There are a couple types of
contractions that we need to consider.
The first is a
A concentric contraction involves a
muscle belly shortening and pulling
on a bony lever and causing it to
move towards the body, such as this.
As you’re contracting the bicep in
this case, you’re pulling the muscle
and it’s pulling on two bony levers
to try to cause this reaction.
Opposite of this is an
And for this, the body is
still trying to contract
the bicep, but it is being
overridden by the weight.
So both concentric and eccentric
contractions involve the same
crossbridge cycling that we’re
going to discuss later on.
The structure of the thick and thin filament
allow us to undergo muscle contraction.
So let’s talk about the
thick filament first.
The thick filament
has myosin heads.
These myosin heads are what's
going to interact with actin.
They are looped together
in this alpha helix,
and so, they form kind of a multi-headed
structure that’s going to be grouped
together to form a lot of these myosin
heads wrapped around in a circle.
The thin filament has a number
of actin filaments tied together.
These actin filaments are
kind of like small balls.
Grouping these together are
tropomyosin rope-like structures.
These tropomyosin molecules also
hold on to a troponin complex
and a troponin complex involved
three different subunits.
There’s a troponin T, which is bound with
tropomycin, a troponin I, and a troponin C.
The troponin C is where
calcium is going to bind.
As soon as calcium binds
to this troponin C,
you’re going to get a conformational
change of the protein,
and what it will do is move tropomyosin
off of the active site on actin
so that there can be an interaction
between myosin and actin.
The thick and thin filaments are arranged
in such a way that there are myosin heads
and actin filaments on all sides, 360
degrees, around these filaments.
Interestingly, you can break down
these units into smaller fields.
We call those smaller
Sarcomere has a number of
different components to it.
The A band is where the
thick filaments are.
The H band is where the thin filaments are
not, but the thick filaments are located.
Then, we have some structural proteins
such as nebulin and titin that are
attached to Z lines to help form the
structural integrity of this dynamic unit.
Looking at it in a
little bit more detail,
you can see the close
proximity of the myosin heads
to the active sites on actin that are
covered up in this case by troponin.