What are the contractile units within muscle fibers called?

The contractile units within muscle fibers are called sarcomeres, which are the fundamental units of muscle contraction. Each sarcomere is defined by its structure, consisting of thin filaments (actin) and thick filaments (myosin) that interact to produce muscle contraction. When a muscle fiber contracts, the sarcomeres shorten, leading to the overall shortening of the muscle. The arrangement of sarcomeres along the length of the muscle fibers is crucial for coordinated movement and force generation.

While myofilaments are the specific proteins (actin and myosin) that make up the sarcomeres, they are not the units themselves; rather, they contribute to the function of the sarcomere. Myofibrils are bundles of sarcomeres that run the entire length of the muscle fiber, but again, they are not the individual contractile units. Actin refers specifically to one of the proteins found in the myofilaments and is essential for the contraction process but does not represent the contracting unit. Therefore, sarcomeres are the correct answer as they encapsulate the structural and functional characteristics necessary for muscle contraction.