Hey, have you been working out? The reason I ask is I noticed there seemed to be a buildup of myofilaments in your muscles, which really only happens if you've been working out. So wait, you don't know what a myofilament is? Well, then it's a good thing I made this video. Don't worry, I'm going to tell you all about it.
Let's jump to the whiteboard and get started. In this video, we're going to look at the structure of a muscle. We're going to start at the organ level, looking at the whole muscle itself, and we're going to break it down piece by piece all the way down to the molecular level, where we'll look at the proteins that actually grab onto each other and pull in order for our muscles to contract. So in this diagram right now, we've got a few organs we can see. We've got a bone here, and attached to that bone is going to be a tendon, and tendons connect bones to muscles, and so we have a whole muscle right here.
Right now being drawn is a cross-section of that muscle. We're going to break it down into the next level of organization, which will be the tissue level. Now the muscle, of course, is the whole organ, and drawn in pink here is something that we call a fascicle.
Each of our skeletal muscles is made up of bunch of fascicles that are bundled together to form the whole muscle so each of those strands coming out of this muscle covered in pink is a fascicle each muscle fascicle is a bundle of cells so we've got several levels here that we're looking at we've got a muscle organ the muscle organs made up of a bunch of these fascicles which each fascicle is a bundle of muscle cells so now in the diagram we're going to zoom in to an actual muscle cell which has a lot of the same properties of any cells that you learned about back in biology. It's going to look a lot different. In this case, for one, it's really long, right? And so it's kind of extending out toward us. It's not actually getting bigger right here.
That's just kind of a three-dimensional representation of it so that we can kind of zoom in on each additional structure here. First thing you'll notice here is that it's covered in blood vessels. Our muscles need lots of blood.
They need lots of oxygen. They need lots of nutrients in order for them to be contracting all the time to do the different movements that we do. And so we've got lots of blood flow to our skeletal muscles.
We call this muscle cell a muscle fiber. So anytime somebody talks about muscle fiber, they're talking about the cellular level. Muscle fiber is a muscle cell.
Now, cells, of course, have organelles. So we're going to look at several organelles in the muscle cell right here. Now, of course, it has a nucleus just like most of our cells do.
I'm not going to draw them in there. I'm going to draw three organelles that are kind of specific to muscle cells. The first is just the cell membrane. We have a special name for that.
That's called the sarcolemma. The sarcolemma is just the cell membrane of the muscle cell. The next organelle that I'm drawing is called the transverse tubules or the T-tubules.
These are spaces on the cell membrane where it kind of folds into itself and there's a little tunnel that goes down into it. And you see the little network of tunnels drawn in purple there that make up the transverse tubules. The purpose of these is to take an action potential, which is going to be a signal from our neurons, our nerve cells, because that's how we control our muscles and tell them to contract.
It's going to conduct an action potential down into the muscle cells. So we can tell all of the little proteins in the muscle to start contracting with each other. So the transverse tubules are sort of a way to get that action potential from the cell membrane down into the muscle cell so it can tell everything in the muscle to start contracting.
So sarcolemma, cell membrane, and the transverse tubules, a way to conduct the signal or the action potential down into the muscle fibers themselves. Now the next organelle I want to talk about is called the sarcoplasmic reticulum. Now you'll notice that I've used the term sarco a couple times now. Sarcolemma, sarcoplasmic reticulum.
Sarco just means flesh in Greek and we use that to refer to muscles. So you'll see the term sarco. You'll also see the term myo which also means muscles.
So anytime sarco or myo we're talking about muscles. The sarcoplasmic reticulum has a lot in common with something you learned about in biology, the endoplasmic reticulum. So this is sort of the ER of the muscle. and it's going to store calcium which is an integral part of the contraction system of the muscle we call that the sliding filament model and we're going to see the filaments here in a little bit okay so those are all organelles and one more organelle i lied there's one more one more organelle that we're going to talk about are going to be these next sort of level of tubes that come out of this now we have the muscle which is sort of tube shaped we had fascicles which are a smaller tube within that and then we have muscle fibers which are kind of a smaller tube within that and now we have what we call the myofibril which is still at the organelle level the myofibril is going to be made up of a bunch of myofilaments which now is down to the protein level and so we see these drawn out right here i've drawn one in purple and one in pink to distinguish between the two now they don't actually have those colors of course and those are the myofilaments that are inside the myofibrils i apologize for all of these names i didn't name them so we've got the muscle We've got the muscle fascicles, which are bundles of muscle fibers, which have bundles of myofibrils.
Fibril just means small fiber. And those myofibrils are made up of myofilaments. The filaments are the proteins and we've got two of those called actin and myosin. Now these myofilaments form this sort of structure like this that we see drawn on the outside of the myofibril that really goes all the way down throughout the myofibril. I just drew it on the outside to show it but before we jump into that let's do a quick recap of everything we have here.
Now those myofilaments remember those are the proteins down at the molecular level of all of this but there's really five levels of organization that we've talked about so far. We've got the muscle. The muscle is going to be the whole organ. We have the fascicles. That's at the tissue level.
There's bundles of fascicles that make up the muscle. Next, we have the muscle fiber. That's going to be the cellular level.
A muscle fiber is a muscle cell, and a bundle of muscle fibers make up one fascicle. Now, each fiber has several organelles in it. We talked about the sarcoplasmic reticulum, which stores calcium.
We've talked about the sarcolemma, the cell membrane, and the transverse tubules, which conduct a signal or an action potential down into the muscle fiber. So those are all organelles and that's our next level of organization. The myofibril which is at the organelle level and then the myofibrils are made of these myofilaments which are proteins down at the molecular level.
Okay now let's zoom in on some of these myofilaments in what we call a sarcomere. So this whole thing I'm showing in this box down here is a sarcomere and I'll talk a little bit more about that in a minute. Drawn right now is one of the filaments called myosin.
And myosin is often called the thick filament because it's a little bit thicker than the actin filament, which is being drawn in purple. I realize now I didn't draw that to scale very well and the myosin looks thinner, but the myosin is the thick filament. And you'll see at the myosin on either end are these little kind of appendages.
We call those myosin heads. And even though we call them heads, they're really for grabbing on and pulling on the actin filament. So here we have the actin filament or the thin filament and the myosin heads which can grab onto it.
They sort of grab on and pull like you're pulling on a rope. Now you can imagine if the myosin filament right here on both sides of it are grabbing and pulling on the actin, it's going to pull this whole actin molecule closer together. And that's what happens whenever our muscles contract.
These myosin heads pull on the actin and sort of squish these Z lines together. Now we call this a z-line because it sort of zigzags back and forth. You see those zigzags right there.
So the myosin heads pull on the actin. They pull both actins closer. And suddenly we have these z-lines are going to be closer to each other on either side. They squish them together. That's a contracted muscle then.
And we would say that the sarcomere length, the sarcomere length being the distance from one z-line to the next, that sarcomere length decreases to contract that muscle. and then whenever we relax the muscle those myosin heads will let go and those z lines can spread back out and then we would relax our muscle and that's kind of a little bit of what's going on at the protein level or the molecular level i'll cover that more in depth in another video we'll talk about atp and calcium and all the role that that plays in all of this but for now we're just focusing on these levels of organization within the muscle we've got the muscle that's the organ level a fascicle which is the tissue level we've got a fiber the cellular level which has all these different organelles We have a myofibril down at the organelle level, and that myofibril is made up of these myofilament proteins, which are the things that are going to pull on each other to contract the muscle. And if you think of protein whenever you think of muscles, this is really what you're thinking of, all of these filaments.
And I just drew a few, actin and myosin, but really in every muscle that we have, we've got millions and millions of these actin and myosin proteins. Each protein is small and has a little bit of pulling force, but when you sum all of those up, millions and millions of them, our muscles can be very strong and cause all of the movements that we do every day. All right, we covered a lot of stuff in this, but the main thing I want you to test yourself on right now are these five levels of organization.
Pause the video, see if you can identify each level of organization as well as what the muscular equivalent of that level of organization is in our diagram. Let's start at the biggest level and work our way down. We've got the muscle, which is the organ level. Muscles are made of bundles of fascicles, which are at the tissue level.
Each fascicle is made up of a bundle of muscle fibers, which is the cellular levels, muscle fiber, muscle cell. Each muscle fiber, of course, contains all these different organelles, as well as some other classic ones you've learned, like the nucleus, as well as, at the organelle level, the myofibrils. And the myofibrils, of course, are going to be bundles of myofilaments, which are the protein molecules that actually pull on each other whenever we contract our muscles.
You know, I'm trying to think of something funny for the end credits, but I'm just running out of material. How do... How do stand-up comedians do this?
I can't stand up, and I'm not a good comedian. 0 for 2. All right, see y'all next time. Actually, that's like the most cliche way to end a video on YouTube, I think.