Final answer:
Ryanodine receptors in cardiac muscle cells release calcium ions from the sarcoplasmic reticulum, which is necessary for muscle contractions. Calcium combines with troponin, enabling myosin to bind to actin, and is then actively transported back into the sarcoplasmic reticulum.
Step-by-step explanation:
The ryanodine receptors in cardiac muscle play a critical role in the physiology of the heart. Specifically, these receptors are responsible for option (A) releasing calcium ions from the sarcoplasmic reticulum into the cytosol. Upon the electrical impulse arrival at the muscle cell, ryanodine receptors open and allow the calcium ions that have been stored in the sarcoplasmic reticulum to flood into the cytoplasm. This surge of calcium ions then combines with troponin, ultimately resulting in the activation of the contractile mechanism of the cardiac muscle fibers.
Calcium ions are crucial because they trigger muscle contractions by combining with troponin which then allows the myosin heads to form cross bridges with actin, providing the power stroke necessary for contraction. After the contraction, ATP-dependent calcium pumps actively transport calcium ions back into the sarcoplasmic reticulum, readying the muscle cells for the next contraction.