Which ion is listed as creating a positive charge in the myocardium?

• A. Potassium
• B. Calcium
• C. Chloride
• D. Sodium

Answer: (D)

Sodium is the ion that creates the positive charge in the myocardium during depolarization. When a stimulus triggers the cardiac cell, fast voltage-gated sodium channels open and sodium ions rush into the cell, making the inside of the cell rapidly more positive. This rapid Na+ influx is what upstrokes the action potential and starts the electrical impulse through the heart. Calcium also contributes later, helping sustain positivity during the plateau phase and enabling contraction, but the initial, rapid depolarization—and the associated positive charge inside the cell—is driven primarily by sodium influx. Potassium moves out to repolarize the cell, and chloride carries a negative charge, so they don’t generate the initial positive charge in this context.