This cytoskeleton is responsible for muscle contraction, and the changes in shape during the development of a vertebrate embryo. These diverse activities depend on three principal types of protein filaments: Actin filaments, microtubules and inter mediate filaments. .
Muscle fibers are huge single cells formed from many separate cells whose nuclei lie just beneath the plasma membrane. The bulk of the cytoplasm of these huge cells is made up of myofibrils, which are sometimes just as long as muscle cells itself. These myofibrils contract instantly if ATP and Ca 2+ is added to them.
"The central and structural and functional unit of a muscle cell is the sarcomere, a cylinder of 1.5 micrometers in diameter and 2 micrometers in length which contains about 2 * 2000 thin protein filaments and 1000 thick protein filaments- (2). Each sarcomere is separated by Z-discs. Two Z-discs bound a sarcomere in the direction of stretching. Thin filaments made of actin are attached to each of these discs and extent toward each other inside the sarcomere. There is a dark band visible between the Z-discs. This is made up of the thick myosin filaments, which overlap partially with the actin filaments, which extend into a light half of the I-band left and right of a dark A-band. .
When myofibrils contract the thin and thick filaments move past each other. Each saromere unit of the myofibrils shortens proportionally to the muscle contraction. Upon contraction, it is the light bands, which shorten whereas the dark bands do not change in length. This is explained by the actin filaments sliding into the dark region of the myosin filaments. In the overlapping regions cross-bridges, "where energized myosin binds to a specific site on actin- (4), extend about 13nm from thick myosin filaments. These cross-bridges are mainly composed of the myosin heads which are attached to the end of two coiled alpa-heclices typically 150nm in length.