 This is the picture where you're reminded of the big picture so that you don't lose track of the fact that all these little, oh, actin' binding site calcium, what, all those little pieces are put together into a cell which is bundled into a group, which is bundled into a group which makes up a muscle organ. So don't forget that. Here's a muscle organ. I'm going to draw on this guy because we need to label our parts. Do you agree that this whole thing is a skeletal muscle organ like biceps brachii, really? Pectoralis major, zygomaticus. Okay, really, I'll just stop right there. Skeletal muscle organs, the whole thing, one end attached to a bone, the other end attached to a different bone, when it shortens, you get a movement. Within every skeletal muscle organ, you have structures called fascicles. So you have the muscle organ which is made up of a bundle of fascicles and muscle fascicles are nothing more than a bundle of myofibers. Are you cool with that? Here's a fascicle. Look how many fascicles I have. One, two, three, four, seriously, yeah, there's a bunch of them. A whole bunch of fascicles in this muscle organ. And then the muscle fascicle is made up of a bunch of muscle fibers, the myofibers. That's just the muscle cells. So there's a fascicle as a bundle of cells. And then you remember, what are our myofibers? What are our cells filled with? They're filled with myofibrils. And those myofibrils were our little, we just made them. Myofibrils are like sarcomeres, stacked end to end to end for the full length of your sartorius. The sarcomeres make up the myofibril. And if you look at a single myofibril, you will see sarcomeres stacked end to end down the line. But what are sarcomeres made up of? Or myofibrils, what are they made up of? They're made up of the myofilaments. And so these are the thick and thin filaments that actually are going to do the work. So we have to look at the myofilaments in order to understand what's actually happening during a muscle contraction. Muscle contraction, next.