Which ion has a higher extracellular concentration relative to inside a neuron?

Which ion has a higher extracellular concentration relative to inside a neuron?

• A. Sodium (Na+)
• B. Potassium (K+)
• C. Calcium (Ca2+)
• D. Chloride (Cl−)

Answer: (A)

Sodium is kept at a high concentration outside the neuron and is much lower inside, a difference actively maintained by the Na+/K+ pump. In many neurons, extracellular Na+ is about 140 mM while intracellular Na+ is around 10–15 mM. This large outside-versus-inside difference is a fundamental feature that underpins the action potential, providing the strong electrochemical driving force for Na+ to rush into the cell when voltage-gated Na+ channels open. Potassium, on the other hand, is more concentrated inside the cell, not outside, so it doesn’t fit the criterion of having higher extracellular concentration. Calcium and chloride are also more abundant outside than inside, but the most emphasized and physiologically important example used in this context is the sodium gradient.

Sodium is kept at a high concentration outside the neuron and is much lower inside, a difference actively maintained by the Na+/K+ pump. In many neurons, extracellular Na+ is about 140 mM while intracellular Na+ is around 10–15 mM. This large outside-versus-inside difference is a fundamental feature that underpins the action potential, providing the strong electrochemical driving force for Na+ to rush into the cell when voltage-gated Na+ channels open.

Potassium, on the other hand, is more concentrated inside the cell, not outside, so it doesn’t fit the criterion of having higher extracellular concentration. Calcium and chloride are also more abundant outside than inside, but the most emphasized and physiologically important example used in this context is the sodium gradient.