Contraction / Power Stroke

Contraction of the Sarcomere

  • Overview of Contraction

    • Focus on the contraction of the sarcomere
    • Integration of the sliding filament theory with the interaction of myosin heads and actin subunits
    • Main objective: Drawing the Z disc towards the Z disc, leading to muscle shortening

  • The Power Stroke

    • Definition: A specific moment during contraction when the myosin head moves the actin towards the M line
    • Functional dynamics: Both ends of the myosin head work to crank the Z discs toward each other

  • Structural Components of a Sarcomere

    • The structure includes:
    • Actin filaments that span from one sarcomere to another, anchored at their midpoint by the Z disc
    • Myosin filaments aligned towards the M line (the middle of the sarcomere)
    • Imagery: Actin is described as resembling "a bead of pearls," indicating its intertwined nature with another filament.

  • Interaction of Myosin Heads with Actin Proteins

    • Detailed visualization:
    • Contains multiple myosin heads interacting with actin subunits
    • Highlighting the binding sites on actin for myosin attachment
    • Structural proteins involved:
    • Tropomyosin: Covers the actin binding site under resting conditions
    • Troponin: A protein that interacts with calcium ions to alter tropomyosin position
      • Mechanism: Calcium binds to troponin, causing tropomyosin to move away from the myosin binding sites, allowing myosin to attach.

  • Myosin Head Positions

    • Three critical positions of myosin during contraction:
    1. Attached Position: Myosin is bound to actin
    2. Released Position: Myosin releases actin after the power stroke
    3. Cocked Position: Myosin head cocks back in preparation for the next cycle
    • Cycle of interaction: Myosin attaches → Executes power stroke → Remains attached → Releases → Cocks back → Repeats

  • ATP and Muscle Contraction

    • Importance of ATP in muscle function:
    • Required for the release of myosin from actin binding sites
    • Lack of ATP results in rigidity or stiffness known as rigor mortis
    • Rigor mortis occurs because muscles remain contracted without ATP supply
    • ATP is necessary for:
      • Myosin head detachment from actin during relaxation
      • Energy release enables the power stroke to occur

  • Molecular Interactions During Contraction

    • Initial state: Actin with exposed binding site; myosin attaches to it
    • Myosin conducts the power stroke, advancing the actin towards the M line
    • The role of ATP in changing molecular structure:
    • ATP hydrolysis leads to myosin head changing shape, thus releasing from actin
      • Resulting two products: ADP and inorganic phosphate (P)
    • Shape change and its dynamics:
      • Changing shape of myosin head allows binding to another actin if available
      • Release of ADP prompts another power stroke as myosin head moves actin towards the M line

  • Summary of Events:

    • Cycle portrayed effectively: ATP → ADP + P → Myosin shape change → Attachment to actin → Power stroke → Release of ADP
    • Feedback loop of actions upon molecule interactions leads to muscle contraction:
    • One molecule touching another changes the shapes of both, fueling further interactions and contractions

  • Clinical Implication: Rigor Mortis and ATP

    • The initial focus on rigor mortis highlights the critical need for ATP in muscle physiology:
    • Illustrates the cessation of ATP supply causing sustained contraction or stiffness
    • Vital for understanding muscle mechanics and potential implications in cadaver studies.