Ions Required for Muscle Contraction

Which of the following sets of ions are necessary for muscle contraction?

Correct Answer: Ca²⁺ and Mg²⁺

Both Calcium (Ca²⁺) and Magnesium (Mg²⁺) are essential for muscle contraction.

Role of Ca²⁺ in Muscle Contraction

• Ca²⁺ is released from the sarcoplasmic reticulum upon nerve stimulation.
• Ca²⁺ binds to Troponin C, causing a conformational change.
• Tropomyosin shifts → exposes myosin-binding sites on actin → contraction begins.

Role of Mg²⁺ in Muscle Contraction

• Mg²⁺ is a cofactor for ATPase activity of myosin.
• The actual substrate for myosin ATPase is Mg²⁺-ATP complex (not free ATP).
• Without Mg²⁺, ATP cannot be hydrolyzed → cross-bridge cycling cannot occur.

Rigor Mortis Connection

After death, Ca²⁺ leaks from sarcoplasmic reticulum → permanent cross-bridge formation → stiffness (rigor mortis). ATP depletion after death means cross-bridges cannot detach.