A sarcomere is one of the segments into which a myofibril of striated muscle is divided. It is smallest functional unit of a muscle. The sarcomere is a functional unit composed of two membranes (Z-lines), which act as boundaries. The Z-lines are attachment points of actin filaments, while the M-band/line is where the myosin filaments attach. The muscle contraction is a result of the actin and myosin filaments sliding over each other, thereby causing the length of the sarcomere to shorten.