As we continue talking about the steps of muscle contraction, remember the events at the neuromuscular junction ended with an action potential spreading out or propagating through the sarcolemma. We now want to link or couple that excited membrane, the sarcolemma, to the actual steps of muscle contraction, and we call this excitation-contraction coupling. And to remind ourselves that this is the second major step we're talking about, we have it labeled here B. So excitation contraction coupling. What we're trying to do here is to connect an action potential. And sometimes you can see here, we just write the action potential as AP. We're trying to connect that action potential with a muscle contraction. Now, we're going to break this up into 5 steps here, but before we dive in, I want to look at these first two steps and just step back and look at this in 3 dimensions in the muscle fiber in a little animation here. So, as we look, we're going to see this sarcolemma, the outside membrane, and this action potential spreading down. And you can see that that action potential spreads down into the transverse or the T tubules. Those t tubules are these tunnels that dive in from the sarcolemma and surround these myofibrils. Now, you see that these T tubules are in really close association with the sarcoplasmic reticulum. They come up really tight together, and where they meet the sarcoplasmic reticulum is referred to as the terminal cisternae. And the 3 there, the terminal cisternae and the T tubule, form what's called the triad. Now, this sarcoplasmic reticulum is filled with these calcium ions. And as this action potential is going to come down, it's going to stimulate this voltage gated channel that's going to open up, and those calcium ions are going to rush out into the sarcomere, into the myofibril, and that's going to lead to the muscle contracting, as you see here. Alright. So let's return to our notes and write this down. So we're going to say that the action potential, or the AP, spreads down the sarcolemma and into the T tubules. In our image here, you can see-we have this section of the muscle fiber with parts cut away. You can see all those myofibrils here. The action potential is going to come down the sarcolemma, it's going to dive into these T tubules here, these pink sort of tunnels that are running down, and this is going to carry that excitation deep within the cell and it's going to stimulate the sarcoplasmic reticulum. The sarcoplasmic reticulum, you can see it in really close association with those T tubules there. So next, we're going to say that the voltage gated channels of the sarcoplasmic reticulum are going to release the calcium, or 2, these calcium ions into the sarcomere, and that's going to be our signal to contract. Alright. So now let's remember exactly how this happens. Before we fill in our notes here though, let's look at another animation. So we have here, we have the myosin on the bottom and we can see that actin filament. This calcium comes in, the calcium is going to come in and bind to that troponin. The troponin is then going to move the tropomyosin, and when that happens the binding sites on the actin are exposed and that myosin can bind. Now it's ready to contract. Alright. Let's look at that one just one more time. So remember here we have the calcium is going to enter in and it's going to bind to the troponin. And remember, the troponin, we said, is there to open the binding sites. It opens the binding sites by moving the tropomyosin. Tropomyosin's normally there saying no to the myosin, but when it moves the binding sites are open, our myosin can bind, and we're ready to contract. Alright, so we'll go back to our page here. And we see here we're going to say: 3, the calcium ions binds to the troponin, and that moves the tropomyosin. In our image here we can see these calcium ions coming in. We can see the troponin here in blue. We see this tropomyosin right now is going to be blocking those binding sites on the actin, so the myosin head can't bind. But once it binds, it's going to move and the myosin binding sites on the actin are exposed. And we can see that down here. Right now, this tropomyosin is out of the way and these actin binding sites are open. If those actin binding sites are open, well, now the myosin head is going to bind to the actin, and that's going to create what we call a crossbridge. Alright. And we can see that here. The myosin head has come up, it's bound to the actin, and now it is ready to pull and do that power stroke. We're not going to talk about that yet. Alright. Finally here, I just want to note, right, we've dumped this calcium into the sarcomere. It's not going to stay there forever. We'll go into all this later on, but I just want to note here that the calcium is only there for a little while. It's going to start leaving the sarcomere, And when the calcium ion exits the sarcomere, gets picked back up, or reenters the sarcoplasmic reticulum, that's going to cause that tropomyosin to move back, and the binding sites will now be blocked. So if the calcium's in the sarcomere, you're going to have contraction. If the calcium all gets picked up and it's removed from the sarcomere, the contraction's going to stop. Ah, forgot the unblockEd. Alright. We're going to talk about the cross-bridge cycle and how this myosin actually pulls on the actin, coming up next. But first, we have an example and some more practice problems. Give them a try.
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9. Muscle Tissue
Steps of Muscle Contraction
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