The power stroke is the conformational change within muscle proteins that generates mechanical force, driving muscle contraction. This microscopic event directly causes the shortening of muscle tissue. It converts stored chemical energy into kinetic energy, pulling the internal structures of the muscle fiber past one another. Understanding this action requires appreciating the biological machinery assembled within every muscle cell.
The Structural Components of Contraction
Muscle tissue is organized into individual muscle fibers, which are packed with cylindrical myofibrils. These myofibrils contain the contractile machinery, arranged into repeating functional units called sarcomeres. The sarcomere is the fundamental unit of muscle contraction, defined by the organization of two types of protein filaments.
The thick filaments are composed primarily of myosin, a large molecule with globular heads that protrude outward. The thin filaments are built from actin, which forms a helical structure containing the binding sites for the myosin heads. The interaction between these overlapping filaments forms the basis of the sliding filament theory of contraction.
Two additional proteins, tropomyosin and troponin, regulate the thin filaments. Tropomyosin is a rod-shaped protein that wraps around the actin helix, blocking the myosin binding sites when the muscle is at rest. The troponin complex is attached to tropomyosin, acting as the sensor that controls site exposure.
Energy and Activation
The power stroke requires two conditions: the myosin head must be energized, and the actin binding site must be uncovered. Energy is supplied by adenosine triphosphate (ATP), which attaches to the myosin head. The myosin head acts as an ATPase enzyme, hydrolyzing ATP into adenosine diphosphate (ADP) and inorganic phosphate (Pi).
This hydrolysis releases energy captured by the myosin head, causing it to change angle and move into a high-energy, “cocked” position. The myosin head is now prepared, holding the ADP and Pi. The second prerequisite, activation, involves a chemical signal that unblocks the thin filament.
This signal is the release of calcium ions (\(\text{Ca}^{2+}\)) from the sarcoplasmic reticulum. The calcium ions bind to a specific component of the troponin complex. This binding changes the shape of troponin, which physically pulls the attached tropomyosin molecule away from the myosin-binding sites on the actin filament.
The Mechanical Action
With the actin binding sites exposed and the myosin head cocked, the cross-bridge cycle begins. The cocked myosin head, holding ADP and Pi, immediately forms a strong bond with the accessible actin site, creating a cross-bridge. This stable bond formation triggers the next dynamic step.
The force-generating event starts with the rapid release of inorganic phosphate (Pi) from the myosin head. This release directly triggers a sudden conformational change in the myosin structure. The myosin head pivots sharply—the power stroke itself—pulling the attached thin actin filament.
This mechanical pivoting pulls the actin filament toward the center of the sarcomere, moving it approximately 10 nanometers (nm) per stroke. Following this movement, the ADP molecule is released, and the myosin head remains tightly bound to the actin in a low-energy state. Synchronized power strokes across all sarcomeres pull the Z-discs closer, shortening the muscle fiber and generating force.
Detachment and Repetition
For contraction to continue or for the muscle to relax, the myosin head must detach from the actin filament. Detachment occurs when a fresh ATP molecule binds to the myosin head. This binding causes a change that weakens the connection, breaking the cross-bridge and allowing the myosin head to release the actin.
If the nervous signal continues, calcium concentration remains high, and the binding sites stay exposed. The newly bound ATP is hydrolyzed into ADP and Pi, which re-cocks the myosin head. The cycle of binding, power stroke, and detachment repeats continuously, maintaining the contraction through a sustained, ratcheting movement.
If the stimulating nerve signal ceases, calcium ions are quickly pumped back into the sarcoplasmic reticulum. Calcium removal causes the troponin-tropomyosin complex to shift back into its blocking position, covering the binding sites. With the sites covered, myosin heads can no longer attach, and the filaments passively slide back to their resting position, resulting in muscle relaxation.