Sliding Filament Theory Explained

The sliding filament theory is a widely accepted explanation for the molecular mechanism of muscle contraction. It describes the process by which actin and myosin filaments interact to generate force and produce muscle contraction. The correct answer to the given question is option 'C', which states that actin and myosin filaments do not shorten but rather slide past each other.

Key Points:
- Sliding filament theory explains how muscles contract.
- Actin and myosin filaments slide past each other.
- Actin and myosin filaments do not shorten but rather slide past each other.

Explanation:
The sliding filament theory proposes that muscle contraction occurs when the actin and myosin filaments within the muscle fibers slide past each other, causing the muscle to shorten. This process allows the muscle to generate force and perform its function.

Actin and Myosin Filaments:
- Actin filaments: Actin filaments are thin filaments composed of actin protein. They are attached to a structure called the Z-line.
- Myosin filaments: Myosin filaments are thick filaments composed of myosin protein. They are located between the actin filaments.

Sliding Filament Mechanism:
1. Initiation: The process of muscle contraction starts when the brain sends a signal to the muscle fibers to contract. The signal triggers the release of calcium ions within the muscle cells.
2. Cross-bridge formation: The released calcium ions bind to troponin, a protein present on the actin filaments. This binding causes a conformational change in tropomyosin, another protein associated with actin filaments, exposing the binding sites on actin.
3. Power stroke: The myosin heads, which are projections from the myosin filaments, bind to the exposed binding sites on actin forming cross-bridges. ATP (adenosine triphosphate) is then hydrolyzed, providing energy for the myosin heads to undergo a power stroke. During the power stroke, the myosin heads pull the actin filaments towards the center of the sarcomere, causing them to slide past the myosin filaments.
4. Release and reattachment: After the power stroke, the myosin heads release the ADP (adenosine diphosphate) and phosphate molecules, allowing them to detach from actin. The myosin heads then reattach to a new binding site on actin, ready for the next power stroke.
5. Sliding of filaments: As more and more myosin heads perform power strokes, the actin filaments slide past the myosin filaments towards the center of the sarcomere. This sliding action shortens the sarcomere, leading to muscle contraction.

Conclusion:
In summary, the sliding filament theory explains muscle contraction as a result of actin and myosin filaments sliding past each other. Contrary to options (a) and (d), both actin and myosin filaments do not shorten during muscle contraction. Option (b) is also incorrect as it implies that both filaments shorten, which is not the case. The correct answer, option 'C', accurately describes the sliding filament theory by stating that actin and myos