>>Dr. Ketchum: The purpose of this video lecture is to learn about the myofilament structure,
and specifically the thin filament and how the thin filament is important to muscle contraction.
So to begin, what I would like to do is just to point out the thin filament to you so that
you understand which structure that we’re talking about in terms of the sarcomere. So
what you’re looking at in this image is one sarcomere, and remember that a sarcomere
extends from a Z disk to a Z disk. So everything in between here is just one sarcomere. And
the sarcomere is made up of thin and thick filaments, and you have several different
I bands, and A bands, and H zones, and M lines, and so on that you learned about earlier.
And so the focus of this is to look specifically at the structure of the thin filament. So
let’s take a look.
So as far as the thin filament goes, remember that this is a type of myofilament. “Myo”
again means muscle, and there are three parts that make up the thin filament. There’s
the actin, the tropomyosin, and the troponin. So let’s take a look at each of these. The
actin is a globular protein. And when you look at this actin molecule, it has a
center dot. That center dot is the myosin binding site. And what I tend to do is I tend
to think about actin as like an olive. So if you think about a green olive, for example,
and then you have the red pimento—the pimento part of the olive is the myosin binding site.
The entire olive is the actin. So really what you have when you look at the thin filament
is a whole string of olives that are shown in the figure, and each olive has this myosin
binding site.
The next structure that makes up the thin filament is called tropomyosin. This is a
fibrous protein. So by fibrous, what we mean is that it’s a long protein; it’s not
a glob, in other words. Tropomyosin is very interesting because it has the ability to
move. It can shift over. And so what it does when it shifts is it can either cover the
myosin binding sites or it can expose them. So in order to form a crossbridge—which
I’ll talk about in a moment—the tropomyosin has to shift over to expose the myosin binding
site. The third structure that makes up the thin filament is called troponin. Troponin
consists of three globular proteins. What I think about troponin is I think kind of
like about a snowman, because a snowman is consists of three parts, right? A big base,
a center and a top. That’s kind of how troponin sits on top of the thin filament. And so let’s
take a look at how we stack the troponin molecules.
The bottom troponin will bind to actin; so this binds to your green olive. Then the middle
troponin molecule binds to tropomyosin, and then the top troponin protein binds calcium.
When we put tropomyosin and troponin together, those are referred to as regulatory proteins.
It’s important that you think about troponin and triple myosin as regular proteins because
they’re very important in regulating skeletal muscle contraction.
This slide represents a crossbridge, and so it’s important to know what a crossbridge
is. So what you’re looking at here is a thin filament. The bottom myofilament is the
thick filament. When these two filaments bind, as you can see here, we call this a crossbridge.
So a crossbridge is binding between your thin and your thick filament. Cross-bridging has
to occur in order for a skeletal muscle to contract. So let’s look at the role of calcium
in forming crossbridges. So we return to our thin filament once again, and the first configuration
that you have for the thin filament is the relaxed configuration. So in the relaxed configuration,
note the position of the tropomyosin. Tropomyosin covers the myosin binding sites. If tropomyosin
is covering your myosin binding sites, a crossbridge cannot form, and hence, contraction cannot
take place. Now in order for the muscle to contract, then, calcium has to bind to troponin.
So when calcium binds to troponin that causes troponin to change shape. And then when troponin
changes shape, that causes tropomyosin to shift over. And when tropomyosin shifts over,
now that exposes the myosin binding sites. So now that the myosin binding site is exposed,
now crossbridge can form. And because a crossbridge can form, the muscle can contract. So that’s
the role of calcium in forming a crossbridge. How would the muscle relax? Would calcium
have to fall off troponin? The answer to that is yes. Calcium would fall off troponin, tropomyosin
would then shift back over and cover your myosin binding sites, and hence, another crossbridge
could not form.