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 resting membrane potential of a neuron is typically about -70 millivolts (mV), measured inside relative to outside. The minus sign shows that the inside is negative.
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:
- Ionic gradients -- the unequal distribution of Na+, K+, and other ions across the membrane, set up and maintained by active transport.
- 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.
The resting membrane potential is not a true equilibrium -- it is a steady state. The constant outward leak of K+ and the constant pumping action of the sodium-potassium pump balance each other, so the potential stays steady at around -70 mV instead of running down over time.
Why This Matters
The resting potential is the starting line for every act of neural communication. When a stimulus arrives, it changes the membrane's permeability to Na+, letting Na+ rush into the cell. This depolarises the membrane -- makes the inside less negative -- and if the change crosses a threshold, it triggers an action potential. Without a stable resting potential to begin from, there would be no baseline from which a nerve impulse could be generated.
In short: the resting membrane potential is the polarised, ready state of a neuron -- stored electrical energy, built by ionic gradients and selective permeability, and continuously maintained by the sodium-potassium pump, waiting to be released as a nerve impulse.
This topic is directly aligned with the NCERT/CBSE Class 11 Biology syllabus, and the -70 mV resting potential across the axonal membrane is a favourite area for both board-exam diagrams and NEET important questions. If you're looking up "Resting Membrane Potential important questions" or "Resting Membrane Potential NCERT notes," the explanation above covers the core mechanism examiners typically test.