Question

An action potential in a nerve fiber is produced, when positive and negative charges on the outside and inside of the axon membrane are reversed, because

• A. More $K^{+}$ enter the axon as compared to sodium ions leaving it.
• B. More $N{a}^{+}$ enter the axon as compared to $K^{+}$ leaving it.
• C. All $K^{+}$ leaving the axon.
• D. All $N{a}^{+}$ enter the axon.

Answer 1

The correct option is B More $N{a}^{+}$ enter the axon as compared to $K^{+}$ leaving it.

Action potentials are generated by special types of voltage-gated ion channels embedded in a cell's plasma membrane. When the channels open, they allow an inward flow of sodium ions, which changes the electrochemical gradient, which in turn produces a further rise in the membrane potential. This then causes more channels to open, producing a greater electric current across the cell membrane, and so on. The rapid influx of sodium ions causes the polarity of the plasma membrane to reverse, and the ion channels then rapidly inactivate.

As the sodium channels close, sodium ions can no longer enter the neuron, and then they are actively transported back out of the plasma membrane.

Potassium channels are then activated, and there is an outward current of potassium ions, returning the electrochemical gradient to the resting state.

In order to maintain the cell membrane potential, cells keep a low concentration of sodium ions and high levels of potassium ions within the cell (intracellular).

The sodium-potassium pump moves 3 sodium ions out and moves 2 potassium ions in, thus, in total, removing one positive charge carrier from the intracellular space.