Which type of myofilament is characterized as a thin contractile protein in muscle fibers?

Actin is the thin contractile protein found in muscle fibers. It plays a crucial role in muscle contraction by interacting with the thick myofilament, myosin. During the contraction process, actin filaments slide over myosin filaments, which causes the muscle to shorten and generate force. This interaction is an essential aspect of the sliding filament theory of muscle contraction.

In contrast, myosin is the thick contractile protein that works alongside actin. Myofibrils are not individual proteins but rather the structures within muscle fibers that contain the contractile proteins (both actin and myosin). Sarcomeres are the functional units of a myofibril, defined as the segment between two Z lines, and they contain both actin and myosin filaments organized in a specific arrangement that facilitates contraction. Understanding the roles of actin and myosin, and the structural organization of myofibrils and sarcomeres is critical to grasping how muscles contract and generate movement.