Ivy Tech APHY 101 – Muscle System Practice Test

The dark band that runs the length of the thick filaments and contains myosin is the A band. It spans the full length of the thick filaments, including regions where thick and thin filaments overlap, which gives it its darker appearance under light microscopy. The lighter region adjacent to it is the I band, made up mostly of thin actin filaments, while the central part within the A band where there’s only thick filaments is the H zone. The Z disc marks the boundaries between sarcomeres and lies at the center of the I band. During contraction, actin slides past myosin, shortening the I band and H zone while the A band remains constant in length.