Explanation

High concentration of $K^{+}$ and low concentration of $N^{+}$ inside the axon. Both A and R true and R is the correct explanation of A. 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 ($Na^{+}$). Similarly, the membrane is impermeable to negatively charged proteins present in the axoplasm. Consequently, the axoplasm inside the axon contains high concentration of $K^{+}$ and negatively charged proteins 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