Depolarization is the initial phase of the neuronal membrane potential characterized by the influx of which ions?

Depolarization is the initial phase of the neuronal membrane potential characterized by the influx of which ions?

• A. Sodium and calcium
• B. Potassium and chloride
• C. Sodium and potassium
• D. Calcium and magnesium

Answer: (A)

Depolarization is driven by inward positive current when voltage-gated cation channels open. The rapid rise of the action potential is mainly due to sodium entering the neuron, making the inside more positive. Calcium can also enter through voltage-gated calcium channels and contribute to depolarization in certain neurons or contexts (for example, during sustained excitation or in dendritic signaling). The other ions mentioned—potassium and chloride—typically move in ways that hyperpolarize or stabilize the membrane rather than initiate the depolarizing upstroke, and magnesium isn’t the depolarizing current involved in the initial phase. So the ions most characteristically involved in initiating depolarization are sodium, with calcium potentially contributing in specific situations, which is why the option listing sodium and calcium is the best among the choices.

Depolarization is driven by inward positive current when voltage-gated cation channels open. The rapid rise of the action potential is mainly due to sodium entering the neuron, making the inside more positive. Calcium can also enter through voltage-gated calcium channels and contribute to depolarization in certain neurons or contexts (for example, during sustained excitation or in dendritic signaling). The other ions mentioned—potassium and chloride—typically move in ways that hyperpolarize or stabilize the membrane rather than initiate the depolarizing upstroke, and magnesium isn’t the depolarizing current involved in the initial phase.

So the ions most characteristically involved in initiating depolarization are sodium, with calcium potentially contributing in specific situations, which is why the option listing sodium and calcium is the best among the choices.