Myofibril

The myofibril is a long contractile unit made of the myofilaments actin and myosin which form the sarcomere. They line up parallel to one another forming cylindrical structures which bundle together into a muscle fibre. The regular arrangement of cross striations of sarcomeres, which can be observed under a low magnification electron microscope, attribute to the alternative name given to skeletal muscles: striated muscle. An electron micrograph of myofibrils shows differently shaded bands. These are representitive of the different density protein filaments:

• Z discs: thick, darkly stained, short filaments which mark the start and end of the sarcomere.
• A bands: thick, darkly stained region which marks the locations of myosin filaments.
• M lines (or midlines): thin, darkly stained, short vertical filaments which link the two myosin II complexes of the sarcomere at their tail regions.
• I bands: thin, lightly stained region which marks the locations of actin filaments .