We now want to talk about the steps of muscle contraction with the cellular and molecular level in much more detail. However, before we examine these step-by-step processes, we just want to step back and look at an overview of what's going on here. So, we're going to start out by saying that muscle contraction involves two major things that need to happen. First up is the transmission of a nervous signal. Skeletal muscle contracts when it receives a signal from the nervous system. So, how is that signal received? How is that signal spread throughout the entire muscle fiber? Then, we also have to think about the contraction of the sarcomere. Now, we said the sarcomere is the fundamental unit of muscle contraction, and so far, we mentioned that the myosin pulls on the actin to make that happen. But we want to talk about, at the molecular level, in much more detail, what's going on in the sarcomere. Now, as you look here, you'll notice we have three boxes we're about to fill in, and we've just talked about two major processes. That third box is going to be about linking these two things. How do we get from an electrochemical signal to the mechanics of contraction? Alright. To start, we're talking about the transmission of a nervous signal, and we're going to talk about the neuromuscular junction. The neuromuscular junction is where the nervous system meets a muscle fiber. We're going to say at the neuromuscular junction, a muscle cell is stimulated by the nervous system, and that's going to result in the initiation of an action potential. An action potential is this wave of electrochemical signal that's going to spread down the muscle fiber through the sarcolemma, the muscle fiber cell membrane. Now, to illustrate this, we can see here we have the axon terminal, the end of this nervous system cell, releasing neurotransmitters into the synapse, the small space between the two cells. They're binding to receptors on the cell membrane, and that's going to start this action potential that's going to flow down the sarcolemma. I have just gone through all those steps quickly. Don't worry. We will go through those steps later on in more detail more slowly. Alright. We now have this action potential, and it's spreading down the sarcolemma, but we need to get it down into the sarcomere to get to the mechanics of contraction. That link, we're going to talk about as the excitation-contraction coupling. We're going to say that that starts with the action potential spreading or, in more technical terms, it propagates along the sarcolemma and enters the t-tubules. Here we can look at our image. We can see the edge of the muscle fiber. We have the sarcolemma here, showing this action potential spreading down, and it's going to dive down into this pink t-tubule, which surrounds like a ring around this myofibril. Alright. That gets that electrochemical signal down deep into the muscle fiber. You also notice right up in close connection with that t-tubule is this blue structure that is the sarcoplasmic reticulum, that highly specialized endoplasmic reticulum of the muscle fiber. When that signal goes down through the t-tubule, that's going to signal the sarcoplasmic reticulum to release calcium ions, and I'm going to write that in shorthand here as Ca2+. Alright. It's going to release the calcium ions, and those are going to go into the myofibril sarcomeres, and that is our link. Those calcium ions are going to result in the myosin binding sites becoming exposed. Here we can look at our image again. We're zoomed way in here. We see in pink, these are the calcium ions coming in. They're going to bind to the troponin. The troponin is going to open the binding site by moving this tropomyosin, this green filament surrounding the actin. We can see the myosin binding sites there on the actin. Those are going to become exposed. Alright. Now we can talk about the mechanics of contraction. Once those myosin binding sites are exposed, the myosin is going to bind to the actin. We're going to call that binding a cross bridge. Once bound, that myosin is going to pull on the actin, and we call that pulling motion the power stroke. Alright. We can see that here. We see the myosin is bound to the actin, and it's pulling the actin that way. Okay. Again, for all of these, we've talked about many steps. We're going to go through all those steps much more slowly and in more detail coming up. I'm looking forward to it, and I hope you are too.