Sliding Filament Model of Contraction

When a muscle contracts, the actin is pulled along myosin toward the center of the sarcomere until the actin and myosin filaments are completely overlapped. In other words, for a muscle cell to contract, the sarcomere must shorten. However, thick and thin filaments—the components of sarcomeres—do not shorten. Instead, they slide by one another, causing the sarcomere to shorten while the filaments remain the same length. The sliding filament theory of muscle contraction was developed to fit the differences observed in the named bands on the sarcomere at different degrees of muscle contraction and relaxation. The mechanism of contraction is the binding of myosin to actin, forming cross-bridges that generate filament movement (Figure 1).

 Part A of the illustration shows a relaxed muscle fiber. Two zigzagging Z lines extend from top to bottom. Thin actin filaments extend left and right from each Z line. Between the Z lines is a vertical M line. Thick myosin filaments extend left and right from the M line. The thick and thin filaments partially overlap. The A band represents the length that the thick filaments extend from both sides of the M line. The I band represents the part of the thin filaments that does not overlap with the thick filaments. Part B shows a contracted muscle fiber. In the contracted fiber, the thick and thin filaments completely overlap. The A band is the same length as in the uncontracted muscle, but the I band has shrunken to the width of the Z line.

Figure 1. When (a) a sarcomere (b) contracts, the Z lines move closer together and the I band gets smaller. The A band stays the same width and, at full contraction, the thin filaments overlap.

When a sarcomere shortens, some regions shorten whereas others stay the same length. A sarcomere is defined as the distance between two consecutive Z discs or Z lines; when a muscle contracts, the distance between the Z discs is reduced. The H zone—the central region of the A zone—contains only thick filaments (myosin) and is shortened during contraction. The H zone becomes smaller and smaller due to the increasing overlap of actin and myosin filaments, and the muscle shortens. Thus when the muscle is fully contracted, the H zone is no longer visible. The I band contains only thin filaments and also shortens. The A band does not shorten—it remains the same length—but A bands of different sarcomeres move closer together during contraction, eventually disappearing. Thin filaments are pulled by the thick filaments toward the center of the sarcomere until the Z discs approach the thick filaments. The zone of overlap, in which thin filaments and thick filaments occupy the same area, increases as the thin filaments move inward.

Note that the actin and myosin filaments themselves do not change length, but instead slide past each other.