What is the primary function of the sarcoplasmic reticulum in skeletal muscle fibers during muscle contraction?

• A. It stores and releases calcium ions needed to trigger muscle contraction.
• B. It transmits electrical impulses along the muscle fiber.
• C. It synthesizes proteins required for muscle growth.
• D. It generates ATP for muscle contraction.

Answer: (A)

Verified step by step guidance

1. Understand the role of the sarcoplasmic reticulum (SR) in skeletal muscle fibers. The SR is a specialized organelle that plays a critical role in muscle contraction by regulating calcium ion (Ca²⁺) levels.
2. Recall that muscle contraction is initiated by the release of calcium ions. When a muscle fiber is stimulated by an action potential, the SR releases stored calcium ions into the cytoplasm.
3. Recognize that calcium ions bind to troponin, a regulatory protein on the actin filaments. This binding causes a conformational change that exposes binding sites on actin for myosin, enabling the cross-bridge cycle and muscle contraction.
4. Note that the SR also reabsorbs calcium ions after contraction, allowing the muscle fiber to relax and reset for the next contraction. This process is facilitated by calcium pumps in the SR membrane.
5. Eliminate incorrect options: The SR does not transmit electrical impulses (this is the role of the sarcolemma and T-tubules), synthesize proteins (this is the role of ribosomes), or generate ATP (this is the role of mitochondria). The correct function of the SR is storing and releasing calcium ions needed for muscle contraction.