IB Sports, exercise and health science B.1.3.4 Sliding Filament Theory and Molecular Basis of Muscle Contraction Notes

Sliding filament theory explains how actin and myosin produce contraction

1. Huxley first proposed the sliding filament theory in 1954.
2. The Sliding Filament Theory describes how muscle contraction occurs at the molecular level.

Organization of Skeletal Muscle

Skeletal muscle is highly organized and follows a hierarchical structure that allows it to generate force efficiently.

Hierarchy of Skeletal Muscle Organization

1. Muscle (Whole Muscle): Composed of multiple fascicles, surrounded by the epimysium.
2. Fascicle (Bundle of Muscle Fibers): A group of muscle fibers, surrounded by the perimysium.
3. Muscle Fiber (Single Muscle Cell): Contains many myofibrils, surrounded by the endomysium.
4. Myofibril (Contractile Units within a Muscle Fiber): Contains repeating units of sarcomeres, which are the fundamental contractile structures of muscle.
5. Sarcomere (Functional Unit of Contraction): Composed of myofilaments (actin and myosin), responsible for contraction.
6. Myofilaments (Protein Filaments in a Sarcomere): Includes actin (thin filament) and myosin (thick filament), which interact to produce muscle contraction.

Structure and Function of Key Muscle Fiber Components

Skeletal muscle fibers have distinct substructures that work together to enable contraction.

The Structure of a Sarcomere

1. Each sarcomere consists of two main protein filaments, actin and myosin, which are the active structures responsible for muscular contraction.
2. These filaments slide past each other to generate force.

Sarcomere Changes During Contraction

During contraction:

1. The Z-discs move closer together, shortening the sarcomere.
2. The I-band and H-zone decrease in size.
3. The A-band remains unchanged because myosin filaments do not change length.
4. The actin filaments slide over myosin filaments, powered by ATP.

Myofilaments: Actin and Myosin

1. Thin Filament (Actin)

Composition of Actin Filament

1. Actin: A protein that forms a helical structure and provides the foundation for muscle contraction.
2. Troponin: A regulatory protein that binds calcium ions and causes a change in the tropomyosin position.
3. Tropomyosin: A regulatory protein that blocks myosin-binding sites on actin at rest, preventing contraction.

Function of Actin Filament

1. Actin provides the binding sites for myosin heads, which are crucial for the cross-bridge cycle.
2. Troponin and tropomyosin work together to regulate contraction by blocking or exposing the myosin-binding sites on actin.