What are the ions present in high concentration inside the axon...

When a neuron is not conducting any impulse, i.e., resting, the axonal membrane is comparatively more permeable to potassium ions (K⁺) and nearly impermeable to sodium ions (Na⁺). Consequently, the axoplasm inside the axon contains high concentration of K⁺ and negatively charged proteins and low concentration of Na⁺. The electrical potential difference across the resting plasma membrane is called the resting potential. The outer surface of the axonal membrane possesses a positive charge while its inner surface becomes negatively charged and therefore is polarised.