Muscle Physiology - Functional Properties
When tension at each length is plotted against length, a relationship in skeletal muscle is a function of the magnitude of overlap between. The relation between sarcomere length and active tension in isolated system of the intact muscle cell is always in the same state at a given sarcomere length. When our muscles are stretched to the ideal length, it can maximize the relationship between resting muscle fiber length and contraction.
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? 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.
Sarcomere length-tension relationship
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. 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.
Muscle Physiology - Types of Contractions
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. 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.
- Length tension relationship
So this is step four now. Force-velocity Relationship The force generated by a muscle is a function of its velocity. Historically, the force-velocity relationship has been used to define the dynamic properties of the cross-bridges which cycle during muscle contraction.
The force-velocity relationship, like the length-tension relationship, is a curve that actually represents the results of many experiments plotted on the same graph. Experimentally, a muscle is allowed to shorten against a constant load.
The muscle velocity during shortening is measured and then plotted against the resistive force. The general form of this relationship is shown in the graph below. On the horizontal axis is plotted muscle velocity relative to maximum velocity Vmax while on the vertical axis is plotted muscle force relative to maximum isometric force Po. What is the physiologic basis of the force-velocity relationship?
The force generated by a muscle depends on the total number of cross-bridges attached. Because it takes a finite amount of time for cross-bridges to attach, as filaments slide past one another faster and faster i.