I Band

The I band is the lighter region of a sarcomere that contains thin, actin-containing filaments but no thick, myosin-containing filaments. It lies on either side of the darker A band in striated skeletal and cardiac muscle. A Z-disc runs through the middle of each I band, so a single I band spans the boundary between two neighboring sarcomeres .^1,2

Thin filaments are anchored at the Z-disc and project from there toward the center of each sarcomere. The portions of these filaments that do not overlap the thick filaments make up the I band. This arrangement distinguishes the I band from the A band, which corresponds to the full length of the thick filaments and includes regions where thick and thin filaments overlap .^1,2

During sarcomere shortening, thin and thick filaments slide past one another rather than appreciably shortening themselves. As the thin filaments move farther into the A band, their non-overlapping portions become smaller. The I band therefore narrows as the sarcomere shortens, while the width of the A band remains essentially constant .^1,2

The alternating light I bands and darker A bands contribute to the characteristic striated appearance of skeletal and cardiac muscle. The term “I band” originated from its relatively isotropic optical properties compared with the anisotropic A band .¹