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