Health and medicine
Get a close-up view of the cardiac cells and see what makes them different from the other (skeletal and smooth) muscle cells. Rishi is a pediatric infectious disease physician and works at Khan Academy. Created by Rishi Desai.
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- 1:21- Why would a cell need two or more nuclei ?(21 votes)
- Also, I think it's so that more protein synthesis can take place because more mRNA strands can be produced with multiple nuclei in a cell.(3 votes)
- 1:33- Is there a significance to the 'position' of the nucleus (ie- middle, periphery,etc.) and if so, what is it? Also, are nuclei fixed and stationary or do they move around within the cell?(10 votes)
- Can heart cells have 3 or more nuclei?(3 votes)
- Muscle cells such has heart cells often form a syncytium. A syncytium is a cell with more than one nucleus, resulting from the fusion of two or more uninuclear cells.(8 votes)
- Where is the SR getting its own calcium to store?(3 votes)
- Why do these cells require so much calcium?(2 votes)
- They are constantly contracting and expanding. They are still muscle cells, and muscle cells need calcium to keep breaking through the tropomyosin so that the myosin can pull the actin.(2 votes)
- why heart never stop ? why heart doesn't need a time period to recovering it cells(2 votes)
- In a matter it does stop after every beat. The cells do need to recover in between beats. This is mainly because of the time it takes for the sodium/potassium pumps to undo the work of the action potentials.(2 votes)
I've drawn for you three heart cells. And one of the first things you should be thinking about is, well, how exactly do we know that it's actually a heart cell? So I'm going to try to prove it to you in some sort of a systematic way. So this is my heart. And my premise is that I've just taken some tissue. Looking under a microscope, this is what it would look like. But let's look at some features that you actually see here that would clue you in as to whether what I'm saying is true or not. So one of the things that you might notice right away is that this cell right here, this guy, is actually branching. And I haven't actually shown you the bottom cell, but just trust me for the moment that there is one. So that's one feature you notice. And this is certainly unique to the muscle cells of the heart, is that they branch. And they don't always branch. You can see that some of these are actually connected in a plain, old-fashioned, boring way, where there's a cell on either side. But some of them do. Now, another interesting thing you can actually pick up from this screenshot, is that you actually have some nuclei. Or some cells with only one nuclei, and some with two. So that's another interesting and unique feature of the heart cells. Is that they have one or two nuclei. And this makes them different than their skeletal and smooth-muscle cousins. And the other thing is that these cells actually keep their nuclei, usually, in the middle. Which specifically is different from the skeletal muscle cells where they're in the edge or periphery. So a couple of things we've picked up already, right? Now let's do something interesting. I'm going to actually erase something here. And so you can actually start looking within the cell. I'm going to pretend we can take a knife and cut away at this cell. What would it look like? So let's just hack away at it. Something like that. And you basically get this fantastic, beautiful inside of the cell, right? So this is something that I actually drew out earlier and I wanted to save it for you. So that you could see what it would look like. So this is the cell on the inside. And there's a bunch of fun stuff we've uncovered, right? So now you can see what these squiggly line actually looks like on the cut surface. And this squiggly line is known as the intercalated disc. And it kind of looks like a disc, right? It's a disc shape. And along this intercalated disc are a couple of-- two interesting things. One that you can see right away are these little, tiny doughnuts. These little doughnuts here. And these doughnut-shaped holes, right here, represent the gap junctions. These are the gap junctions. So these are, of course, within the intercalated disc. And these gap junctions, what they do is they basically allow two cells that are next to each other, to talk to each other. They say OK, well, these are little doughnuts and, of course, if it's a little hole, then things can get through the hole. And so, through this hole, coming from one side to the other, will be something like, let's say, sodium. Or maybe calcium. Or some other ion will actually be able to float through, physically. Go right through that gap junction from one cell to the next. So it's a way of letting two neighboring cells talk to one another, using their ions, right? And this is super important when you're thinking about a wave of depolarization. Because these are literally the gap junctions through which ions go from one cell to another, promoting that depolarization wave that happens every time your heart beats. So these are actually really important for that reason as well. Now, in addition to gap junctions, you've also got little staples. What I think of as staples. Literally holding two cells together, fixed to each other. And I'm drawing them as little green x's. And these green x's, we call them desmosomes. And these desmosomes are important. Because imagine that one cell contracts, right? Let's say this guy, over here, contracts. And let's say this guy contracts. If they both contract and they can pull away from each other, because let's say they don't have any desmosomes, then you basically have two cells that are separate from each other, right? They are no longer attached. And so you would need desmosomes to really keep all these cells attached as they're contracting so that they don't pull away from each other. And what that does, what this physically does, is attaches the cells. The gap junctions, they allow the cells to chemically communicate. So what we're creating, literally, is a community of cells, right? Like a community of cells that are working together. And I say that they're a community and not just one giant cell, because in skeletal muscle you often think of it as one huge cell with many, many nuclei. But here, it's truly a community because there are these connections, or these separations rather, that are the intercalated discs. So I think of these then as a functional-- functioning as a syncytium. So we sometimes call this a functional syncytium. Now the last thing you might notice is that there's a lot of mitochondria in this cell. This cell is chock-full of mitochondria. That's what these little red beans are, they're mitochondria. And if it's loaded with mitochondria, it tells you that this cell is making a lot of energy. And this heart cell is going to be cranking out the energy because, of course, it's contracting every, single day. And so, this gives you a clue as to the fact that it's an energy consuming bit of tissue. And you can see that the mitochondria are literally stuffed into every corner. So a couple things here, intercalated disc, lots of mitochondria. So let me actually change that to a number four. So we have now a few more things here that we can use to identify what kind of cell we're talking about, right.? Number four. Now let me actually go ahead and erase a little bit more. You now can see more of this cell's environment. Actually, I've drawn even more for you here, something like this, right? So you can see now, you've got another cell below. And I've actually drawn lots and lots of details here. And I don't want you to get thrown off by it, but it's actually pretty cool once you dive into it. So what are these things? First, let's talk about these white tube sock looking things. These are T-tubules. And we call them T because the direction is literally transverse to the surface, right? So this is the surface right here. It's going down like that, right.? You can see that in the cut part of it. And so this looks like the letter T. But it's also transverse. So this is why we call them T-tubules. And in blue, over here, you can see that we have this-- almost like a river delta. This huge network of little, tiny tubules. But basically what these are, in agrregate, in total, we call all of this our sarcoplasmic reticulum. And so you might see that word. And now you know what it looks like. In fact, I've actually been gulty-- I'm going to make a little bit of space on here-- i've been guilty in the past of drawing the sarcoplasmic reticulum in an unfair way. I've often drawn it like this, like a lake. In fact, let me just put two side by side. I often draw like this. I say, well, this is my sarcoplasmic reticulum. And oftentimes, you'll see that in a book. But the truth is that sarcoplasmic reticulum looks a lot more like this, like a river delta. And that's because, remember the function of the sarcoplasmic reticulum is to contain calcium. And it wants to get calcium out to all of the different protein. And the best way to do that is to basically drape itself all over the proteins, right? And so you can see how-- in this view of it-- you can see how that's definitely happening. Now, let's go back to T-tubules. What is the purpose of the T-tubule? We haven't actually said anything about why they're there, we just described what they are. But remember, there's an outside of the cell over here, O for outside. And there's an inside of the cell. And that would be where the sarcoplasmic reticulum is. So this would be inside, right? Now, you have ions on the outside. For example, let's say you've got some calcium out here. And this calcium is kind of floating around on the outside. But the sarcoplasmic reticulum needs to know that that calcium is there. In fact, when it crosses over, imagine you could only cross on the surface, right here. There's not a lot of surface area there is there? Just a little bit. But by creating these T-tubules, , what the cell does is it increases surface area. So this is increasing surface area. And it allows calcium to dive deep inside. In fact, if you were imagining that you were little person, let's say you're a person here, and you go and walk and you stumble into a T-tubule, you would literally fall all the way down and end up at the bottom of the T-tubule, down here. Almost like you're falling into a giant tube sock. And so, you could climb your way back out. But the whole time you're inside a T-tubule, you're still on the outside of the cell. And that's actually really important. You're still on the outside of the cell. So once the calcium now crosses the membrane, so this is finally, let's say it finally crosses the membrane and gets into the cell. Let's say calcium here gets into the cell. And, of course, it's crossing all over the place, right? That calcium has an important job. It basically-- what it will do, it will bind two little spots here, little receptors, on the sarcoplasmic reticulum. So the sarcoplasmic reticulum is loaded with these receptors, where the calcium binds. And once it's bound, it opens a channel for the sarcoplasmic reticulum to release its own calcium. So once the calcium binds, the sarcoplasmic reticulum dumps out its own calcium. So I just want to make that very, very clear. Calcium from the extracellular environment, meaning from the outside, comes into the T-tubule, crosses the membrane. And then it gets on the inside of the cell. It binds to these yellow protein, these little yellow docking stations. And then, it allows calcium from the inside of the sarcoplasmic reticulum, right in here, to dump out. And basically get all over the cell. So this is actually an important step. You basically, you need-- and I'll write down here-- you need, step, I suppose we're on five now. You need extracellular calcium to bind to the sarcoplasmic reticulum. So that it can dump out its own calcium, so that it can release its own calcium. So you might be thinking well, why do you have two steps? Well, what happens is that you only need a little bit of extracellular calcium. And you can get a lot of calcium out of that sarcoplasmic reticulum. So this becomes a trigger and amplifies the amount of calcium that's actually going to be released. So that's an important difference then, because the skeletal muscle just has this bit right here. It just immediately releases calcium. It doesn't need extracellular calcium to first bind. OK, now the next thing I want to point out-- actually, bring this back up just a little bit-- is that if you were to look under a microscope, you would actually see interesting differences. You'd see that these actin and myosin bands look different. In fact, that's what these are. I should label them for you. This blue is actin and this red is myosin. And you might have seen this before. But these two, actin and myosin, are different types of protein that are going to crawl over each other and actually allow muscle contraction, right.? We've talked about that. But under a microscope, this actually looks really cool because this part looks really dark and the actin looks really light. And what you get is, you basically get something like this. You might get a red band, right here, and then you have a blue band in between. And because it looks banded-- and the colors, I just made the colors up, red and blue. But you could imagine that if it looks striped like this, then someone might say, wow, this looks striated. And in fact, that's why we say that heart muscle cells are striated. Now the last thing I want to point out, and I know we've talked a lot about these heart cells. And hopefully by now I've proven to you that they are, indeed, heart cells. The last thing is that you might be wondering, what is between these cells? I've drawn a little gap here. What is between these cells? And remember, we have a lot of mitochondria. And that means that there's a lot of energy needs here. And so you must have blood vessels. And so that's what is actually sitting between these cells in the gaps. Coursing through are blood vessels, little capillaries, specifically. And connective tissue, you've also got a lot of connective tissue in here. So this is the extracellular environment on the outside of the cells. You've got connective tissue, you've got blood vessels, you've got some nerves, and you've got all sorts of stuff in here. And that's what fills in all these gaps. In fact, this is actually going to be full of-- make a little bit space again-- endomysium is the name of the connective tissue. So this is actually going to be full of endomysium, and cells, and vessels, and all that good stuff. So that's what's in the gaps. All right. hopefully you feel comfortable with the idea that this is definitely heart tissue.