Q.Why is the axoplasm of a resting axon negatively charged?

Resting Membrane Potential

Imagine a neuron sitting quietly, doing nothing -- no signal coming in, no signal going out. Even in this resting state, the inside of the cell is electrically different from the outside. That difference is the resting membrane potential.

The Intuition: A Battery in the Cell

Think of the neuron's membrane as a wall with selective gates. On either side of this wall, ions (charged particles) are distributed unevenly. The two most important players are:

• Potassium ions (K+) -- more concentrated inside the cell
• Sodium ions (Na+) -- more concentrated outside the cell

Because ions naturally move down their concentration gradients, both K+ and Na+ tend to cross the membrane. But the membrane is not equally permeable to both. At rest, it is far more permeable to K+ than to Na+.

So K+ leaks out through dedicated potassium channels, carrying positive charge with it. This leaves the inside of the cell relatively negative compared to the outside. The cell is now polarised -- like a tiny battery with a negative terminal inside and a positive terminal outside.

The Precise Statement

The resting membrane potential is the steady electrical potential difference across the plasma membrane of a neuron when it is not conducting an impulse. It arises from two main factors:

1. Ionic gradients -- the unequal distribution of Na+, K+, and other ions across the membrane, set up and maintained by active transport.
2. Selective permeability -- the resting membrane is far more permeable to K+ than to Na+, so the outward leak of K+ dominates and sets up the negative interior.

The result is a stable, negative interior that forms the baseline from which every nerve impulse begins.

The Sodium-Potassium Pump: The Battery Charger

If K+ keeps leaking out, why doesn't the gradient run down over time? Because the sodium-potassium pump (Na+/K+ ATPase) constantly works against the leak, using energy from ATP. For every cycle, it pumps:

• 3 sodium ions (Na+) out of the cell
• 2 potassium ions (K+) into the cell

Because it moves three positive charges out for every two it brings in, the pump is electrogenic -- it directly adds a small amount to the negative interior. Its main job, however, is to keep restoring the concentration gradients of Na+ and K+ that make the resting potential possible in the first place. …