What primarily triggers the contraction of cardiac muscle?

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Calcium ions from outside the cellCalcium stored within the sarcoplasmic reticulumSodium ions from outside the cellSodium ions inside the cell

Explanation

Cardiac muscle contraction is mainly initiated by calcium ions entering the cell from the extracellular space, which then stimulate further calcium release inside the cell. This extracellular calcium influx is crucial for the contraction process, unlike skeletal muscle where intracellular calcium plays a larger role.