1. Whole muscle or heart

2. Group of muscle fibers

3. Muscle fiber (cell)

4. Myofibril

5. Sarcomere

6. Myofilaments (thin filament, thick filament)

1. Composed of myosin

2. Projections from myosin are crossbridges (a molecular motor that converts chemical energy into mechanical work)

3. Titin

1. A filamentous protein that spans the M-line and Z-line

2. Attached to myosin filament within the A-band

3.  Highly elastic protein that resists stretch of the sarcomere

1. Composed of actin, troponin and tropomyosin

2. Cyclic attachment and detachment of myosin crossbridges to actin produces muscle active force and/or shortening

3. The troponin and tropomyosin comprise the calcium sensitive regulatory proteins

4. In the absence of calcium, troponin/tropomyosin prevent the interaction of actin and myosin

5. Nebulin is a filamentous protein attached to Z-line and the actin filament along its entire length, and may serve as molecular ruler to regulate actin filament length

As muscle shortens, the following is observed:

1. Sarcomeres shorten

2. A-band length remains constant

3. I-band length becomes shorter

4. Myofilament lengths remain constant (slide past each other)

1. Hydrolysis occurs while myosin is detached (myosin is an ATPase)

2. Hydrolysis of ATP primes myosin for attachment

3. Once attached to actin, the products of ATP hydrolysis are released and the myosin undergoes a conformational change believed to the force generating step, or powerstroke

4. Myosin detaches from actin upon binding of ATP to complete one cycle of the actomyosin ATPase