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This here is a model of a sarcomere.

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A Sarcomere

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is a contractile unit for skeletal muscle fibers.

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This covering

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

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

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which is crucial for muscle contraction.

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

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These are the terminal cisternae.

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

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

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The sarcomere’s

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boundaries

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are shown at

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the left and right. This is the Z discs

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and they're actually shaped like the letter Z.

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OK. So, what a sarcomere is

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creating the striations that you see under a microscope.

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Now, the striations, these light and dark

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bands that the muscle has

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as a result of these thin and thick myofilaments.

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So, this is a thin myofilament containing mostly actine,

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and this is the thick myofilament containing myosine.

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

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

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

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And during the muscle contraction, the thick filaments,

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the myosin, walk the actine towards the middle,

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and it happens on both sides simultaneously.

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

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

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And when you look at this area here,

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just this region here will appear light under a microscope.

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That's called an eye band or light band.

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It appears light because it only has thin myofilaments,

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this area from here to here,

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

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The reason why it appears dark is because

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it contains thick myofilaments and they're thicker,

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of course.

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There is a special area right here,

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where there is no overlap with thick and thin.

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

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That's called the H band.

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

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All right.

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

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You will learn about this in lecture troponin,

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tropomyosin. That's not something we really go over

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in lab.

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But those are the parts of a sarcomere.