- [Voiceover] Each muscle fiber is made of these functional
sub-units known as sarcomeres.
A sarcomere is the basic contractile unit.
It's made up of myosin and actin.
Myosin is larger,
so it's referred to as the thick filament.
Actin's smaller, so it's referred to as the thin filament.
The end of each sarcomere is marked
by these things called Z lines.
They're called Z lines because they actually look like a Z,
a zigzag coming all the way down.
So imagine just taking a straight line
and just starting to curve it back and forth, Z lines.
Now, the arrangement of the filaments,
the myosin, the actin here,
is what gives rise to that striated look of skeletal muscle.
How you can see these dark and light alternating bands.
So look at top figure.
You can see how you have the Z line
really zigzagged back and forth,
and between the two Z lines, that's a sarcomere.
In that sarcomere, the black thin lines are the actin.
The larger red is the myosin.
But look all the way at the bottom.
You see that banding pattern.
You can notice how the left is kind of
a light gray with red.
The center is that very dark, dense yellow,
and then you go into the light grey and red.
This will keep alternating over and over and over again,
creating a banding pattern.
But each muscle can't do a thing without activation.
Acetylcholine is going to be a neurotransmitter.
It's a chemical that is going to help carry
your nervous signal from a neuron into the muscle.
Now as you need to have these chemicals
because, is electricity really efficient
at going through water?
I mean can you pinpoint saying,
we'll start the electrical charge here,
and end it right here.
I don't know about you guys
but I don't want to be anywhere near
having my feet in a lake during a thunderstorm.
That lightning can hit hundreds of feet away,
and still you're gonna be in real trouble.
So electrical charges and fluid aren't really great,
so you need to have a chemical,
something more solid, to carry it from the neuron
to the muscle.
Now once it reaches the muscle,
you get these electrical impulses back.
Same impulses as were in the nervous system.
But they can go down the muscle and head down
these T tubule things.
The T tubules are indents into the muscle.
The T tubules are what allow the signal,
this electrical impulse, to actually go into the muscle,
and trigger the cells.
Because as that electrical impulse goes in,
calcium is released.
Just that same old calcium that's in your bones.
But you have calcium now stored in the muscle.
The calcium comes out of this structure
called a sarcoplasmic reticulum.
It's basically a large, thin, sac,
just containing lots and lots of calcium.
So here's the image.
They label the motor neuron up top,
that's just a neuron carrying an impulse
from the nervous system to the muscle.
The acetylcholine is going to be released,
all those tiny little dots,
that's because an electrical impulse can't really travel
across the fluid very easy.
So once that's released, you create a new signal,
a new electrical impulse on the muscle.
Those kind of yellowish arrows
are showing electrical impulse.
It's carried down the outside of the muscle,
and carried into the muscle on those T tubules,
which is where the calcium gets released.
The calcium, which are those little tiny
kind of pink spheres there,
is released out of the sarcoplasmic reticulum,
that blue stuff.
And that calcium is what is going to help initiate
the actual movement of a muscle.