Which electrolyte abnormality is NOT a cause of hyperpolarization?

Which electrolyte abnormality is NOT a cause of hyperpolarization?

• A. Hypercalcemia.
• B. Hyponatremia.
• C. Hypokalemia.
• D. Hyperkalemia.

Answer: (D)

Membrane potential becomes more negative (hyperpolarization) when the cell’s resting state shifts toward a stronger negative inside atmosphere. Extracellular ion changes influence this balance: elements that make the inside more negative or raise the threshold for firing tend to hyperpolarize or reduce excitability, while factors that reduce negativity or raise the inside potential toward zero depolarize. Increasing extracellular potassium reduces the gradient that drives K+ out of the cell, so the resting potential moves closer to zero and becomes less negative. This depolarizes the cell, not hyperpolarizes it—so high potassium does not cause hyperpolarization. In contrast, higher calcium outside the cell stabilizes the membrane and lowers excitability by increasing the threshold for action potential firing, which aligns with a hyperpolarized or functionally quieter state. Low sodium outside the cell reduces the depolarizing Na+ current during stimulation, also leading to decreased excitability. Low potassium inside or outside (hypokalemia) enhances the outward K+ gradient, making the resting potential more negative and producing hyperpolarization. Therefore, the electrolyte abnormality that does not cause hyperpolarization is the one with high extracellular potassium, since it tends to depolarize rather than hyperpolarize the cell.

Membrane potential becomes more negative (hyperpolarization) when the cell’s resting state shifts toward a stronger negative inside atmosphere. Extracellular ion changes influence this balance: elements that make the inside more negative or raise the threshold for firing tend to hyperpolarize or reduce excitability, while factors that reduce negativity or raise the inside potential toward zero depolarize.

Increasing extracellular potassium reduces the gradient that drives K+ out of the cell, so the resting potential moves closer to zero and becomes less negative. This depolarizes the cell, not hyperpolarizes it—so high potassium does not cause hyperpolarization.

In contrast, higher calcium outside the cell stabilizes the membrane and lowers excitability by increasing the threshold for action potential firing, which aligns with a hyperpolarized or functionally quieter state. Low sodium outside the cell reduces the depolarizing Na+ current during stimulation, also leading to decreased excitability. Low potassium inside or outside (hypokalemia) enhances the outward K+ gradient, making the resting potential more negative and producing hyperpolarization.

Therefore, the electrolyte abnormality that does not cause hyperpolarization is the one with high extracellular potassium, since it tends to depolarize rather than hyperpolarize the cell.