Sliding Filament Model of Contraction
Muscles contract when sarcomeres shorten. The thin and thick filaments that compose sarcomeres do not shorten; instead, they slide past one another, causing the sarcomere to shorten while the filaments remain the same length. The sliding filament theory of muscle contraction is the binding of myosin to actin, forming cross-bridges that generate filament movement (Figure 1).
Process of Movement
Myosin is a molecular motor that acts like an active ratchet. Chains of actin proteins form high tensile passive 'thin' filaments that transmit the force generated by myosin to the ends of the muscle. Myosin also forms 'thick' filaments. Each myosin 'paddles' along an actin filament repeatedly binding, ratcheting, and letting go, sliding the thick filament over the thin filament.
- ATP binds to myosin and is hydrolyzed by ATPase into ADP and phosphate. The energy released by this process activates the myosin head and cocks it into a high-energy, extended position.
- The cocked myosin head binds to a newly-exposed active site on the thin filament, generating a cross-bridge between actin and myosin.
- Myosin releases the ADP and phosphate, returning to a low-energy position, pulling the thin filament along; this movement is called a power stroke. Shortening occurs when the extensible region pulls the filaments across each other (like the shortening of a spring). Myosin remains attached to the actin.
- The binding of ATP destabilizes the myosin-actin bond, allowing myosin to detach from actin. While detached, ATP hydrolysis occurs, "recharging" the myosin head. If the actin-binding sites are still available, myosin can bind actin again.
- The collective bending of numerous myosin heads (all in the same direction), combine to move the actin filament relative to the myosin filament. This results in muscle contraction.
Contraction in all Sarcomeres
The sarcomere consists of a central bidirectional thick filament flanked by two actin filaments, oriented in opposite directions. When each end of the myosin thick filament ratchets along the actin filament with which it overlaps, the two actin filaments are drawn closer together. The A-bands contract toward the I-bands, thereby shortening the I-band as the myosin overlaps with the actin. Thus, the ends of the sarcomere are drawn in and the sarcomere shortens. Sarcomeres are connected by so-called 'Z-lines', which anchor the ends of actin filaments in such a way that the filaments on each side of the Z-line point in opposite directions (with reversed polarity). When a muscle fiber contracts, all sarcomeres contract simultaneously so that force is transmitted to the fiber ends.