This here is a model of a sarcomere.

A Sarcomere

is a contractile unit for skeletal muscle fibers.

This covering

which is acting as a dust cover, but it actually represents

the sarcoplasmic reticulum, which is a modified ER which is storing calcium,

which is crucial for muscle contraction.

The parts of this you should know. This is the transverse tubule or T tubule.

These are the terminal cisternae.

Off to the sides of that, and they make this triad structure.

OK. So, then you can look at this. This is the actual sarcomere.

The sarcomere’s

boundaries

are shown at

the left and right. This is the Z discs

and they're actually shaped like the letter Z.

OK. So, what a sarcomere is

creating the striations that you see under a microscope.

Now, the striations, these light and dark

bands that the muscle has

as a result of these thin and thick myofilaments.

So, this is a thin myofilament containing mostly actine,

and this is the thick myofilament containing myosine.

So, what happens is the acting is attached to the Z discs, right?

You can see that on both sides, the thin myofilament is attached here.

The thick myofilament is connected here in the middle, this is called the M line.

And during the muscle contraction, the thick filaments,

the myosin, walk the actine towards the middle,

and it happens on both sides simultaneously.

So, the whole thing shortens the Z discs actually get closer together.

So, what's creating the striated appearance when you look at it under a microscope?

And when you look at this area here,

just this region here will appear light under a microscope.

That's called an eye band or light band.

It appears light because it only has thin myofilaments,

this area from here to here,

that will appear dark under a microscope. And that's called the A band or dark band.

The reason why it appears dark is because

it contains thick myofilaments and they're thicker,

of course.

There is a special area right here,

where there is no overlap with thick and thin.

You can see just this middle region where it's just thick myofilaments

That's called the H band.

And if you actually look at it from up top, it looks like the letter H.

All right.

Additionally, there's some accessory proteins here with the thin myofilament.

You will learn about this in lecture troponin,

tropomyosin. That's not something we really go over

in lab.

But those are the parts of a sarcomere.