MUSCLE FIBER STRUCTURE & FUNCTION

EXCITATION-CONTRACTION-RELAXATION CYCLE

1. Motor neuron activity triggers sarcolemmal depolarization via NMJ
2. Action potential generated
   • Travels along muscle surface membrane
   • Enters transverse tubule system (t-tubules)
   • Signal transmitted from t-tubules to terminal cisternae at triads
3. Ca⁺⁺ ions released from sarcoplasmic reticulum: Regulated by 2 large membrane protein complexes
   • Dihydropyridine receptor (DHPR)
     • Located in junctional t-tubules
     • Voltage sensing: α1 subunit of DHPR activated by membrane depolarization
     • Activated DHPR interacts with ryanodyne receptor
   • Ryanodyne receptor (RyR)
     • Located in junctional sarcoplasmic reticulum (SR)
     • RyRs release Ca⁺⁺ ions into cytosol after interaction with activated DHPR
     • Other RyRs in SR membrane without DHPR interactions
       • Activated by excess Ca⁺⁺ ions released by neighboring RyRs
       • Amplify & propagate Ca⁺⁺ signal
   • Related molecules: CASQ; Triadin; ATP2A1; STAC3; SPEG
4. Contractile apparatus activated
   • Ca⁺⁺ binds to troponin complex
   • Tropomyosin binding to contractile apparatus changes
   • Actin allowed to bind to myosin heads
   • Muscle contraction occurs via myofilament sliding

MUSCLE FIBER STRUCTURE From UCSD THICK-THIN FILAMENT CROSSBRIDGES From M Ferenczi MUSCLE FIBER CONTRACTION From M Ferenczi