Sliding filament theory

Theory explaining muscle contraction

The sliding filament theory is a scientific explanation of how muscles contract to produce force. It was first proposed in 1954 by Andrew Huxley and Rolf Niedergerke, and independently by Hugh Huxley and Jean Hanson. This theory describes the process by which muscle fibers contract by the sliding of actin and myosin filaments over each other, resulting in the shortening of the sarcomere, the basic unit of a muscle's striated muscle tissue.

Structure of the Sarcomere

The sarcomere is the fundamental unit of a muscle's striated muscle tissue. It is composed of thick and thin filaments. The thick filaments are primarily made of the protein myosin, while the thin filaments are primarily composed of the protein actin. The sarcomere is delineated by Z lines, which anchor the actin filaments. The myosin filaments are located in the center of the sarcomere and overlap with the actin filaments.

Animation of the sliding filament theory in action

Mechanism of Muscle Contraction

The sliding filament theory explains muscle contraction in the following steps:

1. Initiation: The process begins with a signal from the nervous system, which triggers the release of calcium ions from the sarcoplasmic reticulum into the muscle cell.

2. Binding: Calcium ions bind to the protein troponin, causing a conformational change that moves tropomyosin away from the binding sites on actin filaments.

3. Cross-Bridge Formation: The myosin heads, which are part of the thick filaments, bind to the exposed sites on the actin filaments, forming cross-bridges.

4. Power Stroke: The myosin heads pivot, pulling the actin filaments toward the center of the sarcomere. This action is powered by the hydrolysis of adenosine triphosphate (ATP) into adenosine diphosphate (ADP) and inorganic phosphate.

5. Detachment: A new molecule of ATP binds to the myosin head, causing it to detach from the actin filament.

6. Reactivation: The myosin head is re-cocked as ATP is hydrolyzed, and the cycle can repeat as long as calcium ions remain present and ATP is available.

Diagram illustrating the sliding filament theory

Significance

The sliding filament theory is crucial for understanding how muscles generate force and movement. It provides insight into the molecular basis of muscle contraction and has implications for understanding muscle diseases and developing treatments.

Also see

• Muscle contraction
• Sarcomere
• Myosin
• Actin
• Troponin
• Tropomyosin

References

• Huxley, A. F., & Niedergerke, R. (1954). "Structural changes in muscle during contraction: interference microscopy of living muscle fibres." Nature, 173(4412), 971-973.
• Huxley, H. E., & Hanson, J. (1954). "Changes in the cross-striations of muscle during contraction and stretch and their structural interpretation." Nature, 173(4412), 973-976.