Romeo and Juliet, Helen and Paris, Tristan and Isolde. These famous star-crossed lovers bring to mind insatiable longing, forbidden love and tragic separation, and poets and emo rockers love them for it. But you know where else you can find a nice hot romance? Your muscle cells. They've got their own famous coupling, a pretty pair of tiny protein strands called actin and myosin. Romeo and Juliet may have set a chain of tragic events into motion with their infatuation, but deep down in the cells of your muscles, the hot protein action between actin and myosin is actually literally causing all of your motions. All of them.

And I don't just mean voluntary stuff like walking down the street or moving your mouth so you can talk or chew chips, because your muscles also support your weight and help fend off gravity. The amazing thing about your complicated, self-healing, blood-guzzling muscle tissues is that they turn chemical potential energy into mechanical energy or movement, simply by doing two things: contracting and relaxing. And that contracting and relaxing is exactly what's fueled by the constant coupling and separation of biology's greatest lovers. Somebody get these proteins a movie contract. 

(Crash Course Intro Music) 

You will recall from our early lessons on tissues that you're kept alive and moving by three types of muscle tissue - smooth, cardiac, and skeletal. Your smooth muscle tissue is found in the walls of all your hollow visceral organs like your stomach and airways and blood vessels, where it involuntary and very usefully pushes fluid and other material around by contracting and relaxing over and over. 

Your heart is so important that it gets its very own muscle tissue type, cardiac muscle, which looks striped or striated, and also functions involuntarily to keep your blood pumping without you having to think about it. But when you hear the word muscle, you're probably thinking the kind that you see on Chris Evans when he first walks out of that machine in Captain America. 

And those types, the ones you can see and feel and flex, are your 640 skeletal muscles. They're striated like cardiac muscle tissue but they're also mostly voluntary, meaning you have to think about using them and activate them with your somatic nervous system. Most of them attach to your skeleton and create movement by pulling bones in different directions as they contract. Each one of your different skeletal muscles, like your biceps brachii, your vastus lateralis or gluteus maximus is technically its own organ, made up mostly of muscle tissue, but also of connective tissue, blood vessels and nerve fibers. 

And because your muscles are voracious energy hogs, each one is rigged up with its own personal nerve to stimulate contraction, and its own artery and vein to keep it well fed with all the blood and oxygen and nutrients it needs to operate. But to understand those operations, we first need to get a grip on the anatomy of a skeletal muscle, which involves fibers within fibers and lots of layers.

Basically a skeletal muscle is constructed like a really sturdy piece of rope. Thousands of tiny parallel threads called myofibrils squish together to form muscle fibers, which are your actual muscle cells. Cells with mitochondria, multiple nuclei, and a cellular membrane called a sarcolemma. Those muscle fibers then form larger string-like bundles called fascicles, which combine to form the larger rope-like muscle organ like your biceps brachii. Overall this bundles of bundles configuration makes muscle tissue fairly sturdy, but considering how much abuse your muscles take when you do something like pretty simple, like lift a big bag of dog food, it's no surprise they need a little help. 

That's why every muscle contains a few different kinds of supportive sheets of connective tissue, the protective reinforcements to keep that bulging muscle from bursting. So that is the structure part of the story. But if you wanna get into the nitty gritty, the down and dirty of how you actually move? Well there are rules. 

Really just two main rules and they have to do with proteins. And they're both true for a lot of the proteins we talk about, whether they're enzymes or ion channels or receptors or muscle proteins. And these rules are: 1) proteins like to change shape when stuff binds to them. And 2) changing shapes can allow proteins to bind or unbind with other stuff. So keep those rules in mind while we see how a muscle fiber contracts and relaxes. 

Now, remember those tiny myofibrils that bundle up to form your muscle fibers? They're divided lengthwise into segments called sarcomeres, which contain two even tinier strands of protein - two different kinds of myofilaments called actin and myosin. And its their angsty story of star-crossed love that fuels every movement your body could possibly dream up. 

A sarcomere contains both thin filaments made up mostly of two light and twisty actin strands, and thick filaments composed of thicker, lumpy looking myosin strands. Each sarcomere is separated by what's known as a z-line at either end, which is just a border formed by alternating thin filaments in a kind of zig-zag pattern. A muscle contracting is all about sarcomeres contracting bringing those z-lines closer together.

All right, so now comes the romance. When your muscle cells are at rest, your actin and myosin strands don't touch but they really, really want to. Specifically that club-headed myosin wants to get all up close and personal with the actin. When this happens, and it will eventually, it's called the sliding filament model of muscle contraction. But in meantime, like in any good love story, the pair have some obstacles to overcome. 

Namely actin is blocked by a couple of protein bodyguards called tropomyosin and troponin, which keep getting in the way. Luckily, these guards can be bought off with a little ATP and some calcium. I prefer cash and nachos, but whatever. 

Remember ATP is kind of like molecular currency. It contains chemical energy and your muscles are all about converting chemical energy to motion, so they're always hungry for more ATP.  Your muscle cells have lots of nuclei, but some of them also have a lot of mitochondria, whose sole purpose in life is to crank out ATP, and muscle cells also have their own version of an endoplasmic reticulum, the cell's transport and storage system, but in this case, it's specialized so it gets a special name: the sarcoplasmic reticulum.  Its walls are loaded with calcium pumps, which use ATP to save up a bunch of calcium ions, and it's also studded with calcium channels that are linked to voltage sensitive proteins in the membrane of the muscle cell.

Say I want to move my arm.  My brain sends an action potential along the motor neuron until it synapses with a muscle cell in my arm.  The receptors on that muscle cell are ligand-gated sodium channels.  So when the motor neuron releases our old friend acetylcholine into the synapse, the channels open up and create a rush of sodium into the cell as a graded potential which, if it's strong enough, causes nearby voltage-gated sodium channels to open.  Now I want to take a second and point out here that we're still talking about action potential but not in a neuron, this is happening in a muscle cell, people.

So that action potential zips along a muscle cell's membrane, the sarcolemma, which has lots of tubes that run deep inside the cell called T-tubules. When the action potential travels down one of those tubes, it eventually triggers the voltage-sensitive proteins that are linked to those calcium channels in the cell's sarcoplasmic reticulum. When those channels are thrown open the calcium stored inside rushes into the rest of the cell and finally myosin is like, "Yes! Here we go!"  

At this point the myosin is totally stoked because the bodyguards that had been frustrating it are in for a big, irresistible distraction. That's because the protein troponin just loves to bind with calcium. And remember, when stuff binds to proteins, the proteins change shape. So the calcium latches on to the troponin and causes it to pull the other bodyguard protein, the tropomyosin, away from the sites on the actin strands that the myosin really wants to get its paws on. And suddenly, it's all, "Okay?" "Okay."  

But the only myosin heads that can bind to those newly-exposed sites are the ones that are ready for action. That is, the ones that have already grabbed a molecule of ATP that’s been floating around and broken it down into ADP and the leftover phosphate. When a myosin head does that, it moves into an extended position, kind of like a stretched spring, still holding on to the ADP and the phosphate, and still storing the energy that was released when they were broken apart.  

So after all that, with the myosin primed for action and the bodyguards out of the way, the myosin finally binds to actin and it is beautiful. When they bind, the myosin releases all that stored energy, and in the excitement of it all, the myosin changes shape. It pulls on its precious actin strand, kind of like pulling a rope, hand over fist. In the process, it shrinks the whole sarcomere and contracts the muscle. That's the sliding part of the sliding filament model.

Now with its energy spent, that little head has no use for the ADP and the phosphate, so they unbind with it because, remember rule number two, changing shape encourages proteins to bind or unbind with stuff.  That unbinding causes a small change in its shape, which lets a fresh ATP bind there in its place. That binding causes another shape change, but this time it causes the myosin to release from the actin in a tear-jerking scene like some microscopic re-creation of the finale from Titanic.

But fear not, this epic is not quite over.  Because this is when the myosin breaks down its new molecule of ATP into ADP and a phosphate, which moves it into the armed position yet again, getting it ready for its next rendezvous.

And meanwhile those calcium pumps are working hard to restock the calcium and the sarcoplasmic reticulum, so they start grabbing the calcium that's floating around, causing calcium to unbind from the troponin. When it unbinds, the resulting shape change puts the tropomyosin bodyguards back into place.  

It's a circle. Or potentially, a big Hollywood franchise with lots, and lots of sequels. It keeps repeating itself many, many times every moment while I sit here and talk and while you sit there and eat, and text, and take notes, the whole drama replaying itself, over and over.  Kind of like you'll have to play this video over and over again to get all the little steps of the sliding filament model straightened out. But hey, some stories get better the more you hear them.

If you do watch this one again, you will re-learn that your smooth, cardiac and skeletal muscles create movement by contracting and releasing in a process called the sliding filament model. You'd also re-learn that your skeletal muscles are constructed like a rope, made of bundles of protein fibers and that the smallest strands are your actin and myosin myofilaments. It's their use of calcium and ATP that causes the binding and unbinding that makes sarcomeres contract and relax.

Special thanks to our headmaster of learning Thomas Frank for his support of Crash Course and free education, and thank you to all of our Patreon patrons who help make Crash Course possible through their monthly contributions. If you like Crash Course and want to help us keep making great new videos like this one, you can check out patreon.com/crashcourse. 

Crash Course is filmed in the Dr. Cheryl C. Kinney Crash Course studio. This episode was written by Kathleen Yale, edited by Blake de Pastino, and our consultant is Dr. Brandon Jackson. Our director is Nicholas Jenkins, the editor and script supervisor is Nicole Sweeney, our sound designer is Michael Aranda, and the graphics team is Thought Café.