In skeletal muscle fibers, which structure serves as the primary intracellular storage site for calcium ions (Ca$2^{+}$) used during contraction?

• A. Synaptic vesicles at the neuromuscular junction
• B. Sarcoplasmic reticulum (especially the terminal cisternae)
• C. Transverse (T) tubules
• D. Mitochondria in the sarcoplasm

Answer: (B)

Verified step by step guidance

1. Understand the role of calcium ions (Ca^{2+}) in skeletal muscle contraction: Calcium ions are essential for initiating the interaction between actin and myosin filaments, which leads to muscle contraction.
2. Identify the intracellular structures involved in calcium storage and release: In skeletal muscle fibers, calcium ions are stored and released to trigger contraction.
3. Recall that synaptic vesicles at the neuromuscular junction store neurotransmitters, not calcium ions, so they are not the primary calcium storage site.
4. Recognize that transverse (T) tubules conduct action potentials into the muscle fiber but do not store calcium ions.
5. Know that the sarcoplasmic reticulum, especially the terminal cisternae, is the specialized intracellular organelle that stores and releases calcium ions during muscle contraction.