During muscle contraction, the power stroke is the pivoting motion of the myosin head that pulls the actin filament inward to shorten the sarcomere. Given this, which of the following events immediately precedes the power stroke?

Question

During muscle contraction, the power stroke is the pivoting motion of the myosin head that pulls the actin filament inward to shorten the sarcomere.  Given this, which of the following events immediately precedes the power stroke?

Quizplus · Accepted Answer

11ecba2d_0fdc_67ef_a3bf_910e14c07895_MD0008_00 Muscle contraction is dependent on the interaction between the myosin (thick) and actin (thin) protein filaments in the sarcomere, the basic contractile unit of muscle tissue. To generate muscle contraction, the sarcomere functions as follows: When the muscle fiber is at rest, the myosin head is in its high-energy conformation (upright and bound to ADP and P_i), and the actin filaments are bound by the regulatory proteins tropomyosin and troponin. Tropomyosin is an elongated protein that wraps around the actin filament to block myosin-binding sites on this thin filament. Troponin is a small protein complex associated with tropomyosin. Following a depolarizing stimulus by a motor neuron, Ca²⁺ ions are released from the sarcoplasmic reticulum into the cytosol. The abundant cytosolic Ca²⁺ ions bind troponin, causing a conformational change that ultimately pulls on tropomyosin and exposes the myosin binding sites on the actin filaments (Choice D). When the active sites are exposed, the myosin head is able to bind strongly to the actin filament, forming a cross-bridge and promoting the release of ADP and P_i. The dissociation of ADP and P_i causes the power stroke, which is the actual pivot of the actin-bound myosin head that drags the actin filament toward the center of the sarcomere. This directly results in shortening of the sarcomere (Choice C). The myosin head is now in its low-energy conformation. A new ATP molecule binds the myosin head and the cross-bridge disassembles (Choice A). Hydrolysis of the ATP molecule allows the myosin head to shift back into its upright, high-energy conformation in preparation for a new cycle of contraction (Choice B). The cycle of cross-bridge formation and disassembly continues until motor neuron signaling ceases, and Ca²⁺ is sequestered back into the sarcoplasmic reticulum. Educational objective: When ATP bound to a myosin head hydrolyzes, the myosin head shifts back and attaches to the actin filament. Dissociation of ADP and P_i leads to a power stroke, which shortens the sarcomere. A new ATP molecule binds to the myosin head, causing it to dissociate from the actin filament. The action of many myosin heads cycling through this process leads to muscle contraction.

During Muscle Contraction, the Power Stroke Is the Pivoting Motion