What initiates muscle contraction in cardiac muscle cells?

What initiates muscle contraction in cardiac muscle cells?

• A. Calcium enters the sarcoplasm from the interstitial fluid and sarcoplasmic reticulum.
• B. Sodium influx from interstitial fluid
• C. Potassium efflux from cytoplasm
• D. Magnesium influx from extracellular space

Answer: (A)

Contractile activity in cardiac muscle starts when the cytosolic calcium concentration rises. The action potential opens L-type calcium channels in the T-tubules, letting calcium enter from the extracellular space. This entry triggers a larger release of calcium from the sarcoplasmic reticulum via ryanodine receptors, a process called calcium-induced calcium release. The resulting higher cytosolic calcium binds to troponin C, moves tropomyosin away from actin’s myosin-binding sites, and enables cross-bridge cycling that causes contraction. Sodium influx drives the action potential but does not directly initiate contraction, while potassium efflux helps reset the membrane potential; magnesium has other roles and does not initiate contraction.

Contractile activity in cardiac muscle starts when the cytosolic calcium concentration rises. The action potential opens L-type calcium channels in the T-tubules, letting calcium enter from the extracellular space. This entry triggers a larger release of calcium from the sarcoplasmic reticulum via ryanodine receptors, a process called calcium-induced calcium release. The resulting higher cytosolic calcium binds to troponin C, moves tropomyosin away from actin’s myosin-binding sites, and enables cross-bridge cycling that causes contraction. Sodium influx drives the action potential but does not directly initiate contraction, while potassium efflux helps reset the membrane potential; magnesium has other roles and does not initiate contraction.