Which statement about calcium sources for contraction in skeletal and smooth muscle is most accurate?

• A. Skeletal muscle stores Ca2+ in the SR and releases it; Smooth uses extracellular Ca2+ exclusively.
• B. Skeletal muscle stores Ca2+ in the SR and releases it; Smooth uses both SR and extracellular Ca2+ entry.
• C. Skeletal uses extracellular Ca2+ only; Smooth uses SR only.
• D. Both rely on extracellular Ca2+ exclusively.

Answer: (B)

Calcium sources for contraction differ between skeletal and smooth muscle. Skeletal muscle relies mainly on calcium stored in the sarcoplasmic reticulum, which is released into the cytosol when the action potential travels along the T-tubules and couples to the ryanodine receptor on the SR. This rise in Ca2+ binds to troponin C, moves tropomyosin away from actin’s myosin-binding sites, and enables cross-bridge cycling. Extracellular Ca2+ contributes little to initiating skeletal contraction, though it can play a minor modulatory role in some conditions.

Smooth muscle, on the other hand, uses calcium from both the SR and the extracellular space. Calcium enters through various channels in the plasma membrane and also can be released from the SR via IP3 receptors. The cytosolic Ca2+ binds to calmodulin, activating myosin light-chain kinase and enabling contraction.

So the most accurate statement is that skeletal muscle uses Ca2+ primarily from the SR, while smooth muscle relies on both SR Ca2+ release and extracellular Ca2+ entry. The other options misstate the reliance on either SR or extracellular sources for one or both muscle types.