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(At neuromuscular voltage-gated Na channels open which stimulates…

- - - - shortening of enough motor units triggers muscle contraction
      - as all of the myofibrils shorten, the muscle fiber shortens
      - as all sarcomeres are simultaneously shortened, the myofibril shortens
    - - Electrical Signaling at NMJ (G4.3)
        
        action potential in axon opens calcium channels on terminal
        
        influx of calcium triggers exocytosis of acetylcholine
        
        acetylcholine crosses synaptic cleft
        
        acetylcholine binds to chemically gated sodium channels and opens them
        
        acetylcholinesterase breaks down acetylcholine for recycling
      - Excitation (G4.4)
        
        influx of sodium depolarizes motor end plate
        
        adjacent voltage gated sodium channels open
        
        additional sodium crosses sarcolemma
      - Excitation-contraction Coupling and Power Stroke (G4.4)
        
        calcium released into sarcoplasm binds to troponin
        
        troponin changes shape, moving tropomyosin
        
        tropomyosin exposes active sites on actin
        
        myosin binds to actin forming cross-bridge
        
        ADP and Pi are released, myosin head bends
        
        bending of myosin pulls actin toward M-line, called the power stroke
        
        ATP binds to myosin, causing it to release actin
        
        Hydrolysis of ATP places myosin back in its ready position to repeat power stroke
      - Action Potential (G4.4)
        
        voltage gated sodium channels allow sodium influx which open more voltage gated sodium channels further down the membrane
        
        slower voltage-gated potassium channels also open, releasing potassium
        
        release of potassium causes repolarization
        
        sodium potassium pumps quickly restore sodium and potassium gradients
        
        action potential occurs as a wave of depolarization and repolarization down sarcolemma
        
        action potential travels through t-tubules and triggers voltage gated calcium channels on sarcoplasmic reticulum to release caclium into sarcoplasm
      - Sliding Filament Theory (G4.1)
        
        many myosin heads pull on actin in ratcheting motions to draw actin toward the M-line
        
        as all the actin in the sarcomere is pulled toward the M line, the sarcomere shortens