Resting potential

Neurons are excitable because their membranes are polarised. The neural membrane carries different ion channels, each selectively permeable to particular ions.

At rest, the axonal membrane is fairly permeable to K⁺, nearly impermeable to Na⁺, and impermeable to the negatively charged proteins of the axoplasm.

So the axoplasm has a high concentration of K⁺ and negative proteins and a low concentration of Na⁺, while the fluid outside has low K⁺ and high Na⁺: a concentration gradient.

The sodium-potassium pump maintains these gradients by active transport, moving 3 Na⁺ out of the cell for every 2 K⁺ it brings in.

The result is a positive charge on the outside of the axonal membrane and a negative charge on its inner face: the membrane is polarised.

This potential difference across the membrane of a resting neuron is called the resting potential.