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

>>This video lecture will cover the thick
filament, which is a myofilament structure in muscle. So to begin, it’s important for
you to realize when you look at a sarcomere—and again, a sarcomere extends from a Z disk to
a Z disk, and this is one sarcomere—and that sarcomere consists of both thin and thick
filaments. Here we will only focus on the thick filaments. To begin, your thick filament consists of
one myosin molecule. So in this one myosin molecule it consists of two fibrous proteins,
which are called the dimer, and these are the tails. The other structure that makes
up the one myosin molecule are the heads. The heads are also proteins, but they’re
globular proteins. Let’s take a look at what structures you would find on the heads
of the myosin molecule. There are two structures found on each head of the myosin molecule.
These include the ATPase site and the actin-binding site. So let’s take a look at each of these
sites. Let’s focus on the ATPase site first. The ATPase sites, again, there’s an ATPase site on each head. This is where ATP binds, and when ATP binds
to this site, that will break your crossbridge. And then ATP can be hydrolyzed. So when it’s
hydrolyzed, remember, your products are ADP plus a phosphate. And when ATP is hydrolyzed
after breaking the crossbridge, the head of your myosin is in the high energy state. That’ll
be very important when you learn about crossbridge cycling. Now let’s focus on the actin-binding
site. So the actin-binding site is also located
on each head. The actin-binding site binds to the myosin-binding site on the thin filament.
Remember specifically that the myosin-binding site is found on actin (remember, actin is
your green olive.) So when the actin-binding site binds to the myosin-binding site on the
thin filament, what does that form? That forms the crossbridge. And remember that when you
form a crossbridge, what mechanical event can take place? Contraction. So that is the
basic scenario for one myosin molecule, one thick filament. In reality, let’s take a
look at what happens when I join a second myosin molecule. Notice the direction of the heads for each
of these molecules; the heads are pointing in opposite directions. That’s an important
concept, because if you visualize the heads moving straight up and binding to the thin
filament, then cocking inward. When they cock inward, that shortens the muscle. Now the
thick filament is actually more complicated than that of course, correct? Because this
is physiology, and you guys know nothing in physiology is easy, right? Look at how many
myosin molecules make up a thick filament.

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