What Are the Contractile Units of Skeletal Muscles?

According to PearsonHigherEd.com, the sarcomere is the basic unit of contraction within a bundle of muscle fibers. Individual sarcomeres contain clearly defined bands of filaments which have a special terminology.

Each sarcomere is surrounded by Z lines which denote the boundary from one sarcomere to the next. I bands are that portion of the sarcomere which contain thin filaments. H bands contain thick filaments, and A bands contain a mix of thick and thin. BiologyGuide.net states that these differences in thickness produce the characteristic striations of skeletal muscle.

BiologyGuide.net also states that the differences in thickness represent differences in what the filaments are made of. I bands contain filaments made up of actin alone, while H bands contain myosin filaments alone; A bands contain lengths of myosin.

The University of Washington describes how the filaments are typically arranged within a sarcomere; thin filaments extend from the Z lines to the center of the sarcomere, with thick filaments in the middle. The thick filaments never get as far as the Z lines, but they overlap the thin filaments that do. The two kinds of filament slide in respect to one another, causing the Z lines to be pulled toward one another, creating a muscle contraction.