Striated MuscleIn contrast to smooth muscle, cardiac and skeletal muscle types possess an internal ultrastructure of highly organized contractile myofilaments. Actin and myosin myofilaments are stacked and overlapped in regular repeating arrays to form sarcomeres. Dark A bands result where myofilaments overlap with light I bands present in areas of non-overlap. The A and I bands repeat along the length of the muscle fibers to create the striations visible under the light microscope. Some Key Points About SarcomeresSarcomeres repeat along skeletal and cardiac fibers.Components between adjacent Z-lines comprise the sarcomere.Sarcomeres are considered the functional units of striated muscles.I Bands are lighter areas of non-overlap between actin and myosin that contain the Z-lines.A Bands are areas where some overlap occurs and coincide with the length of myosin myofilaments.