Y1-BMF-2024 Regulation of Muscle Contraction and Energy Sources (1)

Excitation-Contraction Coupling

  • Process Overview:

    1. Action Potential Propagation: An action potential travels along the sarcolemma and T-tubules.

    2. Calcium Release: Dihydropyridine receptors (DHPR) sense voltage changes and trigger ryanodine receptors (RyR) on the sarcoplasmic reticulum (SR) to release calcium ions.

    3. Troponin Activation: Calcium binds to troponin, causing a conformational change (crossbridge formation) with actin and release of ADP and phosphate from myosin ATPase.

    4. Cross-Bridge Cycling: Myosin heads bind to actin and perform a power stroke, pulling actin filaments towards to centre of the A band

    5. Resetting Myosin Heads: ATP binds to myosin, releasing it from actin, hydrolyzing ATP, and resetting myosin to a "cocked" position.

    6. Relaxation: occurs when calcium concentration returns to normal by active transport of calcium back into the sarcoplasmic reticulum where it is bound to calsequestrin

Rigor Mortis

  • Rigor Mortis:

    • Stiffening of body muscles after death.

    • ATP is required for the detachment of the myosin crossbridge from actin.

    • No ATP is produced post-mortem, causing myosin and actin to remain bound.

    • Begins 3-4 hours after death and gradually decreases as proteins degrade.

Energy Requirements for Muscle Contraction

  • Energy is expended during contraction:

    • ADP + Pi Released

    • Myosin heads must bound to ATP for detachment from actin.

    • Myosin ATPase hydrolyzes ATP to maintain high-energy, "ready" state.

Energy Sources for Muscle Contraction

  • Creatine Phosphate (CP):

    • Immediate ATP source, donating a phosphate to ADP to quickly regenerate ATP.

    • CP levels in resting muscle are about 5x ATP levels and are replenished during rest.

  • Glycolysis (Anaerobic):

    • Fast but inefficient, providing 2 ATP per glucose without oxygen.

    • Produces lactic acid, used during high-intensity, short-duration exercise.

  • Oxidative Phosphorylation (Aerobic):

    • Generates ~32 ATP per glucose using oxygen and nutrients.

    • Slower but more efficient, supporting sustained, endurance activities.

Muscle Fiber Types

  • Characterized based on speed of contraction and ATP production:

    • Type I: Slow-oxidative (small diameter)

    • Type IIa: Fast-oxidative (intermediate diameter)

    • Type IIx: Fast-glycolytic (large diameter)

Training Effects:

  • Training can improve oxidative capacity, blood supply, and mitochondria but doesn’t change fiber type.

  • High-intensity training increases muscle size (hypertrophy), particularly in fast-glycolytic fibers.

Neuromuscular Disease

  • Neuromuscular diseases result from issues in:

    • Neuropathy: Nerve diseases affecting central or peripheral nervous systems.

    • Junctionopathy: Diseases affecting neuromuscular junctions, like myasthenia gravis (autoimmune where antibodies destroy the communication between nerves and muscle, resulting in weakness)

    • Myopathy: Diseases affecting muscles, such as muscular dystrophies.

Muscle Contraction Types

  1. Isotonic Contraction:

    • Muscle shortens and tension remains constant.

  2. Isometric Contraction:

    • Muscle develops tension without changing length.

  3. Eccentric Contraction:

    • Muscle lengthens while developing tension due to external force.

Length-Tension Relationship

  • Isometric tension developed in a muscle depends on its initial length before contraction.

  • An optimal length allows maximal tension development in skeletal muscle.

Huxley Model and Length-Tension Relationship

  • The optimal length for tension development is where crossbridge overlap is maximized.