Muscles. They’re quite magnificent. The muscular system consists of muscles. And often when we think of muscles, we think about certain ones that you could identify right under the skin like biceps or triceps. But muscles are so much more than that. This video is going to not focus on the names of all kinds of different muscles, which is an exciting thing for you to explore, but more about muscle tissue and how muscle contraction works: the actin-myosin cycling. First, let’s talk about muscle tissue. Muscle tissue is made up of muscle fibers. These fibers are the muscle cells, and they have a structure that aids in their function. There are three types of muscle tissue to discuss. Cardiac Muscle tissue. Like its name suggests, it’s in the heart. The muscle fibers are branched and striated, or striped. Each fiber has a nucleus (note: sometimes two). At the ends of the fibers, you’ll find something called intercalated discs. These are really important because they’re involved in helping the cardiac muscle tissue contract in an organized, wave-like pattern. This muscle tissue control is involuntary – that is, you do not consciously control it. Smooth Muscle tissue. It’s…smooth. Really just meaning here it doesn’t have striations or stripes. Each fiber has one nucleus and they are spindle-shaped, meaning they are wide in the middle then taper at both ends. You’ll find them in the digestive system, in arteries and veins, in the bladder, in the eyes changing the iris size. They’re involuntary which means, you don’t have conscious control of them. Skeletal muscle tissue. This is the one you often think of with the biceps or triceps, because skeletal muscle is what attaches to bone or skin and is involved with voluntary control, meaning you can consciously control it. You can choose to pick up that biology textbook. If you could zoom in to see these, skeletal muscle fibers are striped, or striated. The fibers are long cylinders that are multinucleated. Fancy term for multiple nuclei. All muscle tissue have some characteristics to mention: It can stretch or extend, extensibility. It can retract back to its starting length, elasticity. Muscle tissue also has excitability. This means these cells have the ability to be stimulated and in the case of muscle tissue, their membranes can have electrical changes and send action potentials. Muscle tissue also has the ability to contract – or contractility. There are some differences in how the contraction happens in the three tissue types. For the rest of this video, we’re really going to focus on that last tissue mentioned, skeletal muscle. Let’s look a little bit about how they’re named, how they’re arranged, and how they contract. Many skeletal muscles are named by their location or their shape. Many have Latin or Greek root words in them so checking out a root word definition list can be really helpful. For example: rectus femoris –it’s a muscle on the thigh -or rectus abdominis - that’s a muscle of the abdomen. The Greek letter delta looks like a triangle which is a fitting name for deltoids, a triangular shaped muscle. There are some beautiful diagrams of skeletal muscles where you can explore the many muscle names and locations. As we mentioned, many skeletal muscles can pull on bones. The part that attaches to the bone that will be moved is called the insertion and the part that attaches to a fixed part of the bone is called the origin. There could be several muscles involved in a single action. The main muscle doing the work – the prime mover- is called the agonist. Whereas antagonists are muscles that can do the opposite action which is helpful for keeping position. If we zoom into some skeletal muscle…at the cellular level…how does it do what it does? How does it…contract? This is probably the most exciting part of the video. Ok so imagine you have some skeletal muscle like the biceps muscle. That muscle is made of many muscle fibers, which, remember are muscle cells. By the way, some of these cells are big. Of course cells are generally microscopic, but muscle cells can have lengths up to 30 cm! Anyway, inside a muscle fiber are multiple myofibrils, which are long cylinders. Each myofibril has sections that repeat called sarcomeres. And it’s the arrangement of these sarcomeres that contribute to the skeletal muscle’s striated look. There is a lot to a sarcomere, and that’s where we’re going to focus. So, the sarcomere has a protein known as actin. Actin makes up what is known as thin filaments. The sarcomere also has a protein called myosin. Myosin makes up thick filaments. To try to remember which one is which, I try to remember that the word “thin” is almost in the word actin. Minus the “h” though but close enough to help me remember. Anyways, both of these are essential in causing muscle contraction. Enter the sliding-filament model of muscle contraction. Now as we often tell you, we’re going to do a simplified version – when you’re ready for more to explore, check out our video description. Ok, so we have actin (thin filaments). We have myosin, (thick filaments). We have these Z lines here where a sarcomere ends and these Z lines are where the thin filaments attach. The thick filaments are held by accessory proteins in this area called the M line. The big, super important concept here is this: the sarcomere must shorten for muscles to contract. BUT, the thick and thin filaments themselves do NOT shorten. So the way that is going to happen is that they’re going to slide past each other. And, they do. When the sarcomere contracts, the thin filaments will be pulled by the thick filaments towards the center. There is overlap of the thin and thick filaments. Z lines will be moved closer together. How are they doing that? So, let’s zoom into this here. Showing a thin filament on top. That’s actin. Thick filament on bottom. That’s myosin. Myosin has structures here called myosin heads. Hundreds of them actually, but we’re just going to focus on one. And here, we show the myosin head is bound to ATP. It hydrolyzes the ATP– remember hydrolyzing results in ADP and a phosphate, which are both still bound to the myosin head. The myosin head can now bind to the actin. We call this a cross bridge. Now, the myosin head can release the ADP and phosphate, and the myosin head bends and it performs a power stroke! Power stroke meaning the thin filament slides towards that sarcomere center. A new ATP molecule binds the myosin head, and that is what lets the myosin head detach. Without ATP, the myosin head would not detach and this is actually the reason for rigor mortis – how muscles can be rigid after an organism dies as the organism is no longer producing ATP. ATP is needed for the myosin to separate from the actin. So during muscle contraction, you can imagine all these hundreds of cross bridges forming and breaking and power strokes happening in each sarcomere throughout the time of a muscle contraction. It’s popular in bio to compare the myosin heads to tiny little oars of a boat. And because there would be some attached at any given time during a contraction, it prevents the actin from slipping back to its original position. But there’s more. A muscle isn’t always contracting, there is regulation in that as it’s not just cycle after cycle. A lot of this regulation involves the ability of the myosin heads to bind the actin in the first place. The actin has something called tropomyosin on it. It’s a regulatory protein and it blocks the myosin bonding sites on the actin. We’re drawing it like…a ribbon here, blocking sites. Another thing called troponin or really a troponin complex, is a set of more regulatory proteins. Together, these regulatory proteins block those myosin bonding sites and if the myosin doesn’t bind the actin, the muscle can’t contract because that thin filament isn’t going to be sliding. But when a neuron stimulates a muscle, it can trigger a release of calcium. And those little Ca2+ ions bind to the troponin. The troponin has a conformational change and this lets the tropomyosin move off the myosin bonding sites. Now, the myosin heads can bind! Pretty fascinating way to regulate. Next time you pick up that biology textbook, you might want to pause to reflect on the amazing events occurring in your skeletal muscles. Well, that’s it for the Amoeba Sisters, and we remind you to stay curious.