Translations:AY Honors/Bones, Muscles, and Movement/Answer Key/73/en

1. When the muscle is in a resting state, thin strands of a protein called tropomyosin are wrapped around the actin filaments, blocking the myosin binding sites. This keeps the myosin from binding to actin.
2. Molecules called troponin are attached to the tropomyosin.
3. When calcium is introduced into the muscle cell, calcium ions bind to troponin molecules.
4. Calcium then pulls troponin, causing tropomyosin to be moved as well, therefore causing the myosin binding sites on the actin to be exposed.
5. Myosin binds to the now-exposed binding sites.
6. As soon as the myosin head binds to actin, the head bends at its hinge.
7. Once the head bends, the myosin loses energy, and remains attached to the actin.
8. When re-energized by adenosine triphosphate (ATP), the myosin head detaches from the actin filament, and is ready to attach and bend again.
9. The collective bending of numerous myosin heads (all in the same direction), combine to move the actin molecules closer together. This results in a muscle contraction.