|Figure 10-10 details the molecular events that occur during the contraction cycle. In the resting sarcomere, each cross-bridge is already energizedcharged with the energy that will be used to power a contraction. The cross-bridge functions as ATPase, an enzyme that can break down ATP. At the start of the contraction cycle, each cross-bridge has already split a molecule of ATP and stored the energy released in the process. The breakdown products, ADP and phosphate ( PO43-, often represented as ), remain bound to the cross-bridge.
The contraction cycle involves five interlocking steps (Figure 10-10):
STEP 1: Exposure of active sites. The calcium ions entering the sarcoplasm bind to troponin. This binding weakens the bond between the troponintropomyosin complex and actin. The troponin molecule then changes position, pulling the tropomyosin molecule away from the active sites and allowing cross-bridges to form.
STEP 2: Attachment of cross-bridges. When the active sites are exposed, the myosin cross-bridges bind to them.
STEP 3: Pivoting. In the resting sarcomere, each cross-bridge points away from the M line. In this position, the myosin head is cocked like the spring in a mousetrap. Cocking the myosin head requires energy, and the energy is obtained by breaking down ATP into ADP and a phosphate group. In the cocked position, both the ADP and the phosphate are still bound to the myosin head. After cross-bridge attachment has occurred, the stored energy is released as the myosin head pivots toward the M line. This action is called the power stroke. When this occurs, the ADP and phosphate group are released.
STEP 4: Detachment of cross-bridges. When an ATP binds to the myosin head, the link between the active site on the actin molecule and the myosin head is broken. The active site is now exposed and able to interact with another cross-bridge.
STEP 5: Reactivation of myosin. Myosin reactivation occurs when the free myosin head splits the ATP into ADP and a phosphate group. The energy released in this process is used to recock the myosin head. The entire cycle can now be repeated. If calcium ion concentrations remain elevated and ATP reserves are sufficient, each myosin head will repeat this cycle about five times per second. Each power stroke shortens the sarcomere by about 1 percent, so each second the sarcomere can shorten by roughly 5 percent. Because all the sarcomeres contract together, the entire muscle shortens at the same rate.
To appreciate the overall effect, imagine that you are pulling on a large rope. You are the myosin head, and the rope is a thin filament. You reach forward, grab the rope with both hands, and pull it toward you. This action corresponds to cross-bridge attachment and pivoting. You now release the rope, reach forward, and grab it once again. By repeating the cycle over and over, you can gradually pull in the rope.
Now consider several people lined up, all pulling on the same rope, as in a tug-of-war team. Each person reaches forward, grabs the rope, pulls it, releases it, and then grabs it again to repeat the cycle. The individual actions are not coordinated: At any one moment, some people are grabbing, some are pulling, and others are letting go. The amount of tension produced is a function of how many people are pulling at any given instant. A comparable situation applies to tension in a muscle fiber, where the myosin heads along a thick filament work together to pull a thin filament toward the center of the sarcomere.