sliding filament theory

Skeletal Muscle Fibers and Myofibrils

  • Basic Structure

    • Skeletal muscle fibers consist of myofibrils, which are the functional units of muscle contraction.

    • Myofibrils are made up of repeating units called sarcomeres, extending from Z disk to Z disk.

  • Filament Types

    • Thick Filaments: Composed of myosin protein.

    • Thin Filaments: Composed primarily of actin, along with regulatory proteins troponin and tropomyosin.

  • Arrangement

    • Myofibrils consist of organized sarcomeres that line up end to end.

    • Filaments are kept in proper orientation due to anchoring proteins at the M line and Z disks, allowing overlap.

Sliding Filament Theory

  • Overview

    • The sliding filament theory explains how muscle contraction occurs through interactions between thick and thin filaments.

    • Contraction results in shortening of the sarcomere, which leads to shortening of the myofibril, muscle fiber, and ultimately, the muscle itself.

  • Historical Context

    • The sliding filament theory emerged in the 1950s after advancements in microscopy showed that thin filaments slide past thick filaments during contraction.

Sarcomere Dynamics During Contraction

  • During Contraction

    • Z disks move toward the M line, causing the overall sarcomere to shorten.

    • Multiple sarcomeres shorten together, resulting in the overall shortening of myofibrils and muscle fibers.

Muscle Contraction Mechanism Steps

  1. Activation

    • Calcium binds to troponin, moving tropomyosin away from the actin's active site, allowing actin and myosin interaction.

  2. Energizing the Myosin Head

    • ATP is hydrolyzed to ADP and inorganic phosphate, which 'cocks' the myosin head into a high-energy state.

    • Analogy: Like cocking a gun or a baseball pitcher winding up before a throw.

  3. Cross Bridge Formation

    • Myosin head attaches to the exposed active site on actin to form a cross bridge.

  4. Power Stroke

    • Myosin head pivots and pulls the thin filament toward the M line, releasing ADP and inorganic phosphate in the process.

    • Analogy: Similar to a baseball pitcher releasing the ball toward the batter.

  5. Detachment

    • ATP binds to the myosin head, causing it to detach from the actin.

    • This promotes the cycle to repeat as ATP hydrolysis re-cocks the myosin head.

Sarcomere Features During Contraction

  • A Band: Stays the same length as it represents the length of thick filaments.

  • H Zone: Narrows as the thin filaments slide toward the M line.

  • I Band: Narrows as the thin filaments are pulled further into the A band, overlapping with thick filaments.