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

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

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

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

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