Musculoskeletal System: Study with Video Lessons, Practice Problems & Examples

The musculoskeletal system comprises muscles and bones, facilitating locomotion. Muscle fibers, made of myocytes, contract through the sliding filament model involving actin and myosin. Motor units consist of motor neurons and muscle fibers, with acetylcholine triggering contractions. Muscle types include fast-twitch fibers for rapid, short bursts of energy and slow-twitch fibers for endurance. Bones, composed of osteoblasts and osteoclasts, support the body and produce blood cells. Joints, like ball-and-socket and hinge joints, enable movement, while antagonistic muscle pairs, such as biceps and triceps, work together to control motion.

Muscle System and Skeleton

Hi. In this lesson, we'll be talking about the musculoskeletal system, which is made up of the muscle system and the skeleton. Now the muscle system is an organ system that is made up of muscle, and it's going to include skeletal, cardiac, and smooth muscle, which are different types of muscle. Muscle is just a tissue that can contract due to an interaction between actin and myosin that we'll take a closer look at later. Muscle cells are called myocytes, and that prefix "myo" generally refers to muscles. Now there are specialized types of myocytes that you'll find in skeletal, cardiac, and smooth muscle, but you don't need to worry about that just yet.

Now, here we have a nice overview of the human muscle system, and here we can see just a few individual muscles, and how they are connected to bones. Behind me here, we have a myocyte. This is a muscle cell, and you might notice that there is a nerve that is forming a connection with it. We'll talk more about why that is later. Now the skeleton is going to be the support structure of an organism and it's going to work with the muscles to produce locomotion. Some organisms have an external skeleton called an exoskeleton. This is common in arthropods, like this awesome rhinoceros beetle you see here. We humans, on the other hand, have an endoskeleton, which you can see here. And this is an internal skeleton that's going to be made up of mineralized tissues, like bone. Bone is this rigid mineralized organ we'll talk a little bit more about later. And muscles and bones are going to be connected to allow for voluntary movement. We call this movement locomotion. You can see a pretty awesome example of that in this cheetah right here behind me.

With that, let's go ahead and flip the page.

Muscle Fibers and Sarcomeres

Muscle is made up of many muscle fibers, as you can see here in this diagram. These are long, thin myocytes that actually contain multiple nuclei, and they're going to be chock-full of what are known as myofibrils. Myofibrils are these rod-like protein structures that are going to be made of actin and myosin, and these are what are going to do the contracting. Now they're going to be arranged into units called sarcomeres, and these are essentially the contractile units that will be chained together and form the longer myofibrils. So each sarcomere is going to individually be contracting so that the longer myofibril itself contracts. Now, striated muscle, like skeletal muscle and cardiac muscle, gets its name because of these repeated units known as sarcomeres, that give it a sort of lined pattern, hence, striation. Now sarcomeres are going to be made of what are called thick and thin myofilaments, and these are going to interact by sliding past each other to cause contraction. Now, myofilaments take 2 forms. There are thin filaments, which are made of actin, and thick filaments that are made of myosin. They're going to be joined together, these sarcomeres, at what are known as Z discs. So here you can see our sarcomere. Hopefully, you can see the lined pattern in there, the striation, if you will. And these dark lines are the Z lines. So what's between these Z lines, or Z discs, either name really works, what's between these is one sarcomere, and it's going to be made up of these thick filaments that you see in blue here, and these thin filaments that you see in red. And those are what are going to slide past each other to cause contraction, like you see happening behind me. Here the thin and thick filaments are a little more spread out. Right? There's a little more distance here and here, they are contracted. That's gotten a little bit smaller because essentially what's happened is this, they've moved past each other in this direction. And we're actually going to flip the page right now, and take a look at how that happens.

Sliding Filament Model

The mechanism that explains how the thin and thick filaments slide past each other to cause contraction is known as the sliding filament model. Now, it's going to involve some proteins that interact with actin filaments. You see, these actin filaments are actually going to have binding sites for myosin along their length. But, normally, these myosin binding sites are going to be covered by this protein called tropomyosin, which is basically wrapped around actin to block these myosin binding sites. And it's also going to have this attached protein called troponin. And this is going to be a calcium sensitive protein that when it binds calcium, it causes the tropomyosin to actually move and expose the myosin binding site on the actin filament. Now, we'll talk in a little bit about where this calcium comes from. So, for now, I don't want you to worry too much about it. Just know that it's going to be due to an action potential hitting the neuromuscular junction, and that that's essentially just think of it as a synaptic signal that's going to trigger this calcium release. And we'll go into the details in just a little bit. Now, the myosin filaments have what are called myosin heads. They're I like to think of them more as like grabby arms or something, and these are what are going to be used to sort of pull the myosin along the actin filament. Now, when the troponin binds calcium, causes the tropomyosin to expose those binding sites on actin, the myosin is going to attach to the actin filament. Now it's actually only going to attach if it has ATP bound. And, if it has ATP bound, it will go ahead and attach to the actin filament, like you see happening here. Let me actually go ahead and number these for us. So, that's going to be the first step, second step, and then right behind me, jump out of the way, you'll see step 3. This is where that ATP is going to be hydrolyzed and trigger what's known as the power stroke. Now that's basically going to be this sort of shape change in the myosin protein that you see here, where it hydrolyzes the ATP and releases the phosphate and the ADP, and it causes the head to essentially move in such a way that it pulls itself along the actin filament. So, here you can see they're trying to show the difference in the angle that it takes. Same with what's going on here. So essentially that head is going, you know, from this position to this position, so it'll move, you know, this distance more or less. And once that ADP gets released, like we see happening right here in panel 5, the myosin is actually going to, bind another ATP and release from that actin filament, like we see happening here in panel 6. Now, this is, you know, essentially one little step that will have to be repeated many many many times to cause, you know, the big muscle contractions like, you know, I mean, even just this is, needs, you know, a bunch of those power strokes to, you know, get my muscles to crank like that, you know. So, the point is this is going to be repeated many many times. This just causes like one little movement but you add a ton of these together, you know, in all those sarcomeres and then you actually see a big movement. So with that, let's go ahead and flip the page.

Motor Unit and Neuromuscular Junction

A motor unit is composed of a single motor neuron and the muscle fibers that it controls. The connection between the motor neuron and the muscle fibers is known as the neuromuscular junction. It's basically a synapse between a neuron and muscle, and here in this image, you can see an example of a motor unit. Here we have our motor neuron. This is our neuromuscular junction right here. Inside, you can see all of these little myofilaments, and this whole thing here is a muscle fiber. Now, the muscle fibers of a motor unit are going to contract as a group. The nerve is going to release acetylcholine when an action potential comes, and this is going to stimulate muscle contraction. And the way that works is basically acetylcholine will bind to receptors on the other side of the neuromuscular junction. These are called nicotinic receptors, in case you're curious, and this is going to lead to depolarization through excitatory postsynaptic potentials, and that is going to cause an action potential that will actually travel into these indentations in the membrane called transverse tubules, and that's going to pull these action potentials into an area known as the sarcoplasmic reticulum. And this is basically a special type of smooth endoplasmic reticulum that's found in myocytes. And the action potential will travel down these transverse tubules and cause the sarcoplasmic reticulum to release calcium. That's how this calcium is going to get in there to bind to the troponin and cause the tropomyosin to move out of the way for the myosin heads to bind to the actin filaments. Now after this is done, the calcium has to be pumped back into the sarcoplasmic reticulum, because, you know, you need to be able to essentially make these discrete contractions. Right? The nerves are going to be sending discreet signals, and so we need to be able to have discrete muscle contractions. We also don't want to have permanent contraction, permanent tension. Now here you can see an example of that action potential moving down this transverse tubule, or t-tubule, as it's sometimes called, and this is going to lead to calcium release that will lead to contraction, which you can see here. So with that, let's go ahead and turn the page.

Fast and Slow Twitch Muscles

An action potential will cause muscle fibers to twitch, as it's called. So a single action potential results in a single twitch. Now the tension, the difference in tension that you can create in your muscle is due to action potential frequency. So the higher the frequency of the action potentials, the more tense the muscle will become. And this can actually lead to something known as tetanus, which is sustained muscle contraction because the twitches fuse together. Now you can see a model of that happening here where we have an action potential, this red spike, and then here in blue we have the twitch of the muscle. And here you can see the frequency of action potentials, each one of these lines is an action potential coming in. And here you can see the resulting tension, how the frequency of the action potentials is actually going to cause the tension to increase.

Now, there are actually different types of fibers designed for different types of twitches. There are what we call fast twitch fibers that contract very rapidly, and slow twitch fibers that contract slowly. Now, the reason you want both of these is they serve different purposes. Those fast twitch fibers contract very quickly, but they'll tire out quickly as well. And this is in part because they rely on ATP from glycolysis instead of aerobic respiration, which is going to allow them to get a lot of ATP really quickly, but it's not going to be as sustainable because aerobic respiration provides a much higher yield of ATP. Now these slow twitch fibers can sustain longer contractions. They're not going to tire out as quickly, and they are going to contract more slowly due to the rate of ATP hydrolysis in mice, and that's what's actually going to control that, or what's going to cause it to actually slow down.

Now muscles contain a special oxygen binding protein called myoglobin, like hemoglobin, for example. You know, a similar idea there. Of course, it's going to be a bit different from hemoglobin. However, it's an oxygen binding protein, and its purpose is to actually store oxygen. So that when the oxygen demand in muscles is very high and, you know, perhaps the hemoglobin can't provide everything they need, we have this store in myoglobin.

Now, there are different types of muscles, and they can be grouped based on whether they're controlled by conscious thought or unconscious signals. So voluntary muscles, which are controlled by conscious thought, are skeletal muscles, and these are going to be those striated muscles made of long myofibrils. Now involuntary muscles are, as we said, controlled by unconscious signals, and these are going to be regulated by a division of the nervous system known as the autonomic nervous system.

Now, there are going to be two types of involuntary muscles, smooth muscle, which actually does not contain myofibrils, and these are going to line blood vessels, the digestive tract, and they're going to be responsible for peristalsis. So these are actually a very important type of muscle, just made of little individual cells that will contract in a different way than these myofibrils. They'll still use those actin and myosin interactions, but they're not going to have these long chains of sarcomeres that are, you know, going to chain together those contractions.

Now cardiac muscle is striated, and it contains sarcomeres. However, it is an involuntary muscle. You don't have conscious control over your heart rate. And it's going to have this special feature known as intercalated discs. These are basically, like, they're going to have gap junctions, more or less. There's a little more to them, but, they involve gap junctions, which are those direct connections between cells, and that's going to allow action potentials to spread through the cardiac muscle. And this is going to be very important for heart contraction, which relies on the spread of these action potentials, flowing through the heart to, create the rhythmic heartbeat.

Endoskeleton

The vertebrate endoskeleton is composed of bone, cartilage, tendons, and ligaments. Bones are made of cells in a very special hard extracellular matrix that contains calcium. As you can see, there are many different types of bones: short bones, flat bones, and structural bones. We're going to take a look at long bones. You can see a long bone right here. What's special about long bones is that they have a medullary cavity, which contains red and yellow bone marrow. This is very important for the production of blood cells.

There are some special cell types in bones that I want to mention, which are osteoblasts and osteoclasts. Osteoblasts form bone, and osteoclasts reabsorb bone. These cells are involved in blood calcium homeostasis, the endocrine regulation of it, through the hormones parathyroid hormone and calcitonin. If you want to know more about that, I recommend you check out our videos on the endocrine system.

With that, let's go ahead and flip the page.

Joints

Cartilage is this elastic tissue containing collagen proteins, and for some organisms, it's actually going to be the main component of their skeleton. For example, sharks. Now it's also going to be an important component in tendons and ligaments, which are types of connective tissue that, in the case of tendons, link muscle to bone, and in the case of ligaments, link bones together, or link bone to bone. And you can see an example of that here, where we have a tendon connecting this muscle to this bone, and here where we have a ligament connecting this bone to this bone. Now joints are what we call these connections between bones, and they're going to actually allow for specific types of movements. Now there's many different types of joints, however, I want to mention too, those are ball and socket joints. These are going to be like your hips and shoulders, where you basically have a ball that fits into a socket. And it's going to rotate around in there, and what's nice about ball and socket joints, as I'm sure you know with your arm, is you can really get a lot of range of motion on that. Now, in contrast, hinge joints limit movement to a single plane, but these are usually able to generate more force. And a nice example of a hinge joint is this joint we see, right here in your leg. Right? Now, as you can see inside this joint, we're going to have some nice cartilage that's going to protect those bones from rubbing on top of each other, which would have been very painful, and is actually the cause of some types of arthritis. Now, muscles and bones work together in basically what you can think of as antagonistic pairs. Now this is not always the case, but many muscles work together in these antagonistic pairs. So I want to show you one example of how, you know, muscle and bone come together to produce different types of movement. So here we have a wonderful example of an antagonistic pair that we're all familiar with, and that is the biceps and the triceps. And you can see that the biceps, when these muscles tense, right? When they pull, they're going to pull these bones closer together. That's why we call them flexors. It's a muscle that basically bends a limb and pulls it close together. Now these are going to work in opposition to extensors. This is the that antagonistic pair. The extensor is the antagonist to the flexor. The extensor, when it contracts, is going to straighten and extend a limb. So here in this case, let me just erase this for a second, you can see that if the triceps contract, if it pulls in like this, it's going to result in pulling the arm straight like that. And that is why we call that an extensor because it extends the limb out, and a flexor, you know, it's like you're flexing. Think about it that way. Alright. That's all I have for this one. I'll see you guys next time.

Here’s what students ask on this topic:

What is the sliding filament model in muscle contraction?

The sliding filament model explains how muscles contract by the interaction of actin and myosin filaments. Actin filaments have binding sites for myosin, which are usually blocked by tropomyosin. When calcium ions bind to troponin, it causes tropomyosin to move, exposing the binding sites. Myosin heads, which have ATP bound, attach to these sites and hydrolyze ATP, causing a power stroke that pulls the actin filaments past the myosin. This process shortens the sarcomere, leading to muscle contraction. The cycle repeats as myosin binds new ATP, detaches, and reattaches to another actin site, resulting in a coordinated contraction of the muscle fiber.

What are the different types of muscle fibers and their functions?

Muscle fibers can be categorized into fast-twitch and slow-twitch fibers. Fast-twitch fibers contract rapidly and are used for short bursts of power and speed, such as sprinting. They rely on ATP generated from glycolysis, which provides quick energy but is not sustainable for long periods. Slow-twitch fibers, on the other hand, contract more slowly and are used for endurance activities like long-distance running. They rely on aerobic respiration, which produces ATP more efficiently and allows for sustained muscle activity. Slow-twitch fibers also contain more myoglobin, an oxygen-binding protein that helps store oxygen for prolonged use.

How do motor units and neuromuscular junctions work in muscle contraction?

A motor unit consists of a single motor neuron and the muscle fibers it controls. The neuromuscular junction is the synapse between the motor neuron and muscle fibers. When an action potential reaches the neuromuscular junction, it triggers the release of acetylcholine, which binds to nicotinic receptors on the muscle fiber membrane. This causes depolarization and generates an action potential that travels down transverse tubules to the sarcoplasmic reticulum, releasing calcium ions. The calcium binds to troponin, causing tropomyosin to move and expose myosin-binding sites on actin, leading to muscle contraction.

What roles do osteoblasts and osteoclasts play in bone homeostasis?

Osteoblasts and osteoclasts are crucial for bone homeostasis. Osteoblasts are cells that form new bone by secreting collagen and other proteins, which mineralize to become bone matrix. They are essential for bone growth and repair. Osteoclasts, on the other hand, break down bone tissue by resorbing the bone matrix, releasing calcium and phosphate into the bloodstream. This process is vital for bone remodeling and calcium homeostasis. The activities of osteoblasts and osteoclasts are regulated by hormones such as parathyroid hormone and calcitonin, ensuring a balance between bone formation and resorption.

What are the differences between voluntary and involuntary muscles?

Voluntary muscles, such as skeletal muscles, are controlled by conscious thought and are responsible for movements like walking and lifting. They are striated, containing long myofibrils with repeating sarcomeres. Involuntary muscles, such as smooth and cardiac muscles, are controlled by unconscious signals from the autonomic nervous system. Smooth muscles, found in blood vessels and the digestive tract, do not have striations and contract slowly. Cardiac muscle, found in the heart, is striated and has intercalated discs that allow for synchronized contractions. Unlike voluntary muscles, involuntary muscles function without conscious control.