Recall that each myosin head has a region that binds to actin and a region that binds to ATP. Myosin cannot release from actin until ATP also binds, and the hydrolysis of ATP into adenosine diphosphate (ADP) and inorganic phosphate (P_i) then releases energy needed for the myosin head to reposition or re-cock.

Cross-bridge formation occurs when the myosin head attaches to the actin while adenosine diphosphate (ADP) and inorganic phosphate are still bound to myosin. P_i is then released which causes the myosin head to move toward the M-line, pulling the actin along with it. As actin is pulled, the filaments move approximately 10 nm toward the M-line. This movement is called the power stroke. ADP is then released which causes myosin to form a stronger attachment to the actin, which is referred to as the rigor state. In the absence of ATP, the myosin head will not detach from actin.

ATP binding causes the myosin head to detach from the actin. After this occurs, ATP is converted to ADP and P_i by the intrinsic ATPase activity of myosin. The energy released during ATP hydrolysis changes the angle of the myosin head into a cocked position. The myosin head is now in position for further movement.

Note that each thick filament of roughly 300 myosin molecules has multiple myosin heads. These myosin heads cycle asynchronously to maintain constant tension in the activated myofiber. During a muscle contraction, many cross-bridges form and break continuously. Multiply this by all of the sarcomeres in one myofibril, all the myofibrils in one muscle fiber, and all of the muscle fibers in one skeletal muscle, and you can understand why so much energy (ATP) is needed to keep skeletal muscles working. In fact, it is the loss of ATP that results in the rigor mortis observed soon after someone dies. With no further ATP production possible, there is no ATP available for myosin heads to detach from the actin-binding sites, so the cross-bridges stay in place, causing the rigidity in the skeletal muscles.

Contraction and Relaxation

The sequence of events that result in the contraction of an individual muscle fiber begins with a signal—the neurotransmitter, ACh—from the motor neuron innervating that fiber. The local membrane of the fiber will depolarize as positively charged sodium ions (Na⁺) enter, triggering an action potential that spreads to the rest of the membrane will depolarize, including the T-tubules. This triggers the release of calcium ions (Ca²⁺) from storage in the sarcoplasmic reticulum (SR). The Ca²⁺ then initiates contraction, which is sustained by ATP (Figure 26.2). As long as Ca²⁺ ions remain in the sarcoplasm to bind to troponin, which keeps the actin-binding sites “unshielded,” and as long as ATP is available to drive the cross-bridge cycling and the pulling of actin strands by myosin, the muscle fiber will continue to shorten to an anatomical limit.

Muscle contraction usually stops when signaling from the motor neuron ends, which repolarizes the sarcolemma and T-tubules, and closes the calcium channels in the SR. Ca²⁺ ions are then pumped back into the SR, which causes the tropomyosin to re-cover the binding sites on actin (Figure 26.2).