muscle contraction events
1. Acetylcholine is released at
   neuromuscular junction
2. AP is propagated along
   membrane & down T-tubule
3. Ca released from SR via a
    voltage gated Ca channel
4. Ca binds to Troponin-C -
    conformation changes
    favor tropomyosin opening
    actin myosin binding sites.
5. myosin cross-bridges
   attach-detach from
    actins... pulls filament
    toward M-line.
6. Ca is removed by Ca-pump
       (uptake by SR)
7. tropomyosin blocks actin
   sites and muscle relaxes