Our example tells us that below is a list of structures that are found in the muscle fiber, and we need to mark the structure with a t if its primary role is involved in transferring a signal through the cell, mark the structure with an r if it's directly involved in regulating whether the sarcomere contracts, and mark the structure with a c if it's directly involved in the mechanics of contraction. Alright. So we have this big long process, and we started by breaking it up into these groups, transferring a signal, regulating contraction, and then actually contracting. So let's look at our structures here. We're going to start off with the actin. Which group would you put that in? The transferring a signal, regulating contraction, or actually the mechanics of contraction? Well, actin, I'm going to mark it with a c. The actin gets pulled on by the myosin, and that's what causes the sarcomere to contract. That is directly involved in the mechanics of contraction.
Alright. Let's look at the calcium ions. Which group would you put the calcium ions in? Oh, I'm going to mark the calcium ions with an r. Calcium ions get dumped into the myofibril, into the sarcomere, and that results in the binding sites on the actin opening. So if the calcium's in the sarcomere, there can be contraction. If the calcium's not in the sarcomere, there can't be contraction. That's directly involved in regulating. Next, we have the myosin. What do you think about the myosin? Well, if I marked actin with a c, then myosin's getting a c as well. Myosin holds on the actin, and that's how the sarcomere contracts. That's directly involved in the mechanics of contraction.
Next, we have the sarcolemma. Which group would you put the sarcolemma in? Well, I'm marking the sarcolemma with a t. The sarcolemma is the cell membrane of the muscle fiber, and it spreads that action potential throughout the muscle fiber, spreading that signal so the entire muscle fiber knows to contract at the same time. Alright. Next up, we have the sarcoplasmic reticulum. Which group would you put that in? Well, this is the one that gives me a little trouble. I can see arguments for 2 groups, but I'm definitely marking it with an r for regulating contraction. When the sarcoplasmic reticulum gets the signal, it's going to release the calcium ions. The releasing of the calcium ions is what signals the muscle to contract. That's what opens the binding site and allows the myosin to pull on the actin. Now I can see an argument for putting it in the transferring a signal, section, because transferring a signal to sarcoplasmic reticulum reticulum gets the that action potential, and that action potential signals the sarcoplasmic reticulum to release the calcium ions. But again, because it's releasing the calcium ions, I think it's better placed in the regulation group.
Alright. That brings me to the troponin. Which group would you put the troponin in? Well, I'm marking troponin with an r. Troponin, we said troponin opens the binding site. So if it's involved in opening the binding site, that's allowing whether the contraction can occur or not. When the calcium binds to the troponin, that troponin kind of changes shape, and that opens the binding site. Next, we have the tropomyosin. Well, if the troponin is a regulating group, then the tropomyosin is in that regulating group as well. The troponin moves the tropomyosin, because remember the tropomyosin is there saying no to the myosin. It's this filament that's there blocking the binding site. So if the tropomyosin is blocking the binding site, the myosin can't bind. The cell can't contract. Once it moves, contraction can occur. And then finally, we have the T tubule. Alright. The T tubule, I'm marking that with a t. The action potential moves down the sarcolemma, and then it goes into those tubes, the T tubules, which are these extensions of the sarcolemma, which dive deep into the cell and surround in a little bit of a ring around the myofibrils. That action potential down through the T tubule then tells the sarcoplasmic reticulum to release the calcium ion. So it is definitely there transferring that signal deep within the cell. Alright. Understanding these roles is going to be really important going forward. We have more practice problems to follow. I'll see you there.