Muscle contraction

Myosin (thick filament)

Actin (thin filament)

Resting Sarcomere Myosin head has stored energy Troponin Tropomyosin ADP P Troponin Tropomyosin ADP Active sites of actin are covered P

Step 1 In Muscle Contraction Ca++ Bind to Troponin, exposing active site of actin ADP P Ca++ Tropomyosin Ca++ ADP P Troponin Active sites of actin are covered

Step 2 In Muscle Contraction Myosin cross bridge binds to the active site on the actin filament ADP P Ca++ ADP Ca++ P

Step 3 In Muscle Contraction The myosin head pivots toward the center of the sarcomere The ADP and the Phosphate group are released ADP P Ca++ Ca++ ADP P

Step 4 In Muscle Contraction Another ATP molecule attaches and the myosin head detaches from the active site of the actin ATP Ca++ Ca++ ATP

Step 5 In Muscle Contraction The detached myosin head is reactivated as it splits the ATP into ADP and a phosphate group ADP P Ca++ Ca++ ADP P

The cycle repeats until the calcium ions are pumped out of the myofibril http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter10/animation__sarcomere_contraction.html