Neural Control And Coordination

The resting potential of a neuron depends on the unequal distribution of ions across the axonal membrane maintained by selective permeability and the sodium-potassium pump. NCERT Class 11, Chapter 21 (Neural Control and Coordination) states that in a resting axon the membrane is comparatively more permeable to potassium and nearly impermeable to sodium, and the ionic pump transports three sodium ions outward for every two potassium ions moved inward. Consequently the axoplasm inside has high potassium and low sodium, while the fluid outside has high sodium and low potassium. Therefore sodium ions are concentrated outside the resting axon. Potassium ions are incorrect because they are the cation accumulated inside the axoplasm, the opposite distribution. Chloride ions are not the focus of the active pumping described, and the question specifically concerns the actively extruded cation. Calcium ions, while important in synaptic transmission and muscle activity, are not the principal ion governing the resting axonal potential here. Hence sodium ions is the correct answer. An electrochemical plausibility check confirms it: a pump expelling sodium against its gradient must produce a higher sodium concentration outside than inside, exactly matching the chosen option.