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Updated: Feb 1 2018

Muscle Conduction to Contraction

  • Introduction
    • Skeletal muscle cells are composed of muscle fascicles
      • muscle fascicles are composed of multi-nucleated muscle fibers
        • muscle fibers are composed of myofibrils
          • myofibrils contain sarcomeres, where actin and myosin filaments slide and produce contractions
    • Muscle cell transverse tubules (T-tubules), invaginations of the plasma membrane, are juxtaposed with the sarcoplasmic reticulum’s terminal cisternae to contract
      • T-tubules are responsible for conducting the action potential to the cisternae of the sarcoplasmic reticulum
      • skeletal muscle triad
        • 1 T-tubule and 2 terminal cisternae
      • cardiac muscle dyad
        • 1 T-tubule and 1 terminal cisterna
    • Sliding filament theory
      • sliding of thick and thin filaments form a contraction
      • thick filaments
        • composed of protein myosin
        • anchored to M line
      • thin filaments
        • composed of actin, tropomyosin, and troponin
          • actin contains binding sites for myosin
          • tropomyosin covers these binding site grooves
        • anchored to Z line
  • Excitation-Contraction Coupling

    • Action potential depolarizes plasma membrane
      • presynaptic voltage-gated Ca2+ channels open and neurotransmitters are released from the neuron
      • post-synaptic ligand is bound, causing depolarization of the motor end plate of the muscle
    • Depolarization spreads to the T-tubule
      • depolarization causes a conformational change in the voltage-sensitive dihydropyridine receptor
      • this causes a conformational change in the ryanodine receptor (also a calcium channel protein)
      • Ca2+ is released from the sarcoplasmic reticulum
        • this initiates the contraction
  • Contraction
    • Free Ca2+ binds to troponin C
      • this causes a conformational change and moves tropomyosin out of a myosin-binding groove
    • Myosin releases ADP and Pi, causing displacement of myosin on actin filament
      • this produces a power stroke and contraction shortens H and I bands, resulting in shortening between the Z lines (HIZ shrinkage)
      • A band remains the same length
    • Myosin head is detached from the actin filament because of binding of a new ATP molecule
    • Hydrolysis of ATP to ADP causes myosin head to adopt a high-energy position, in preparation for the next contraction
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