Updated: 1/3/2021

Skeletal and Cardiac Muscle Contractions

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  • 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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