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Current time:0:00Total duration:8:50

Video transcript

I'm going to draw up the length tension relationship this will be kind of the 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 kind of talking about how if you stretch out heart cells and all the things within heart cells all the proteins then 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 now specifically the length that we're talking about is the length of a sarcomere so I'm going to write a sarcomere here and a sarcomere just keep in mind is really going from one z disk to another z disk so to draw this out to actually kind of write it out maybe we can start with myosin 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 it will 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't write below one another and we'll do a total of let's say five I think by the time we get to the fifth one you'll get an idea of kind 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 I'm going to show a very crowded situation so this will be kind of what happens when really nothing is spread out it's very very crowded and you recall that you have actin kind of this box or this half box that I'm drawing is our actin and then you have two of them right 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 Titan which is in green is really not allowing any space or there is no space really and so these ends we remember these are our Z disks right here this is Z and this is Z over here our Z disks 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 disk any closer so because there's no space for them to work there they really can't work and so they really if you give them ATP and say you know go to work they're going to turn around and say well you know we've we've got no work to do because the Z disk 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 and you may be low is not a good route 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 disk 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 oh 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 Titan Titan is kind of helpful because it helps demonstrate that there's now a little bit of space there where there wasn't before and so now there is some space between the Z disk and this myosin right here so there is some space between these myosins and the z disks 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 myosin is 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 when 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 Acton's 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 the myosins can get to work so the z disks are now out here my overall sarcomere of course as I said was from Z disk to Z disk so my start computer is getting longer and you can also see that because now there's more tighten right and there isn't actually more tight nishan use that phrase but the tighten 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 kind of also demonstrated another point which is that the first issue getting us from kind of point one to point two really helped a lot really I mean that was the big big deal because you needed some space here against this space really was necessary right 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 Tinh 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 Titan is just a little bit more stretched out than it was before and our force of contraction is going to be maximal you're going to have and so here I'm drawing the Z disks again they're very spread out our star commuter 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 - and heads working 20 out of 20 up here you had something like 16 out of 20 working here you 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 the myosins working but there's no difference between 0.3 and point 4 because again all the myosins are working you can't you can't do better than hundred-percent right so now in stage 5 we kind of take this a little too far right so let me actually just make a little bit of space here but we take this a little bit too far in the sense that our actin you know is going to kind of 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 Titan is completely stretched out is about as stretched out as our Titan is going to get this green Titan protein and now the question is of course would you get any force and you know the answer is probably no because the myosins aren't even touching the Acton's anymore so really again you have zero out of 20 minus ins 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 longer things were good for a while but then they kind of 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