---

- Myosin:
    - largest of myofibrillar proteins, and it makes up > 50% of muscle mass;
    - it constitutes thick filament of the contractile unit;
    - myosin molecule resembles a thin rod with two small, globular heads;
    - heads of chains protrude from thick filament;
    - it has significant ATPase activity in presence of ionic calcium, and it is this portion of molecule that serves as a cross-bridge 
          binding myosin w actin;
- Actin:
    - I Band, or thin filament, is made up of three proteins:
    - major component of thin filament is actin, consisting of two polymers;
          - G-actin, or globular actin;
          - on binding with ATP, it polymerizes to the F form, or fibrous actin;
          - F-actin binds calcium tightly;
    - has long, thin fibers on its surface, called tropomyosin fibers;
          - each tropomyosin strand has a globular molecule of protein troponin
          - tropomyosin strands may interfere w/ the cross-bridges between actin and myosin, which inhibiting contraction of the muscle;
          - troponin molecule in some way locks the tropomyosin in place;
    - calcium is required to negate this action;
    - calcium ions combine w/ troponin molecules, neutralizing their effect and "unlocking" the tropomyosin, which moves to a 
         neutral position, allows coupling of actin and myosin, and muscle contraction results;
- A Band :
    - consists of entire thick filament, myosin proteins, which are considered to be stationary;
    - it also contains one end of the thin filament;
    - under appropriate conditions, discussed later, heads of myosin molecules form strong bond with units of actin in thin filament;
- I Band :
    - consists of the remainder of the thin filament;
    - at Z line, it is attached to branches of transverse tubules and is closely associated w/ terminal cisternae of sarcoplasmic reticulum;
         - Sarcoplasmic Reticulum:
    - stores calcium in its interior;
    - depolarization of muscle membrane along the T tubular system causes a release of calcium in the closely adjacent sarcoplasmic  
         reticulum;
         - these calcium ions are then available to uncouple the tropomyosin;
    - following muscle contraction occurs, there is reuptake of calcium into sarcoplasmic reticulum and which then allows reactivation of 
          inhibitory effect of tropomyosin;
    - reuptake by the sarcoplasmic reticulum will not occur if there is insufficient ATP available, which may lead to muscle contraction 
          w/o electrical stimulation, as is seen in rigor mortis