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1-2 Muscle Contraction

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Ca++ and Skeletal Muscle

  • Ca++ is essential for muscle contraction
  • Released from the SR during excitation contraction coupling and initiates the contraction cycle
    • Total [Ca++] in SR is 40,000-400,000X higher than in the sarcoplasm!

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Ca++, Troponin, and Tropomyosin

  • At rest, tropomyosin blocks MBS on thin filaments.

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  • Released Ca++ binds to troponin causing a structural change that rotates tropomyosin

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  • This allows myosin heads to bind to MBS on G-actin subunits, forming a cross-bridge.

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The Contraction Cycle

  • Five Steps of the Contraction Cycle
    1. Exposure of active sites
    2. Formation of cross-bridges
    3. Pivoting of myosin heads
    4. Detachment of cross-bridges
    5. Reactivation of myosin

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The Contraction Cycle

  • Five Steps of the Contraction Cycle
    1. Exposure of active sites
      • Ca++ released by SR has bound

to troponin causing MBS to be

exposed

    • Formation of cross-bridges
    • Pivoting of myosin heads
    • Detachment of cross-bridges
    • Reactivation of myosin

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The Contraction Cycle

  • Five Steps of the Contraction Cycle
    1. Exposure of active sites
    2. Formation of cross-bridges
    3. Pivoting of myosin heads
    4. Detachment of cross-bridges
    5. Reactivation of myosin

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The Contraction Cycle

  • Five Steps of the Contraction Cycle
    1. Exposure of active sites
    2. Formation of cross-bridges
    3. Pivoting of myosin heads
      • Power stroke, ADP+Pi still bound
    4. Detachment of cross-bridges
    5. Reactivation of myosin

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The Contraction Cycle

  • Five Steps of the Contraction Cycle
    1. Exposure of active sites
    2. Formation of cross-bridges
    3. Pivoting of myosin heads
    4. Detachment of cross-bridges
      • Binding of new ATP releases myosin

    • Reactivation of myosin

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The Contraction Cycle

  • Five Steps of the Contraction Cycle
    1. Exposure of active sites
    2. Formation of cross-bridges
    3. Pivoting of myosin heads
    4. Detachment of cross-bridges
    5. Reactivation of myosin

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The Sliding Filament Theory

  • Myofilaments slide past each other, rather than physically contracting and expanding each cycle (like gross muscles appear to do)

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Length-Tension Relationship

  • The force of a muscle contraction depends on the length of the sarcomeres prior to the contraction

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Rigor Mortis

  • Deteriorating SR releases Ca++ (starts 3 hours after death)
  • Activates troponin🡪tropomyosin🡪 myosin binding🡪contractions (peaks 12 hours after death)
  • BUT no new ATP
  • Relaxation requires ATP
  • Fibers remain contracted until myofilaments decay (48-60 hours after death)