The power stroke is a fundamental process in muscle contraction, generating force and enabling movement. This microscopic event occurs within individual muscle fibers, allowing for all macroscopic movements. It is a crucial step where myosin heads pull on actin filaments.
Building Blocks of Muscle Contraction
Muscle fibers contain structures called myofibrils, composed of repeating units known as sarcomeres. The sarcomere is the basic functional unit of a muscle fiber, defined by Z-discs at its ends. It contains two primary types of protein filaments: thin and thick.
Thin filaments are primarily actin, arranged in a double helical structure. Thick filaments are composed of myosin, a motor protein with a tail and globular heads. Myosin heads extend outward, interacting with the thin filaments.
The arrangement of actin and myosin within the sarcomere allows for their interaction, the basis of muscle shortening. When a muscle contracts, these filaments slide past one another. This “sliding filament theory” explains how force is generated as actin filaments are pulled towards the center of the myosin filaments, causing the sarcomere to shorten.
The Power Stroke Mechanism
The power stroke is the key force-generating step in muscle contraction, where the myosin head pulls the actin filament. This process begins when the myosin head, already in a high-energy, “cocked” position, forms a cross-bridge by binding to an exposed site on the actin filament. The binding of myosin to actin results in the release of inorganic phosphate (Pi) from the myosin head.
Following the release of inorganic phosphate, the myosin head undergoes a significant conformational change, often described as a pivot or “swinging” motion of its lever arm. This change in shape pulls the attached actin filament towards the center of the sarcomere, a movement of approximately 10 nanometers. This pulling action is the power stroke itself, directly generating the force that shortens the sarcomere and leads to muscle contraction.
Subsequently, adenosine diphosphate (ADP) is released from the myosin head, leading to a strong bond between myosin and actin. The myosin head remains tightly bound to the actin filament in a low-energy state after the power stroke. This entire sequence of binding, pivoting, and releasing products represents the mechanical work performed by the myosin motor protein.
Energy and Regulation of the Power Stroke
Muscle contraction, including the power stroke, requires energy, which is primarily supplied by adenosine triphosphate (ATP). ATP is known as the “molecular currency” for energy transfer within cells. ATP’s role in the power stroke cycle is not directly in the pulling motion itself, but rather in preparing the myosin head for subsequent cycles and enabling muscle relaxation.
Specifically, ATP binding to the myosin head causes it to detach from the actin filament, breaking the strong cross-bridge formed during the power stroke. After detachment, the ATP is hydrolyzed into ADP and inorganic phosphate (Pi) by an enzyme called ATPase located on the myosin head. The energy released from this hydrolysis “re-cocks” the myosin head, changing its angle and returning it to a high-energy position, ready to bind to actin again for the next power stroke. Without ATP, the myosin heads would remain bound to actin, leading to a contracted state.
Calcium ions (Ca2+) act as the primary “on” switch for muscle contraction. In a resting muscle, regulatory proteins, tropomyosin and troponin, are associated with the actin filaments, blocking the myosin binding sites. Tropomyosin physically covers these sites, preventing myosin from forming cross-bridges.
When a muscle receives a signal to contract, calcium ions are released into the muscle cell’s cytoplasm. These calcium ions then bind to troponin, a protein complex on the actin filament. This binding causes a conformational change in troponin, which in turn moves tropomyosin away from the myosin binding sites on actin. With the binding sites now exposed, the myosin heads can attach to actin, initiating the power stroke cycle and leading to muscle contraction.