Now that you have an overview as to how muscle contraction works, here Leslie now discusses in more detail how it is affected in the presence of calcium. What really happens when these ions are released?
Watch to learn more and enjoy!
Transcript of Today’s Episode
Hello and welcome to another episode of Interactive Biology TV where we’re making Biology fun! My name is Leslie Samuel and in this episode, Episode 42, I am going to talk about how the release of calcium ions results in muscle contraction. So, let’s get right into it.
So, here we’re looking at a muscle and there are a few terms that I want you to know. This is called the fascicle, so this section right here, that’s a fascicle and that is basically a bundle of muscle cells. And, this of course then would be individual muscle cells or, as I said in the last episode, you can also call it muscle fibers. What I’m going to do now, is we’re going to take this muscle cell and we’re going to look at it much larger here. Here you can see we have the muscle cell, the muscle fiber and that is made up of these individual myofibrils. So, this would be a myofibril.
We looked at the myofibril in the previous episode and we showed how they’re made up of sarcomeres and I’m going to call a sarcomere from right here, you see this part here, to here, that is one sarcomere. As I said in the previous episode, this is the functional unit of contraction. We’re going to look at how calcium ions is responsible for the contraction of this sarcomere. And, we’re going to look at an animation of how that contraction looks.
So, let’s go to the next slide. Here, we’re looking at the sarcomere and we looked at the parts of the sarcomere. We said that we had a thick filament and that thick filament was myosin, and then we had a thin filament, and that thin filament is called actin.
Now, when muscle contraction happens, it’s because of the sarcomere becoming shorter, this is moving in and I’m going to animate that for you. This is contraction happening, and then the muscle relaxes and it goes back to how it was before. Contraction happens, the muscle relaxes, and then it goes back to how it was before. To put that in perspective, this is me working out in the gym, and as I contract the muscles in my arms, this is what happens. So, my bicep muscles contract, I pull it up, you can see it shortening. The sarcomere is getting shorter. We looked a little bit at how that happens. We said that there are myosin heads on the myosin that actually pulls along and pulls the actin so that this entire unit gets shorter. As the sarcomeres get shorter, and you have many of them along the myofibrils, as they get shorter, the muscle contracts and that causes my lower arm to move up in that direction.
So, now let’s take that and look at a little more detail. So, we’re going back to this picture where we’re looking at the muscle fiber so, that’s this part here again, and as we look at the muscle fiber, there’s something that I want you to notice. Here, we have the membrane that surrounds the muscle fiber and that membrane, we’re going to call that the sarcolemma. Now, you’re probably noticing that I’m using this prefix sarco- a lot. That prefix sarco- refers to the muscle. So, the sarcolemma, the sarcomere… anytime you hear sarco-, you can assume that we’re talking about something relating to muscle. The interesting thing about the sarcolemma is that you have these little openings where the membrane actually goes deep into the cell. And you can see it coming here and you can see it going through there. Where the membrane goes deep into the cell, that is called T-tubules. So, they’re basically these little tubes that go deep into the cell. And they serve a very important purpose. This is how it works.
Last time we looked at the fact that axons come in and make synaptic connections with the muscle cells. This is called the neuromuscular junction. So, when a signal comes down, and it releases the neurotransmitter, in this case the neurotransmitter is acetylcholine, and releases that neurotransmitter, it binds to the receptors that causes the signal in the muscle cell membrane, in the sarcolemma. That signal then travels deep into the cell via these T-tubules and something very important happens. Now, you can see that it looks like it’s one tube that’s going down deep into the cell. But, that tube, I’m going to take that and draw it over here and it’s not by itself. So, here we have the T-tubule and then surrounding the T-tubules, next to the T-tubules, we have the sarcoplasmic reticulum. So, I’m going to draw those here and it’s just going to look like tubes coming down next to the T-tubule. It’s not shown here but, I’m going to show that over here. And, as I said that is called the sarcoplasmic reticulum. The sarcoplasmic reticulum stores calcium ions. So, we have calcium ions inside the sarcoplasmic reticulum. So, let me illustrate that here so, Ca2+, Ca2+, and that’s all throughout the sarcoplasmic reticulum, it’s being stored there for when it needs to be used.
So, once again, we have a signal that’s coming down the axon causing a signal in the sarcolemma. That signal then goes deep into the muscle via the T-tubules. On the T-tubules, we have a receptor that we call the dihydropyridine receptor and on the sarcoplasmic reticulum, we have a receptor that we call the ryanodine receptor. So, let me write those over here. The red is the dihydropyridine receptor (hopefully, I‘m spelling this right) and here we have in blue, the ryanodine receptor. All right, so we have our signal, the signal comes along the sarcolemma, that signal spreads deep into the muscle cell via the T-tubules, that’s going to cause the dihydropyridine receptor to interact with the ryanodine receptor, that it opens the channel and let calcium ions flow out into the cell. Okay so, calcium is flowing out into the cell, out of the sarcoplasmic reticulum, and when that calcium flows out, that then causes muscle contraction. I’m not going to go through all the details as to how it causes muscle contraction in this video but, I’m going to do that in the next episode.
The take-home message is, the signal comes via the axon, causes a signal in the sarcolemma, that signal travels deep into the muscle cell via the T-tubles. Because of the relationship between the dihydropyridine receptor and the ryanodine receptor, that causes calcium that is stored in the sarcoplasmic reticulum to be released, and the calcium released then causes muscle contraction. So, we can look at it here again and we can see here, this is where calcium is being released, and then the calcium is then pumped back out, calcium is being released, calcium is pumped back out.
Now, there’s one thing I didn’t mention and that’s the second part with calcium being pumped back out. You have the T-tubule, you have the sarcoplasmic reticulum, and in the membrane of the sarcoplasmic reticulum, you also have calcium pumps and once the signal is over, the calcium pumps pump the calcium right back into the sarcoplasmic reticulum. So, that’s what’s happening here, calcium being released, calcium being pumped back in, calcium being released, calcium being pumped back in.
That’s all the content for this video. If you have any questions, of course you can ask them in the comments section below, and as usual, you can visit the website at interactive-biology.com for more Biology videos and other resources. That’s it for this video and I’ll see you on the next one.