myosin filaments. What you'll notice here is we've got a Z disc here, Z disc here, and then over here, we have another Z disc. Essentially, what we have are 2 sarcomeres side by side here. The first sarcomere goes from this region to this region, and then the second one goes from this region to this region. You can see how the Z disc marks the end and the start of a new sarcomere. The region that only contains the thin filaments here, which would be this entire region, is going to be the I band since I is a thin letter and only contains thin actin microfilaments. We can label this region as G to represent the I band and then, of course, the length of the thick myosin filaments. The full length of the thick myosin filaments, which would be right here in this region indicated by this bracket, is going to be the A band. We can go ahead and indicate this down below as the A band and cross it off. Last but not least, because H is such a thick letter that reminds us that the region that only contains the thick myosin filament, indicated by this bracket right here is going to be the H zone. We can go ahead and label this one as letter H for that. Now, we've got all of the regions of the sarcomere labeled. That concludes this practice problem and I'll see you guys in our next video.

Here’s what students ask on this topic:

What is the structure and function of a sarcomere in skeletal muscle?

A sarcomere is the fundamental unit of muscle contraction in skeletal muscle. It is composed of thick myosin filaments and thin actin filaments arranged in a specific pattern. The sarcomere is divided into several regions: the H zone (only myosin), I bands (only actin), A band (overlap of both), M line (center anchor for myosin), and Z discs (boundaries of sarcomeres). The interaction between myosin and actin filaments during muscle contraction shortens the sarcomere, leading to muscle movement. Understanding the structure of the sarcomere is crucial for grasping how muscle contractions occur.

What role does the sarcoplasmic reticulum play in muscle contraction?

The sarcoplasmic reticulum (SR) is a membranous structure that surrounds myofibrils in muscle fibers. It plays a crucial role in muscle contraction by storing and releasing calcium ions (Ca²⁺). When a muscle fiber is stimulated, the SR releases Ca²⁺ into the cytoplasm, which binds to troponin on the actin filaments. This binding causes a conformational change that allows myosin to bind to actin, initiating the contraction process. After contraction, Ca²⁺ is pumped back into the SR, allowing the muscle to relax.

How do myofibrils contribute to muscle contraction?

Myofibrils are long, cylindrical structures within muscle fibers composed of repeating units called sarcomeres. Each sarcomere contains thick myosin and thin actin filaments. During muscle contraction, myosin heads bind to actin filaments and pull them towards the center of the sarcomere, shortening the myofibril. This collective shortening of myofibrils within a muscle fiber leads to the overall contraction of the muscle. Myofibrils are essential for translating the molecular interactions between myosin and actin into macroscopic muscle movement.

What is the difference between the H zone, I band, and A band in a sarcomere?

The H zone, I band, and A band are distinct regions within a sarcomere. The H zone is the central part of the sarcomere that contains only thick myosin filaments and no actin overlap. The I band is the region that contains only thin actin filaments and no myosin overlap. The A band encompasses the entire length of the thick myosin filaments and includes areas where myosin and actin filaments overlap. These regions are crucial for understanding the structural organization of the sarcomere and its role in muscle contraction.

What are Z discs and their function in a sarcomere?

Z discs, also known as Z lines, are the boundaries of a sarcomere. They mark the end of one sarcomere and the beginning of the next. Z discs serve as vertical anchors for the thin actin filaments, ensuring their proper alignment and stability during muscle contraction. The Z discs are crucial for maintaining the structural integrity of the sarcomere and facilitating the transmission of force generated during contraction to the entire muscle fiber.