What is the ion released from the terminal cisternae that combines with troponin and removes the blocking action of tropomyosin, resulting in the formation of cross bridges?

• A. Cl^-
• B. Na⁺
• C. K⁺
• D. Ca²⁺
• E. H⁺

Answer: (D)

Verified step by step guidance

1. Understand the role of the terminal cisternae in muscle contraction. The terminal cisternae are part of the sarcoplasmic reticulum, which stores calcium ions (Ca²⁺).
2. Recognize the importance of calcium ions (Ca²⁺) in muscle contraction. When a muscle fiber is stimulated, Ca²⁺ is released from the terminal cisternae into the cytosol.
3. Identify the interaction between Ca²⁺ and troponin. Ca²⁺ binds to troponin, a regulatory protein associated with the actin filaments in muscle cells.
4. Explain how the binding of Ca²⁺ to troponin affects tropomyosin. This binding causes a conformational change in troponin, which moves tropomyosin away from the myosin-binding sites on actin filaments.
5. Understand the formation of cross bridges. With tropomyosin moved aside, myosin heads can attach to actin, forming cross bridges, which is essential for muscle contraction.