sarcomere Any of the functional units that make up the myofibrils of voluntary muscle. Each sarcomere is bounded by two membranes (Z lines), which provide the points of attachment of actin filaments; another membrane (the M band or line) is the point of attachment of the myosin filaments. The sarcomere is divided into various bands reflecting the arrangement of the filaments (see illustration). During muscle contraction the actin and myosin filaments slide over each other (see sliding filament theory) and the length of the sarcomere shortens: the Z lines are drawn closer together and the I and H bands become narrower.

sarcomere That region of a myofibril of striated muscle that lies between the Z-lines.