- [Voiceover] Now that calcium that was released
from the sargoplasmic reticulum,
well, it's gonna start what's called a sliding filament.
The myosin and actin are gonna slide past each other.
Thick filaments of myosin and thin filaments of actin
are gonna slide past, causing a contraction.
A contraction in the muscular sense
is when the thick filaments, myosin,
and the thin filaments, actin, bind together.
They form what's called a cross-bridge.
The cross bridge will momentarily link the two filaments
until they're being pulled past each other.
So up in Figure A,
you can notice that nothing is really attached there.
The thick filaments and thin filaments aren't attaching.
But in the presence of calcium,
go down to Figure B.
The thick filaments myosin can actually attach
to the thin filaments actin.
This myosin head can reach out and latch on.
But it doesn't just latch on,
it'll start to pull it inward,
pulling the actin, or thin filaments,
towards the center of the sarcomere.
By pulling it towards the center,
it's becoming shorter.
So remember, calcium comes from that
sarcoplasmic reticulum.
It binds to a structure called troponin.
Now this troponin is also attached to something
called tropomyosin.
The whole thing just gets pulled out of the way.
Calcium actually triggers this movement.
A myosin binds so it's exposed,
which means it's time for the myosin to bind to actin.
Cross-bridges are formed.
So the role of calcium here is to bind troponin.
Troponin pulls tropomyosin out of the way,
that allows myosin to bind.
When myosin binds to actin,
it forms a cross-bridge.
The actin filaments are then pulled towards the center
of a sarcomere.
As they're pulled towards a center
that's shortening the sarcomere.
If you have dozens of sarcomeres in a row,
each one being a little bit shorter,
it all adds up to the entire muscle movement.
So what we're looking at here again,
muscle cell up top,
can see the thick and thin filaments in the middle,
and the bottom.
The gray are the actin filaments,
the brown are the myosin.
But you notice the green.
The green kind of almost looks like
life-saving preservers,
have that little indent,
perfect for calcium.
Once calcium binds,
that causes troponin
and tropomyosin to move.
Once it moves, it opens up the binding sites.
Myosin actin can now bind.
The cross-bridges are formed.
And once those cross-bridges are formed,
now you can finally start to move the muscle.
Now what's really amazing is all this stuff
we just talked about,
from that electrical impulse coming down,
to when these cross-bridges form
and start to move the muscle,
that takes a whopping four milliseconds.
Man, that's faster than snapping your fingers.
Just goes to show how fast and how quickly
all these can occur.
You've had signal coming down from your brain to your muscle
and it's practically instantaneous.
OK, traveling at hundreds of miles per hour.
So all of this is very quick,
and very precise.
Now how do you relax?
Well, not a trick question.
I mean how do you relax your muscles?
You can't just simply say "Muscles relax."
Doesn't quite work that way.
What you have to do is stop any nervous impulse.
Stop the nerve.
If the nerve is not sending any signals,
the contraction ends.
Once it ends, calcium is pumped back into that
sarcoplasmic reticulum.
It's pulled back in.
Well, if it's pulled back in,
that means it's not bound troponin.
So if the calcium's moved from troponin,
is there any reason for troponin to have a different shape?
No, so it moves back over to cover the myosin binding sites.
Once that occurs, are the myosin heads binding anymore?
No, so no calcium, no cross-bridge.
No cross-bridge, no muscular contraction.
But all this movement requires energy.
In the case of humans,
we're talking ATP, adenosine triphosphate.
That's where your cells obtain their energy from.
ATP is an energy molecule that your body produces.
You can take carbohydrates or proteins or lipids,
and convert them into ATP.
You need ATP for contraction,
the muscle can't move without it.
You need ATP for relaxation:
again, the muscle can't move without it.
It can't contract, it can't relax.
So now here's a question.
I'm willing to bet everyone's probably seen road kill
at some point on the side of the road.
Now, a lot of times with road kill,
you have the legs sticking straight out,
they're rock-solid.
Well, when they're that stiff,
why aren't they moving?
Why don't they just sag down to the ground?
It's because of this ATP.
Rigor mortis is caused by a lack of energy.
A dead organism cannot create ATP,
therefore the muscles cannot relax.
So the whole idea of rigor mortis
is based on this principle of muscles needing ATP.
Now you can replenish this ATP, you can make more of it.
You can use creatine phosphates,
I've heard creatine maybe with working out.
You've have this stored glycogen.
Glycogen is just a whole big chain of glucose,
a lot of carbohydrates.
You can use aerobic metabolism.
Now metabolism is just basically chemical reactions
in your body.
Aerobic means you have oxygen.
So you can take glucose, you can take fatty acids,
anything else that really is high-energy.
You can take these high energy molecules,
run them through this aerobic process,
and create energy out of it.
You can create ATP.