Which ion is primarily responsible for the depolarizing current at the start of an action potential?

Which ion is primarily responsible for the depolarizing current at the start of an action potential?

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

Answer: (D)

The start of an action potential is driven by a rapid inward current of sodium ions through voltage-gated sodium channels. When the membrane reaches the threshold, these channels open quickly, and Na+ rushes into the cell due to its strong electrochemical gradient. This influx brings positive charge into the neuron, pushing the membrane potential toward the sodium equilibrium potential and creating the characteristic sharp, upward spike. After this initial depolarization, potassium channels open to let K+ exit, which helps repolarize the membrane. Calcium can contribute to depolarization in certain cell types or during longer or plateau phases, but it is not the primary driver of the initial depolarizing phase in most neurons. Chloride typically acts to stabilize or inhibit rather than initiate depolarization.

The start of an action potential is driven by a rapid inward current of sodium ions through voltage-gated sodium channels. When the membrane reaches the threshold, these channels open quickly, and Na+ rushes into the cell due to its strong electrochemical gradient. This influx brings positive charge into the neuron, pushing the membrane potential toward the sodium equilibrium potential and creating the characteristic sharp, upward spike. After this initial depolarization, potassium channels open to let K+ exit, which helps repolarize the membrane. Calcium can contribute to depolarization in certain cell types or during longer or plateau phases, but it is not the primary driver of the initial depolarizing phase in most neurons. Chloride typically acts to stabilize or inhibit rather than initiate depolarization.