Final answer:
As membrane potential increases, Sodium's inward current increases until it peaks, then decreases; Potassium's outward current gradually increases as the membrane potential moves back towards resting level.
Step-by-step explanation:
As membrane potential increases, different things happen to the membrane currents of Sodium (Na+) and Potassium (K+). The opening of voltage-gated sodium channels causes Na+ to rush into the cell, creating a depolarization effect that moves the membrane potential toward zero. Even after reaching and surpassing zero potential, sodium ions continue to flow in because of the strong concentration gradient and the attraction to the negative proteins inside the cell, eventually causing the membrane potential to peak around +30 mV.
Following this, voltage-gated potassium channels open, and K+ begins to leave the cell, taking positive charge with it and moving the membrane potential back toward its resting state of -70 mV in a process called repolarization. As the membrane potential increases, Sodium's inward current increases until reaching its peak, after which it starts to decline as the sodium channels close, while Potassium's outward current gradually increases as its channels open to correct the electrical imbalance.
The correct answer would be A: Sodium's inward current increases up until zero and then starts to decrease until a membrane potential of +52, Potassium's outward current gradually increases.