1. The Contraction Cycle Process Diagrams Step-by-Step Copyright © 2007 by John Wiley & Sons, Inc.

2. 1 Myosin heads hydrolyze ATP and become reoriented and energized ADP P = Ca 2+ Key: Contraction cycle continues if ATP is available and Ca 2+ level in the sarcoplasm is high

3. 1 Myosin heads hydrolyze ATP and become reoriented and energized Myosin heads bind to actin, forming crossbridges ADP P P = Ca 2+ Key: 2 Contraction cycle continues if ATP is available and Ca 2+ level in the sarcoplasm is high

4. 1 Myosin heads hydrolyze ATP and become reoriented and energized Myosin heads bind to actin, forming crossbridges Myosin crossbridges rotate toward center of the sarcomere (power stroke) Contraction cycle continues if ATP is available and Ca 2+ level in the sarcoplasm is high ADP P P = Ca 2+ Key: 2 3

5. 1 Myosin heads hydrolyze ATP and become reoriented and energized Myosin heads bind to actin, forming crossbridges Myosin crossbridges rotate toward center of the sarcomere (power stroke) As myosin heads bind ATP, the crossbridges detach from actin Contraction cycle continues if ATP is available and Ca 2+ level in the sarcoplasm is high ADP ATP P P = Ca 2+ Key: ATP 2 3 4