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- [Voiceover] Each muscle fiber
is made of these functional

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sub-units known as sarcomeres.

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A sarcomere is the basic contractile unit.

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It's made up of myosin and actin.

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Myosin is larger,

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so it's referred to as the thick filament.

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Actin's smaller, so it's
referred to as the thin filament.

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The end of each sarcomere is marked

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by these things called Z lines.

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They're called Z lines because
they actually look like a Z,

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a zigzag coming all the way down.

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So imagine just taking a straight line

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and just starting to curve
it back and forth, Z lines.

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Now, the arrangement of the filaments,

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the myosin, the actin here,

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is what gives rise to that
striated look of skeletal muscle.

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How you can see these dark
and light alternating bands.

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So look at top figure.

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You can see how you have the Z line

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really zigzagged back and forth,

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and between the two Z
lines, that's a sarcomere.

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In that sarcomere, the black
thin lines are the actin.

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The larger red is the myosin.

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But look all the way at the bottom.

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You see that banding pattern.

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You can notice how the left is kind of

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a light gray with red.

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The center is that very
dark, dense yellow,

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and then you go into
the light grey and red.

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This will keep alternating
over and over and over again,

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creating a banding pattern.

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But each muscle can't do a
thing without activation.

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Acetylcholine is going
to be a neurotransmitter.

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It's a chemical that
is going to help carry

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your nervous signal from
a neuron into the muscle.

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Now as you need to have these chemicals

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because, is electricity really efficient

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at going through water?

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I mean can you pinpoint saying,

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we'll start the electrical charge here,

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and end it right here.

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I don't know about you guys

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but I don't want to be anywhere near

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having my feet in a lake
during a thunderstorm.

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That lightning can hit
hundreds of feet away,

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and still you're gonna be in real trouble.

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So electrical charges and
fluid aren't really great,

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so you need to have a chemical,

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something more solid, to
carry it from the neuron

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to the muscle.

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Now once it reaches the muscle,

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you get these electrical impulses back.

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Same impulses as were
in the nervous system.

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But they can go down
the muscle and head down

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these T tubule things.

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The T tubules are indents into the muscle.

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The T tubules are what allow the signal,

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this electrical impulse, to
actually go into the muscle,

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and trigger the cells.

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Because as that electrical
impulse goes in,

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calcium is released.

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Just that same old calcium
that's in your bones.

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But you have calcium now
stored in the muscle.

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The calcium comes out of this structure

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called a sarcoplasmic reticulum.

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It's basically a large, thin, sac,

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just containing lots and lots of calcium.

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So here's the image.

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They label the motor neuron up top,

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that's just a neuron carrying an impulse

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from the nervous system to the muscle.

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The acetylcholine is going to be released,

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all those tiny little dots,

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that's because an electrical
impulse can't really travel

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across the fluid very easy.

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So once that's released,
you create a new signal,

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a new electrical impulse on the muscle.

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Those kind of yellowish arrows

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are showing electrical impulse.

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It's carried down the
outside of the muscle,

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and carried into the
muscle on those T tubules,

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which is where the calcium gets released.

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The calcium, which are those little tiny

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kind of pink spheres there,

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is released out of the
sarcoplasmic reticulum,

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that blue stuff.

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And that calcium is what
is going to help initiate

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the actual movement of a muscle.