axonal membrane is impermeable to Na+. Then how does it allow 3 Na+ to flow from the axonal membrane?

At the resting stage (when the neuron is not conducting any impulse), the axonal membrane is comparatively more permeable to potassium ions (K+ ) and nearly impermeable to sodium ions (Na+ ). 

As a result of this, the axoplasm inside the axon contains high concentration of K+ and low concentration of Na+. 

In contrast, the fluid outside the axon contains a low concentration of K+, a high concentration of Na+ and thus form a concentration gradient. 

These ionic gradients across the resting membrane are maintained by the active transport (energy is utilized) of ions by the sodium-potassium pump which transports 3 Na+ outwards for 2 K+ into the cell .3 Na+ are transported outwards for 2 K+ because K+ is more electropositive than Na+. So, the transfer of ions occurs to maintain this ionic gradient. As a result, the outer surface of the axonal membrane possesses a positive charge while its inner surface becomes negatively charged and therefore is polarised.

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