Which ion has the highest concentration inside neurons?

• A. Potassium
• B. Sodium
• C. Calcium
• D. Magnesium

Answer: (A)

Potassium is the ion present at the highest concentration inside neurons. In resting neurons, the inside is rich in K+, with about 140 mM inside compared to only a few millimoles per liter outside. In contrast, sodium is kept at far lower levels inside (roughly 10–15 mM) while the outside is high (around 140 mM). Calcium is kept extremely low inside, often in the nanomolar to low micromolar range, because even small increases can trigger signaling and damage. Magnesium is present intracellularly at about half a millimolar to a millimolar.

This large intracellular potassium concentration is maintained by the Na+/K+ ATPase, which pumps potassium into the cell while pumping sodium out. Potassium leak channels also allow K+ to move according to its gradient, but the pump steadily sustains the high internal K+. This gradient is a major contributor to the resting membrane potential, keeping the neuron ready to respond to stimuli.