Source: Brain-Mind Odyssey, UC Berkeley
Tags: membrane potential, resting membrane potential, ion distribution, sodium-potassium pump, Na+/K+-ATPase, Nernst equation, equilibrium potential, ion channels, selective permeability, concentration gradient, electrical gradient, electrochemical gradient, neuron voltage, resting potential -70mV
Neurons maintain a voltage difference across their membrane (the resting membrane potential, typically around -60 to -80 mV) by keeping specific ions at different concentrations inside and outside the cell. The sodium-potassium pump actively maintains these gradients, and the balance between diffusion and electrical forces at selective ion channels sets the resting potential. This voltage is the foundation for all neuronal signalling.
Membrane potential
The voltage difference across a neuron's cell membrane, created by unequal distributions of ions on either side. Measured in millivolts (mV).
Resting membrane potential
The stable voltage of a neuron when it is not actively firing, typically between -60 and -80 mV. The inside of the cell is negative relative to the outside.
Sodium-potassium pump (Na+/K+-ATPase)
An active transporter that uses one molecule of ATP to move three Na+ ions out of the cell and two K+ ions into the cell, working against their concentration gradients. Consumes roughly 30% of the neuron's energy budget.
Nernst equation
The formula used to calculate the equilibrium potential for a single ion species, based on its concentration on each side of the membrane.
Equilibrium potential
The membrane voltage at which the electrical force on an ion exactly balances the diffusional (concentration gradient) force, so there is no net movement of that ion.
Selective ion channels
Protein channels in the membrane that allow only specific ions to pass through. Their selectivity is a key reason the resting potential exists.
Concentration gradient
The difference in concentration of a given ion between the inside and outside of the cell. Ions tend to move down their concentration gradient (from high to low).
Electrochemical gradient
The combined influence of the concentration gradient and the electrical gradient on the movement of an ion across the membrane.
The extracellular space (outside) is rich in sodium (Na+) and chloride (Cl-) ions
The intracellular space (cytoplasm) is rich in potassium (K+) and organic anions (A-)
This asymmetry is maintained actively and is the physical basis of the resting membrane potential
The resting potential sits at roughly -60 to -80 mV, meaning the inside of the neuron is negatively charged relative to the outside
Pumps 3 Na+ out and 2 K+ in per cycle, using one ATP molecule each time
Works against the natural concentration gradients for both ions
Accounts for about 30% of total cellular energy expenditure in humans
This is a useful exam figure to remember: it underscores how critical the pump is
Without the pump running, the ion gradients would gradually collapse, the membrane potential would dissipate, and the neuron would lose its ability to signal
Ions move through selective channels down their concentration gradients
As charged ions move, they create an electrical potential across the membrane
At a certain voltage, the electrical force pushing the ion back equals the diffusional force pulling it forward
This point is the equilibrium potential for that ion
The Nernst equation calculates this equilibrium potential for any single ion, given the concentrations on each side
The actual resting membrane potential reflects the combined influence of several ions (primarily K+, Na+, and Cl-), weighted by the membrane's relative permeability to each
The result is a non-zero resting membrane potential, which is what allows the neuron to fire when stimulated
Nernst Equation (simplified, for a monovalent cation at body temperature):
E_ion = (61.5 mV) × log10([ion]outside / [ion]inside)
For K+: high inside, low outside, so E_K is strongly negative (around -80 to -90 mV)
For Na+: high outside, low inside, so E_Na is strongly positive (around +50 to +60 mV)
The resting membrane potential sits closer to E_K because the membrane at rest is far more permeable to K+ than to Na+
⚠️ The 3:2 ratio of the sodium-potassium pump (3 Na+ out, 2 K+ in) is a commonly tested detail. Know the ratio and that it consumes ATP.
⚠️ Students often confuse equilibrium potential (single ion, Nernst equation) with the resting membrane potential (multiple ions, Goldman equation). The resting potential is a weighted average, not the equilibrium potential of any one ion.
⚠️ The resting potential is closer to the potassium equilibrium potential because the membrane at rest is most permeable to K+. This is a frequently examined point.
⚠️ "Against the concentration gradient" means the pump is doing active transport. Ion channels, by contrast, allow passive movement down the gradient.
Q: Why is the resting membrane potential closer to the equilibrium potential for potassium than for sodium?
A: Because the neuronal membrane at rest is far more permeable to K+ than to Na+. The resting potential is a weighted average of ion equilibrium potentials, and K+ dominates the weighting.
Q: What happens to a neuron if the sodium-potassium pump is inhibited?
A: The ion gradients gradually dissipate. Na+ accumulates inside, K+ leaks out, and the membrane potential collapses toward zero. The neuron loses its ability to generate action potentials.
Q: How much of a neuron's energy does the sodium-potassium pump consume?
A: Approximately 30% of the cell's total ATP expenditure in humans.
Q: What does the Nernst equation calculate?
A: The equilibrium potential for a single ion species, based on its concentration ratio across the membrane. It gives the voltage at which there is no net movement of that ion.
Q: What is the difference between a concentration gradient and an electrochemical gradient?
A: A concentration gradient considers only the difference in ion concentration across the membrane. An electrochemical gradient combines both the concentration gradient and the electrical gradient (membrane voltage) to determine the net driving force on an ion.
membrane potential, resting potential, resting membrane potential, ion gradient, sodium-potassium pump, Na/K ATPase, Na+/K+-ATPase, Nernst equation, Goldman equation, equilibrium potential, reversal potential, selective permeability, ion channels, passive transport, active transport, concentration gradient, electrochemical gradient, neuron voltage, intracellular ions, extracellular ions, potassium leak channels, neuronal signalling basics