1. IN CLASS NOTES = STEPS OF A MUSCLE CONTRACTION

2. STEP 1 Calcium ions present Ca+ binds to troponin which makes tropomyosin move out of way for myosin head to attach

3. STEP 2 Cross-bridge attaches. ATP breakdown provides energy to ready the myosin head for a power stroke Myosin head attaches to exposed binding site on actin and the power stroke is accomplished

4. STEP 3 Cross-bridge (myosin head) springs from raised position and pulls on the actin filament.

5. STEP 4 Cross bridges break  ATP binds to cross-bridge (but is not yet broken down)  Myosin heads are released from actin

6. STEP 5 As long as calcium ions and ATP are present, this walking continues until the muscle fiber is fully contracted