Which ion is primarily responsible for the depolarization phase described?

Which ion is primarily responsible for the depolarization phase described?

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

Answer: (B)

Depolarization is driven by a rapid influx of positively charged ions through voltage-gated sodium channels. When a stimulus reaches threshold, these sodium channels open quickly, allowing Na+ to rush into the cell. This influx makes the inside of the cell less negative, moving the membrane potential from around -70 mV toward about +30 mV and creating the rapid upstroke of the action potential. Potassium does the opposite later, exiting the cell to repolarize. Calcium can contribute to depolarization in certain tissues (notably cardiac muscle during the plateau phase), but for the fast depolarization most commonly discussed in nerves and skeletal muscle, sodium is the primary driver. Chloride isn’t responsible for the depolarization phase.

Depolarization is driven by a rapid influx of positively charged ions through voltage-gated sodium channels. When a stimulus reaches threshold, these sodium channels open quickly, allowing Na+ to rush into the cell. This influx makes the inside of the cell less negative, moving the membrane potential from around -70 mV toward about +30 mV and creating the rapid upstroke of the action potential. Potassium does the opposite later, exiting the cell to repolarize. Calcium can contribute to depolarization in certain tissues (notably cardiac muscle during the plateau phase), but for the fast depolarization most commonly discussed in nerves and skeletal muscle, sodium is the primary driver. Chloride isn’t responsible for the depolarization phase.