A sarcomere is the basic structural and functional unit of a muscle fiber. It is composed of long protein filaments called actin and myosin, which slide past each other to produce muscle contraction. Sarcomeres are arranged end-to-end within myofibrils, giving muscle tissue its striated appearance. Each sarcomere is defined by its boundaries, known as Z lines, which anchor the actin filaments. The region between two Z lines contains overlapping actin and myosin filaments, creating a pattern that is essential for muscle function. When a muscle contracts, the sarcomeres shorten, leading to overall muscle shortening and movement.