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Sarcomere
The sarcomere is the fundamental unit of muscle structure. Its capacity for contraction is the essential trait that makes muscles work.
A skeletal muscle is a bundle of cylindrical fibers that run the length of the muscle (see a picture). Each cylinder contains a bundle of myofibrils, linear organelles that are themselves composed of chains of sarcomeres (see a picture), which have two primary components (1) thin filaments (each of which contains two strands of actin and a single strand of regulatory protein); and (2) thick filaments made of myosin.
In the diagram at right, the A band is the region where thick filaments are present. The I bands are the two regions where thin filaments exclusively are present. The H zone is the region where only thin filaments are present. The Z lines, which have a jagged appearance under a microscope, are the boundaries between adjacent sarcomeres.
During muscle contraction the thick filaments quickly slide along the thin filaments to shorten the myofibrils. The myofilaments themselves, however, do not contract.
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