Action Potential and Muscle Contraction

Introduction

• Discussing how skeletal muscles work on a molecular level
• Focus on the muscle fibers and myofibrils

Muscle Structure

• Fascicles: Made of muscle fibers (multinucleated muscle cells)
• Myofibrils: Comprised of myofilaments arranged into sarcomeres
• Sarcomeres: Contractile units

Sarcomere Anatomy

• A bands: Darker regions
  • Contain H zones, split by M line (protein: myomesin)
• I bands: Lighter regions, split by Z disc
• Thick filaments: Contain myosin, extend across A band
• Thin filaments: Contain actin, extend across I band into A band
• Elastic filaments: Made of titin, span from Z disc to thick filament

Myosin and Actin

• Myosin: Protein with globular heads (ATP & actin binding sites)
• Actin: Two actin filaments twist together, each subunit has a myosin binding site
• Regulatory Proteins
  • Tropomyosin: Blocks myosin binding sites in relaxed muscle
  • Troponin: Complex of three polypeptides (binds actin, tropomyosin, calcium)

Sarcoplasmic Reticulum (SR) and T Tubules

• Surround the myofibrils
• Regulate calcium levels (necessary for contraction)
• T tubules: At A band - I band junction, help signals reach every muscle region

Sliding Filament Model of Contraction

• Nervous system stimulates muscle fibers
• Myosin heads on thick filaments interact with actin (create cross bridges)
• Thick filaments pull thin filaments towards the sarcomere center (Z discs towards M line)
• I bands shorten, H zone disappears
• A bands of adjacent sarcomeres move closer, muscle cell shortens

Neuromuscular Junction

• Interface between the nervous system and skeletal muscle
  • Axon terminals: Nearly touch muscle fiber (separated by synaptic cleft)
  • Synaptic vesicles: Contain acetylcholine (neurotransmitter)
  • Release Acetylcholine into synaptic cleft -> binds to receptors in junctional folds

Ion Channels and Depolarization

• Acetylcholine binding causes ion channels to open
  • Sodium ions enter, potassium ions leave (more sodium enters)
• Depolarization: Membrane potential changes, threshold voltage generates action potential
• Action potential propagates along sarcolemma and down T tubules
• Acetylcholinesterase: Breaks down acetylcholine, closes ion channel (no further contraction)

Excitation-Contraction Coupling

• Action potential propagation -> calcium release from SR
• Calcium binds to troponin -> tropomyosin moves, binding sites exposed for myosin
• Cross Bridge Cycling
  • Myosin heads bind actin, use ATP, pull actin filaments, detach, repeat
• Calcium levels drop -> troponin returns to original shape, muscle relaxes

Concept Recap

• Signal arrives at neuromuscular junction, acetylcholine released and binds to sarcolemma
• Ion channels open, sodium and potassium ions move, depolarization initiates action potential
• Action potential travels along sarcolemma and T tubules, releasing calcium
• Calcium allows myosin and actin to bind, muscle contraction begins

Next Steps

• Examine the muscular system as a whole to understand movement.