SATHEE: Exemplar Problems

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Question:

Define sliding filament theory of muscle contraction.

Answer:

The sliding filament theory is a widely accepted explanation for how muscle contraction occurs at the molecular level. It describes the mechanism by which muscles contract and generate force. The theory is based on the interactions between two major protein filaments within muscle cells: actin and myosin.

Muscle Structure: Muscles are composed of many individual muscle fibers, and each muscle fiber is made up of smaller units called myofibrils. Myofibrils contain repeating units called sarcomeres, which are the functional units of muscle contraction.
Actin and Myosin: Within each sarcomere, there are thin filaments made of actin and thick filaments made of myosin. Actin is a thin, thread-like protein, while myosin is a thicker, filamentous protein.
Resting State: In a relaxed muscle, actin and myosin do not overlap significantly. The myosin heads are in a low-energy position.
Nerve Signal: When a nerve signal (action potential) reaches the muscle, it triggers the release of calcium ions from the sarcoplasmic reticulum (a membrane system in muscle cells). Calcium ions bind to the regulatory protein troponin, causing a conformational change.
Tropomyosin Shift: This conformational change in troponin shifts the position of tropomyosin, another regulatory protein. Tropomyosin covers the binding sites on actin that myosin would attach to during muscle contraction.
Cross-Bridge Formation: With the binding sites on actin exposed, myosin heads can now attach to actin, forming cross-bridges.
Power Stroke: Using energy from ATP hydrolysis, myosin heads undergo a conformational change (power stroke) that pulls the actin filaments toward the center of the sarcomere. This shortens the sarcomere and results in muscle contraction.
Relaxation: When the nerve signal stops and calcium ions are actively transported back into the sarcoplasmic reticulum, troponin and tropomyosin return to their original positions, blocking the binding sites on actin. The myosin heads detach, and the muscle relaxes.
Repeat Cycle: Muscle contraction occurs as myosin heads repeatedly form and break cross-bridges with actin, causing the thin filaments to slide over the thick filaments. This sliding of filaments shortens the sarcomeres and, ultimately, the entire muscle fiber.

The sliding filament theory explains how muscle cells generate force and contract by the interaction between actin and myosin filaments. This process continues as long as calcium ions, ATP, and nerve signals are present to stimulate muscle contraction.

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