Which ion channel is primarily responsible for the rapid depolarization phase of the action potential in neurons?

• A. Voltage-gated K+ channels.
• B. Voltage-gated Ca2+ channels.
• C. Voltage-gated Na+ channels.
• D. Ligand-gated Na+ channels.

Answer: (C)

The rapid depolarization of a neuron's action potential is driven by a sudden, large inward Na+ current through voltage-gated sodium channels. When the membrane potential reaches threshold, these channels open quickly, allowing Na+ to rush into the cell due to its strong electrochemical gradient. This rapid influx drives the interior toward the sodium equilibrium potential and creates a steep upstroke in membrane potential. The channels then inactivate within a millisecond or so, stopping Na+ entry and letting voltage-gated K+ channels (which open with a delay) repolarize the membrane. Ca2+ channels contribute to other cellular processes or slower depolarizations in certain cells, while ligand-gated Na+ channels mediate synaptic potentials rather than the fast, all-or-none upstroke of the action potential.