Question

Calcium is important in skeletal muscle contraction because it

• A. prevents the formation of bonds between the myosin cross bridges and the actin filament
• B. binds to troponin to remove the masking of active sites on actin for myosin.
• C. detaches the myosin head from the actin filament.
• D. activates the myosin ATPase by binding to it

Answer 1

The Correct Option is B

The question focuses on the role of calcium in the process of skeletal muscle contraction, which is a fundamental concept in physiology. Let's explore the options and identify why the correct answer is suitable:

1. Option 1: Prevents the formation of bonds between the myosin cross bridges and the actin filament.
   This is incorrect. Calcium does not prevent bond formation; rather, it facilitates the interaction between myosin and actin by exposing the binding sites on actin.
2. Option 2: Binds to troponin to remove the masking of active sites on actin for myosin.
   This is the correct answer. Let's explore why:
   • In skeletal muscle contraction, calcium ions (Ca²⁺) play a crucial role by binding to troponin.
   • Troponin is a part of the protein complex on the actin filaments and, upon calcium binding, causes a conformational change.
   • This change moves the tropomyosin away from actin's myosin-binding sites, thus exposing these active sites for the myosin heads to attach, allowing for muscle contraction to occur.
3. Option 3: Detaches the myosin head from the actin filament.
   This is incorrect. Detachment of myosin heads involves the hydrolysis of ATP, not the action of calcium.
4. Option 4: Activates the myosin ATPase by binding to it.
   This is incorrect. Myosin ATPase is involved in the energy release for contraction, but its activation does not directly involve calcium binding.

Conclusion: Hence, the correct answer is Option 2: Calcium binds to troponin to remove the masking of active sites on actin for myosin. This understanding is essential for explaining the coupling of excitation and contraction in muscle fibers.