Inside skeletal muscle cells, calcium ions (Ca2+) are primarily stored in which structure before being released to initiate contraction?

• A. Cytosol (sarcoplasm) surrounding the myofibrils
• B. Transverse (T) tubules
• C. Sarcoplasmic reticulum (especially the terminal cisternae)
• D. Mitochondria

Answer: (C)

Verified step by step guidance

1. Understand the role of calcium ions (Ca2+) in muscle contraction: Calcium ions are essential for initiating the contraction process in skeletal muscle cells by binding to regulatory proteins on the myofibrils.
2. Identify the main intracellular compartments in skeletal muscle cells: These include the cytosol (sarcoplasm), transverse (T) tubules, mitochondria, and the sarcoplasmic reticulum.
3. Recall the function of the sarcoplasmic reticulum: It is a specialized form of the endoplasmic reticulum in muscle cells that stores calcium ions and releases them when the muscle is stimulated to contract.
4. Recognize that the terminal cisternae are enlarged areas of the sarcoplasmic reticulum adjacent to the T tubules, serving as the primary storage sites for calcium ions before release.
5. Conclude that calcium ions are primarily stored in the sarcoplasmic reticulum (especially the terminal cisternae) before being released into the sarcoplasm to trigger muscle contraction.