For muscle contraction, which of the following is true? A. Charged particles must be at rest. B. There must be thin filaments extending into the Z line. C. Repolarization must be happening. D. Fibers must be activated.
The Correct Answer and Explanation is:
The correct answer is D. Fibers must be activated.
Muscle contraction is a complex process that involves several key steps, one of which is the activation of muscle fibers. The activation of muscle fibers is a crucial prerequisite for the initiation of contraction. When a motor neuron stimulates a muscle fiber at the neuromuscular junction, it leads to the release of acetylcholine (ACh). This neurotransmitter binds to receptors on the muscle cell membrane (sarcolemma), triggering an action potential that travels down the T-tubules into the muscle fiber. This action potential causes the release of calcium ions (Ca²⁺) from the sarcoplasmic reticulum into the cytoplasm, which is necessary for muscle contraction.
Once calcium ions are released, they bind to troponin, a regulatory protein on the actin filaments. This binding causes a conformational change in tropomyosin, another protein that blocks the binding sites for myosin on actin. The movement of tropomyosin exposes these sites, allowing myosin heads to attach to actin, initiating the cross-bridge cycle. This cycle leads to muscle contraction.
Let’s break down the other options:
- A. Charged particles must be at rest: This statement is false because for muscle contraction to occur, charged ions (such as calcium and sodium) must move actively across the muscle cell membrane to generate action potentials and facilitate contraction. Resting conditions do not allow contraction.
- B. There must be thin filaments extending into the Z line: Thin filaments (actin) do extend to the Z line in a muscle fiber, but this is a structural feature of the sarcomere, not a condition for activation or contraction. Activation and contraction are about the movement of these filaments relative to each other.
- C. Repolarization must be happening: Repolarization is the process where the muscle fiber returns to its resting potential after depolarization. While repolarization is important for resetting the muscle cell, it is not a direct requirement for the initial contraction itself. It is part of the cycle but not an immediate requirement for contraction.
Therefore, fibers must be activated for muscle contraction to occur.
