Final answer:
The net flux of an ion across the membrane can be zero when the opposing concentration gradient and voltage gradient are of equal magnitude but in opposite directions. This balance is known as electrochemical equilibrium and is essential for the function of neurons during the establishment of action potentials.
Step-by-step explanation:
The concentration gradient and voltage gradient can indeed oppose each other, and if they are of equal magnitude but opposite direction, the net flux of the ion across the membrane can indeed be zero. This situation is known as the electrochemical equilibrium. In neurons, the sodium-potassium pump, for example, creates a concentration gradient with higher levels of Na+ outside the cell and hence, a net movement of Na+ into the cell is favored. Meanwhile, the negatively charged interior of the cell (because of other ions and proteins) creates a voltage gradient that opposes the influx of Na+. If these two forces counterbalance each other precisely, no net movement of Na+ occurs.
In regards to part B of the question, when the membrane potential reaches a threshold voltage, voltage-gated Na+ channels open, allowing Na+ ions to flow into the cell. This influx of Na+ ions creates a positive feedback loop that further depolarizes the membrane and intensifies the action potential, which is crucial for nerve impulse transmission.