The Sliding Filament Theory of Muscle Contraction
The fundamental process of muscle contraction is explained by the sliding filament model, where muscle fibers shorten as thick (myosin) and thin (actin) filaments slide past each other, pulling the Z-discs closer together within the sarcomere.
Key Proteins and Structures Involved
This intricate process relies on specialized proteins: actin, which forms the thin filaments, and myosin, which forms the thick filaments. These filaments are precisely organized into repeating units called sarcomeres, which are the basic contractile units of muscle tissue.
The Role of ATP and Calcium in Muscle Movement
Muscle contraction is initiated by a nerve impulse, leading to the release of calcium ions (Ca²⁺) within the muscle cell. Calcium binds to regulatory proteins on actin, exposing binding sites for myosin. Myosin heads then attach to actin, pivot (the 'power stroke'), pulling the actin filament, and detach using energy derived from ATP hydrolysis.
Steps of Contraction and Overall Muscle Shortening
This cyclical process of myosin attachment, pivoting, and detachment, continuously fueled by ATP, causes the actin filaments to progressively slide further past the myosin filaments. The synchronized shortening of countless sarcomeres across an entire muscle fiber results in the observable contraction and shortening of the muscle.