Which of the following statements regarding contraction in skeletal and smooth muscle is TRUE? A. In skeletal muscle, calcium initiates contraction by binding to troponin, while in smooth muscle calcium initiates contraction by binding directly to myosin. B. In skeletal muscle, calcium initiates contraction by binding to troponin, while in smooth muscle calcium initiates contraction by binding to calmodulin. C. In skeletal muscle, calcium initiates contraction by binding to myosin light-chain kinase, while in smooth muscle calcium initiates contraction by binding directly to tropomyosin. D. All of the above are TRUE.

B. In skeletal muscle, calcium initiates contraction by binding to troponin, while in smooth muscle calcium initiates contraction by binding to calmodulin.

Explanation:

The Calcium ions (Ca2 +) have a key role in skeletal muscle contraction. Moreover, the Troponins are regulatory proteins that also play roles in skeletal contraction. In this muscle, troponin is attached to tropomyosin, when Ca2 + binds to troponin it induces a conformational change, thereby exposing this protein to its binding to myosin filaments on the actin filaments, which initiates muscle contraction.  

On the other hand, calmodulin (calcium-modulated protein) is a protein which mediates the contractile state of the smooth muscle by binding Ca2+ ions. The function of this protein is to stimulate the development of force (contraction) in response to Ca2+ ions by activating the phosphorylation of myosin.