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# Sarcomere length-tension relationship

## Video transcript

So I'm going to draw up the
length-tension relationship. This will be the
key idea we're going to talk about in this video. And it's very
related to some stuff we've already talked about. So we've talked
about, for example, the Frank-Starling curve. And that was talking about
how if you stretch out heart cells, and all of the
things within heart cells-- all the proteins--
that it actually changes the force
of contraction. And actually, force
of contraction is very much related to this
length-tension relationship as well. So I'm going to
put that up here. Force of contraction. And instead of using
that terminology, though, we're going to
use the term tension. I mean, you can essentially
think of them the same way. But classically, the word
tension is what everyone uses. So we're going to
use that same word. And then, as far as length,
specifically the length that we're talking about is
the length of a sarcomere. So I'm going to
write sarcomere here. And the sarcomere,
just keep in mind, is really going from one
z-disc to another z-disc. So to draw this out, to
actually write it out maybe, we can start with myosin. And so maybe this is
our myosin, right here. And I'll draw some
myosin heads here. And maybe some myosin heads
on this side, as well. And, of course,
you know it's going to be symmetric looking,
roughly symmetric. So this is our myosin. And actually, I'm going
to make some copies of it now, just to make
sure that I don't have to keep drawing
it out for you. But something like that. And we'll move it to be just
below so that you can actually see, when I draw
a few of them, how they differ from one another. So I'm going to put them, as
best I can, right below one another. And we'll do a total
of, let's say, five. And I think, by the time
we get to the fifth one, you'll get an idea of what this
overall graph will look like. So these are our five myosins. And to start out at
the top, I'm going to show a very
crowded situation. So this will be
what happens when really nothing is spread out. It's very, very crowded. And you recall that
you have actin, this box, or this half box
that I'm drawing, is our actin. And then you have
two of them, right? And they have their
own polarity, we said. And they kind of go like that. And so, in this first scenario,
this very, very first one that I'm drawing, this
is our scenario one. We have a lot of
crowding issues. That's kind of the
major issue, right? Because you can see that our
titin, which is in green, is really not
allowing any space. Or there is no space, really. And so, these ends,
remember these are our z-discs right here. This is Z and this
is Z over here. Our z-discs are right
up against our myosin. In fact, there's almost
no space in here. This is all crowded
on both sides. There's no space for the
myosins to actually pull the z-disc any closer. So because there's no
space for them to work, they really can't work. And really, if you give them
ATP and say, go to work. They're going to turn
around and say, well, we've got no work to do, because
the z-disc is already here. So in terms of force of
contraction for this scenario one, I would say, you're going
to get almost no contraction. So when the length is very
low, so let's say this is low. Maybe low is not a
good word for length. Let's say this is, I'll
use the word short. The sarcomere is short. And here the sarcomere is long. So when it's short,
meaning this distance is actually very short, then we
would say the amount of tension is going to be actually zero. Because you really
can't get any tension started unless you have
a little bit of space between the z-disc
and the myosin. So now in scenario two, let's
say this is scenario two. And this is my one
circle over here. In scenario two, what happens? Well, here you have a little
bit more space, right? So let's draw that. Let's draw a little
bit more space. Let's say you've got
something like that. And I'm going to
draw the other actin on this side, kind of
equally long, of course. I didn't draw that correctly. Because if it's
sliding out, you're going to have an extra
bit of actin, right? Something like that. And it comes up
and over like that. So this is kind of what
the actin would look like. And, of course, I want to
make sure I draw my titin. Titin is kind of helpful,
because it helps demonstrate that there's now a
little bit of space there where there
wasn't any before. And so now there is some
space between the z-disc and this myosin right here. So there is some space between
these myosins and the z-discs. In fact, I can draw
arrows all the way around. And so there is a little
bit of work to be done. But I still wouldn't say
that it's maximal force. Because look, you still
have some overlap issues. Remember, these myosins, right
here, they're not able to work. And neither are these,
because of this blockage that's happening here. This blockage. Because of the fact
that, of course, actin has a certain polarity. So they're getting blocked. They can't do their work. And so even though you get
some force of contraction, it wouldn't be maximal. So I'll put something like this. This will be our second spot. This will be number two. Now in number three, things
are going to get much better. So you'll see very
quickly now you have a much more
spread out situation. Where now these are
actually-- these actins are really not going to be
in the way of each other. You can see they're not
bumping into each other, they're not in the way
of each other at all. And so all of the
myosins can get to work. So the z-discs are now out here. My overall sarcomere,
of course, as I said, was from z-disc to z-disc. So my sarcomere
is getting longer. And you can also see
that because now there's more titin, right? And there isn't
actually more titin. I shouldn't use that phrase. But the titin is stretched out. So here, more work
is going to get done. And now my force, I
would say, is maximal. So I've got lots, and
lots of force finally. And so it would be
something like this. And so based on my
curve, I've also demonstrated
another point, which is that, the first issue,
getting us from point one to point two,
really helped a lot. Really. I mean, that was
the big, big deal. Because you needed
some space here. Again, this space really was
necessary to do work at all. And now that we've gotten
rid of the overlap issue, now that we've gotten these
last few myosins working, we have even more gain. But the gain was really--
the biggest advantage was in that first step. Now as we go on,
let's go to step four. So this is step four now. As we go here, you're
going to basically see that this is going to
continue to work really well. Because you have your
actin, like that, and all of your
myosins are still involved in making sure
that they can squeeze. So all the myosins are working. And our titin is just
a little bit more stretched out than
it was before. And our force of contraction
is going to be maximal. And you're going
to have-- and so here, I'm drawing
the z-discs again. They're very spread out. Our sarcomere is getting
longer and longer. And our force of
contraction is the same. Now let's just
take a pause there and say, why is it the same? Why did it not go up? Well, it's because
here, in stage three, you had 20 myosin heads working. 20 out of 20. Up here, you had something
like 16 out of 20 working. Here, we said maybe
zero out of 20 right? And here, you again
have 20 out of 20. So you still have an
advantage in terms of all of the myosins working. But there's no difference
between 0.3 and 0.4. Because again, all the
myosins are working. You can't do better
than 100%, right? So now in stage five, we kind
of take this a little too far, right? So let me actually just make
a little bit of space here. We take this a little
bit too far in the sense that our actin is going to
slip out all the way over here. And it's going to be out
all the way over here. So we've got a
huge, huge gap now. And, of course, our titin
is completely stretched out. It's about as stretched out
as our titin is going to get. This green titin protein. And now the question
is, of course, would you get any force? And the answer's probably no. Because the myosins aren't even
touching the actins anymore. So really, again, you have
zero out of 20 myosins at work. And of course, that means
that then the amount of force would be zero. So we go back down to zero. So this is part five. So you can see now, as we've
gotten longer and longer, things were good for
a while, but then they drifted all the way back down. And this curve that I'm showing
you, this tension-length curve, is now based on exactly
what you see on the right. It's based completely
on the idea that as you stretch things
out, the amount of force changes depending on the
length of the sarcomere.