Which ion is more abundant inside a neuron?

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

Answer: (A)

Resting ionic gradients maintain a high potassium concentration inside the neuron. The membrane is leaky to K+, so potassium tends to diffuse out, which helps make the inside negative. The Na+/K+ pump keeps potassium inside and sodium outside, sustaining this gradient. Calcium is kept very low inside to prevent signaling, and chloride is regulated in a way that often keeps it lower inside than outside. Because of these conditions, potassium is the most abundant ion inside a neuron.