What initiates the release of calcium from the sarcoplasmic reticulum in cardiac muscle cells?

• A. Decreased sodium ion concentration
• B. Action potential reaching the T-tubules
• C. Binding of acetylcholine to receptors
• D. Increased levels of ATP in the cytoplasm

Answer: (B)

Verified step by step guidance

1. Understand the role of the sarcoplasmic reticulum in cardiac muscle cells. It stores calcium ions, which are crucial for muscle contraction.
2. Recognize that the release of calcium ions is triggered by an action potential. This electrical signal travels along the cell membrane and into the T-tubules.
3. Learn that T-tubules are invaginations of the cell membrane that help transmit the action potential deep into the muscle cell, ensuring a coordinated contraction.
4. Identify that when the action potential reaches the T-tubules, it causes voltage-sensitive receptors to open calcium channels in the sarcoplasmic reticulum.
5. Note that the influx of calcium ions into the cytoplasm initiates the contraction process by interacting with the contractile proteins within the muscle cell.