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Health and medicine
Course: Health and medicine > Unit 2
Lesson 2: Heart muscle contractionHeart cells up close!
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.
Want to join the conversation?
- - Why would a cell need two or more nuclei ? 1:21(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)
- - 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? 1:33(10 votes)
- They are stationary... as for the position, I don't know.(3 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)
- why is the cardiac muscle not one long cell(6 votes)
- Do the mitochondria from cardiocytes differ from those from other body cells, or they are the same?(2 votes)
- Where is the SR getting its own calcium to store?(3 votes)
- Via pumps. It is explained in another videoe here on khan.(2 votes)
- So, the only organelles it contains are mitochondrion and nuclei?(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)
- Do the gap junction do the same job as protein channels in the cell membrane. 03:08(2 votes)
Video transcript
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.