The basic subunit of muscle (skeletal). See diagram below:
|                                                                   |
|B-------------                           -------------B|
|~~~~~~~~=======+======~~~~~~~|
|B-------------                           -------------B|
|~~~~~~~~=======+======~~~~~~~|
|B-------------                           -------------B|
|~~~~~~~=======+=======~~~~~~~|
|B-------------                           -------------B|
|                                                                    |
In the diagram above (limited to simplistic ASCII art):
The |'s represent the boundary between sarcomeres which connect end to end. This boundary is called the z-line ore the z-disk and it's dark color causes muscle to have dark bands (between sarcomeres) which are responsible for skeletal muscle also being known as "striated muscle".
The -'s represent F-actin microtubules which are capped by the CAP-Z protein represented by the B's. These filaments are called "thin filaments" because they appear thin next to the larger "thick filaments" made up of a polymer of Myosin II proteins (represented here by the + and ='s). The thick filaments (the + and ='s) are attached to the z-line (the |'s) by a strong, but very stretchy protein called TITAN (represented here by a sequence of ~'s although there is infact only one TITAN for each side of each thick filament).

Contraction occurs when the protein heads (found facing opposite directions depending on which side of the center (+) they are) on the thick filaments walk from binding site to binding site along the thin filament. For more detail on sarcomere contraction and it's regulation by calcium and proteins like troponin and tropomyosin, look up information on muscle contraction. (The diagram is of a muscle that is a scrcomere that is not contracted.... when it contracts, the thin filaments slide closer together as they are pulled by the myosin heads on the thick filaments.)

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