During cardiac action potential propagation, sodium channel activation is typically of what duration?

• A. Short
• B. Long
• C. Moderate
• D. None

Answer: (A)

The fast sodium channels open to produce the rapid upstroke of the cardiac action potential, but they do not stay open for long. When the membrane depolarizes, these channels activate quickly and let Na+ rush in, driving a sharp rise in voltage (phase 0). They then inactivate within roughly a millisecond, so the sodium current is brief and the upstroke is rapid. This short activation time supports fast conduction through the myocardium. The plateau phase that follows is due to calcium entry and potassium currents, not sustained sodium channel activity. If sodium channels remained open longer, the upstroke would be broadened and conduction would slow, which isn’t characteristic of normal cardiac propagation.