Which statement best describes calcium-induced calcium release (CICR) and its role in muscle contraction?

• A. In skeletal muscle, CICR is the primary mechanism for Ca2+ release.
• B. CICR occurs only in smooth muscle.
• C. CICR has no role in muscle contraction.
• D. In cardiac muscle, Ca2+ entry triggers release of more Ca2+ from the SR via ryanodine receptors, amplifying contraction; in skeletal muscle, CICR is less prominent.

Answer: (D)

Calcium-induced calcium release means that Ca2+ entering the cell triggers the release of even more Ca2+ from the sarcoplasmic reticulum, via ryanodine receptors, boosting the contraction signal. In cardiac muscle, this happens when Ca2+ flows in through L-type Ca2+ channels during depolarization; that Ca2+ then binds to RyR2 on the SR and opens it, releasing a large amount of Ca2+ to amplify the contraction. In skeletal muscle, however, the release of Ca2+ from the SR is primarily driven by direct, voltage-activated coupling between the dihydropyridine receptors and the ryanodine receptors, so extracellular Ca2+ influx is not the main trigger and CICR is less prominent. The released Ca2+ binds troponin C to enable cross-bridge cycling and muscle shortening.