Human Physiology – Myofilament Structure: The Thin Filament
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Human Physiology – Myofilament Structure: The Thin Filament


>>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.

3 thoughts on “Human Physiology – Myofilament Structure: The Thin Filament

  1. I hardly comment on any videos. Very rarely, to be precise.

    I thank you from the bottom of my heart. This video has helped me learn everything I need to know about the thin filament. Thank you so much. May God bless you always.

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