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 P_i, 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