Skeletal muscle contraction is a fundamental biological process that allows for movement and posture maintenance. This action is a highly coordinated mechanical event occurring at the microscopic level within each muscle cell. The basic functional unit responsible for generating force and causing muscle shortening is the sarcomere.
Anatomy of the Sarcomere
The sarcomere is the repeating structural unit that gives skeletal muscle its characteristic striated, or striped, appearance. Each sarcomere is delineated by two Z lines, which serve as anchoring points for the thin filaments. Extending inward from these Z lines are the thin filaments, primarily composed of the protein actin.
Lying in the center of the sarcomere are the thick filaments, which are bundles of the protein myosin. The entire region spanned by the thick filaments is called the A band, which appears darker in microscopic images. The area containing only thin filaments, located between the A bands of adjacent sarcomeres, is the lighter-staining I band. Within the A band, there is a central region called the H zone, which contains only thick filaments and no thin filament overlap in a relaxed muscle.
The Sliding Filament Model
Muscle contraction is driven by a mechanism known as the sliding filament model, which proposes that the filaments do not shorten, but instead slide past one another. This movement is initiated when an electrical signal causes the release of calcium ions from storage within the muscle cell. Calcium binds to regulatory proteins on the thin filaments, which then shifts the position of another protein, exposing the binding sites on the actin molecules.
Once the binding sites are revealed, the heads of the thick myosin filaments attach to the thin actin filaments, forming cross-bridges. The energy for this action is supplied by the hydrolysis of adenosine triphosphate (ATP), which cocks the myosin head into a high-energy position. The myosin head then executes a power stroke, pulling the thin filament toward the center of the sarcomere. This continuous, rapid cycle of attachment, pulling, and detachment causes the filaments to slide past each other.
Changes to Muscle Bands During Contraction
The sliding of the thin filaments toward the center of the sarcomere has distinct, observable effects on the different bands. The H zone is defined as the central region of the A band that contains only thick filaments. As the thin filaments are pulled inward, the area of overlap with the thick filaments increases, causing the H zone to narrow. During a maximal contraction, the thin filaments meet at the center, and the H zone can disappear entirely.
Similarly, the I band also shortens. This band represents the area of thin filaments not overlapped by thick filaments, and as the Z lines are pulled closer together, this non-overlapped region decreases in size.
The A band, however, remains constant in length throughout the contraction process. This stability is a direct consequence of the thick filaments themselves not changing length, which supports the sliding filament model.