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Updated: Jan 3 2021

Skeletal and Cardiac Muscle Contractions

  • Introduction
    • The primary contractile unit of skeletal muscle is the sarcomere
    • Skeletal and cardiac muscle contraction is explained by the sliding filament theory with four key steps
      • attachment
      • power stroke
      • release
      • cocking
    • Requirements for contraction
      • stimulatory impulse (action potential) from a motor neuron
      • high calcium concentration within muscle cells
      • ATP for energy
    • Definitions
      • motor unit is defined as the individual motor neuron and the muscle fibers it stimulates
      • motor end plate (neuromuscular junction) is defined as the junction between the motor neuron and its associated muscle fibers
  • Sarcomere Structure
    • One sarcomere is defined as the segment between two Z-lines
    • Important defining structures
      • Z-line
        • anchoring point for actin filaments (thin filaments)
        • distance between Z-lines shortens with contraction
      • I-band
        • zone of thin filaments not superimposed by thick filaments
        • decreases in size with contraction
      • A-band
        • entire length of one thick filament
        • stays constant in size with contraction
      • H-zone
        • zone of thick filaments not superimposed by thin filaments
        • decreases in size or disappears entirely with contraction
      • M-line
        • midline of the sarcomere
        • does not change with contraction
    • Important proteins
      • actin
        • thin filament
        • anchored to the Z-line
        • extend from the Z-line into the A-line
      • myosin
        • thick filament
        • extends across the A-band
        • linked at the center by the M-line
      • tropomyosin
        • actin-binding protein
        • at rest, is bound tightly to actin to prevent cross-bridge formation with myosin
        • during contraction, calcium binding to troponin triggers a conformational change that releases tropomyosin from actin, allowing cross-bridge formation to occur
      • troponin
        • complex of three proteins (C, I, and T)
        • troponin C is a calcium-binding protein that regulates the conformational state of tropomyosin
      • titin
        • links the Z-line to the thick filaments
  • Sliding Filament Theory
    • An action potential triggers calcium release from the sarcoplasmic reticulum
    • Calcium ions bind to troponin, inducing a conformational change in the troponin-tropomyosin complex
      • tropomyosin is released from actin, exposing actin binding sites
      • allows cross-bridge formation to occur between myosin heads and actin binding sites
    • Contraction
      • attachment
        • myosin head is "cocked" and bound to actin, forming a cross-bridge
        • ADP and Pi are bound to myosin
      • power stroke
        • myosin head pivots centrally, pulling the actin toward the M-line
        • ADP and Pi are released
      • release
        • myosin head is "uncocked" and not bound to nucleotide
        • ATP binds to myosin, triggering myosin detachment from actin
          • rigor mortis, also known as postmortem rigidity, is caused by ATP deficiency secondary to loss of oxygen and glucose in death
            • without ATP, myosin can not detach from actin, leading to muscle rigidity
      • cocking
        • myosin head hydrolyzes ATP, using the energy from hydrolysis to undergo a conformational change from uncocked (low energy) to cocked (high energy)
        • myosin head is now ready to bind actin again and repeat the contraction cycle
        • ADP and Pi are bound to myosin
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