Sarcomere

A sarcomere is the basic unit of contraction in skeletal and cardiac muscle. Each muscle fiber is packed with long strands called myofibrils, and every myofibril is a chain of sarcomeres lined up end to end, each about 2 micrometers long. A sarcomere runs from one Z-disc, or Z-line, to the next. Thin filaments, made mainly of actin, are anchored to the Z-discs and point toward the middle, where they overlap thick filaments made of myosin, which are held in place at the central M-line.

The regular overlap of thick and thin filaments creates alternating light and dark bands, the I bands and A bands, which give these muscles their striped, or striated, look. Contraction follows the sliding filament model. Myosin heads bind actin, pull, let go and bind again, using energy from ATP, so the thin filaments slide toward the center. The Z-discs move closer together and the I band and central H zone narrow, while the A band, which matches the length of the thick filaments, stays the same. The process starts when calcium released inside the fiber binds troponin, which shifts tropomyosin off the binding sites on actin.

Because thousands of sarcomeres in a fiber shorten together, their tiny movements add up to a visible contraction.