Our example says that the term excitation-contraction coupling refers to the events that turn an action potential into a muscle contraction, and we want to put the steps in order. And so, we see five steps here, and these steps, though, start with an action potential propagates through the muscle fiber, and they end with the cross bridges forming and the muscle contracting.
Alright. So the action potential propagates through the muscle fiber. What's going to come after that? Well, as I look here and I think about the excitation-contraction coupling, I'm definitely looking for something that talks about excitation. And as I look here, there's one thing that talks about an action potential. That's c. It says the action potential travels down the sarcolemma and the T tubules. Alright. That's that wave of depolarization spreading through the cell, spreading that signal, so I'm almost certain that comes next. Going to put a c there, and I'm also going to just cross it out up here for a little bookkeeping.
Alright. So we have the action potential now going down into the T tubules. What's going to come after that? Well, as I think what's going to come after that, I'm going to look for something about the sarcoplasmic reticulum, because the action potential goes down through the T tubules and it stimulates the sarcoplasmic reticulum. So look, do you see something about the sarcoplasmic reticulum? I do. D here, voltage gated channels open releasing calcium into the sarcomere. Now it doesn't say sarcoplasmic reticulum, but this is what it's talking about. That's what the sarcoplasmic reticulum does. When it gets that signal from the T tubules, it releases the calcium. So d is going to come next.
Now, where does the calcium go after that? Well, the calcium binds to troponin, so I'm almost certain a is going to come next. I'm going to write that down. And what happens when the calcium binds to the troponin? Well, remember the troponin kind of changes its shape and when it does that it pulls on the tropomyosin and pulls it out of the way. There we go. E. The troponin changes conformation, moving the tropomyosin. So that's coming next.
And that leaves me with one option. Let's make sure it comes last. So, the last option is, myosin binding sites on actin are exposed. Alright. Is that the last step? Absolutely. That tropomyosin moves, that moves out of the way of the binding sites so the myosin can bind to the actin. So B comes last. And once those myosin binding sites on actin are exposed, well, then the cross bridges can form and the muscle can contract.
Alright. We're going to go into those in a step-by-step fashion coming up, but first, we've got some more practice problems tomorrow.