Arrangement of myofilaments in striated muscle
The thick myosin filaments account for the darker A-bands, and the thin actin filaments account for the light I-bands. The I-band is divided by a dark Z-line, which may or may not be visable with the light microscope. The actin filaments are anchored to the Z-line. A sarcomere runs from one Z-line to the next Z-line, and therefore, it consists of two I-band halves, and an A-band between them. Upon contraction, the actin filaments slide towards the center of the myosin filaments, and the I-bands and overall sarcomere length get smaller.