Which area of the sarcomere contains only actin filaments?

The area of the sarcomere that contains only actin filaments is the I band. The I band is the region that appears lighter under a microscope and is located on either side of the Z line. This band consists solely of thin filaments made of actin, without any overlapping thick filaments, which are primarily composed of myosin.

In the context of muscle contraction, the I band shortens as the sarcomere contracts; however, the actin filaments do not change in length. This distinct composition of the I band is essential in understanding the sliding filament model of muscle contraction, where the actin filaments slide over the myosin filaments, facilitating muscle shortening.

In contrast, other regions of the sarcomere contain different combinations of actin and myosin. The A band contains the length of the thick filaments (myosin) and overlaps with the thin filaments (actin) at both ends, the Z line serves as the boundary between adjacent sarcomeres and is where the actin filaments are anchored, and the H zone is the central part of the A band where only thick filaments are present without overlapping actin. Understanding these distinctions helps clarify the structural organization of muscle fibers and their function