Which of these is a factual statement and not a major difference between smooth muscle and skeletal muscle? a. Ca2+ binds to calmodulin in smooth muscle and skeletal muscle b. Smooth and skeletal muscle can only be activated by depolarization of the membrane c. Phosphorylation of myosin leads to binding to actin d. Cytosolic Ca2+ controls contractile activate in both skeletal and smooth muscle
The correct answer and explanation is:
Correct Answer: d. Cytosolic Ca²⁺ controls contractile activity in both skeletal and smooth muscle
Explanation (300 words):
Muscle contraction in both smooth and skeletal muscle is triggered by an increase in cytosolic calcium ions (Ca²⁺). This is a factual similarity, not a major difference, which is why option D is correct.
When a muscle is stimulated to contract, cytosolic Ca²⁺ concentration increases, leading to activation of the contractile machinery. However, the mechanism of activation differs between smooth and skeletal muscle:
- In skeletal muscle, Ca²⁺ binds to troponin C, which causes a conformational change in the troponin-tropomyosin complex, exposing binding sites on actin filaments and allowing myosin heads to attach and initiate contraction.
- In smooth muscle, troponin is absent. Instead, Ca²⁺ binds to calmodulin, forming a Ca²⁺-calmodulin complex that activates myosin light chain kinase (MLCK). MLCK then phosphorylates the myosin light chains, enabling myosin to bind to actin and initiate contraction.
Now, let’s break down why the other options are incorrect:
- a. “Ca²⁺ binds to calmodulin in smooth muscle and skeletal muscle”: Incorrect. This is true only for smooth muscle. In skeletal muscle, Ca²⁺ binds to troponin, not calmodulin.
- b. “Smooth and skeletal muscle can only be activated by depolarization of the membrane”: Incorrect. While skeletal muscle contraction depends heavily on depolarization, smooth muscle can be activated by various stimuli including hormones, stretch, and local chemical signals, not just membrane depolarization.
- c. “Phosphorylation of myosin leads to binding to actin”: This is true for smooth muscle, but not for skeletal muscle, where phosphorylation is not required for myosin to bind actin.
Therefore, statement D is the only factual similarity, not a distinguishing difference, between the two muscle types.