Final answer:
The relative Na+ and K+ concentrations stay constant during an action potential due to the action of the Na+/K+-ATPase pump, which restores ions after their temporary flow during the depolarization and repolarization phases of the action potential. The correct option is b. they are pumped back across the membrane using the Na+/K+-‐‑ATPase pump.
Step-by-step explanation:
The relative concentrations of Na+ and K+ remain constant during a single action potential because they are pumped back across the membrane using the Na+/K+-ATPase pump. After depolarization, the cell undergoes repolarization resulting in the closure of sodium channels and the continued outflow of potassium at a high conductance. This phase restores the negative charge inside the cell relative to the outside.
The plasma membrane sodium-potassium ATPase always operates to transport Na+ out of the cell and K+ back into the cell, thus restoring and maintaining the concentration gradients required for the resting potential and future action potentials.
During an action potential, a slight shift in ion concentrations occurs, but it is not significant enough to deplete the ions required to generate future action potentials. The refractory period ensures that the voltage-gated ion channels return to their resting configurations before another action potential can be initiated.
Both the sodium and potassium ion channels play integral roles in depolarization and repolarization, ensuring the transmission of the action potential along the neuron. This synergy between ion flow and ion transport mechanisms enables nerve and muscle cells to maintain excitability and conduct multiple successive action potentials.