2. Skeletal Muscle Microscopic Anatomy
The myofibrils are chains of tiny contractile units called sarcomeres, which are aligned end to end
It is the arrangement of even smaller structures (myofilaments) within the sarcomere that produce a banding pattern

4. Skeletal Muscle Banding Pattern
Alternating light (I band) and dark (A band) bands along the length of the perfectly aligned myofibrils give the muscle cell as a whole its striped appearance.
The I band has a midline interruption, a darker area, called the Z line.
The dark A band has a lighter central area called the H zone

7. Two types of Myofilaments
myosin filaments: thick
actin filaments: thin

8. Myosin Filaments larger thicker filaments
Extend the entire length of the dark A band
The midparts of the thick filaments are smooth, but their ends are studded with small projections, or myosin heads.
These myosin heads are sometimes called crossbridges because they link the thick and thin filaments together during contraction

9. Actin Filaments Anchored to the Z line
The light I band is an area that includes parts of two adjacent sarcomeres and contains only the thin filaments
Although the thin filaments overlap the ends of the thick filaments, they do not extend into the middle of a relaxed sarcomere, and thus the central region (the H zone) looks a bit lighter