What triggers the release of calcium ions from the sarcoplasmic reticulum during muscle contraction?

• A. The binding of acetylcholine to receptors on the muscle cell membrane
• B. The depolarization of the muscle cell membrane
• C. The hydrolysis of ATP by myosin heads
• D. The binding of calcium ions to troponin

Answer: (B)

Verified step by step guidance

1. Understand the role of the sarcoplasmic reticulum (SR) in muscle contraction: The SR stores calcium ions (Ca²⁺) and releases them when a muscle cell is stimulated, which is crucial for muscle contraction.
2. Identify the initial trigger for muscle contraction: The process begins with a nerve impulse that reaches the neuromuscular junction, causing the release of the neurotransmitter acetylcholine.
3. Explain the role of acetylcholine: Acetylcholine binds to receptors on the muscle cell membrane, leading to the opening of ion channels and an influx of sodium ions (Na⁺), which depolarizes the muscle cell membrane.
4. Connect depolarization to calcium release: The depolarization of the muscle cell membrane triggers the opening of voltage-gated calcium channels in the sarcoplasmic reticulum, leading to the release of Ca²⁺ into the cytosol.
5. Clarify the role of calcium ions: The released calcium ions bind to troponin, causing a conformational change that allows myosin heads to bind to actin filaments, ultimately leading to muscle contraction.