MCAT content library / Biology & biochemistry / Resting membrane potential: balance forces, currents and gradients
Predict an ion’s movement and the membrane’s voltage by keeping concentrations, conductance, and time scale separate.
About 16 minutes, including the worked examples and checks.
From a membrane to a neural signal · Lesson 4 of 6 · Continue in this path
Useful foundations
On this page
- Concept: voltage is measured across a selective boundary
- Pattern: calculate the voltage that balances one ion
- Worked application: equilibrium voltage predicts direction
- Rule: a resting membrane can carry balancing currents
- Changed case: inhibit a pump instead of opening a channel
- Check your reasoning
- Continue learning
- Sources
Concept: voltage is measured across a selective boundary
Membrane potential is the electrical potential inside a cell relative to outside: Vm = Vinside − Voutside. A value of −60 mV therefore means that the inside is lower in electric potential. It does not mean that every intracellular ion is negative, or that the cell has lost most of its positive ions. Only a small separation of charge near the membrane is needed to produce a voltage.2
Ions experience both a concentration-related force and an electrical force. If potassium is more concentrated inside, its chemical tendency is outward. A sufficiently negative interior attracts this positively charged ion inward. These tendencies can oppose one another. A channel permits movement; it does not decide the direction independently of the electrochemical conditions.4
Pattern: calculate the voltage that balances one ion
An ion’s equilibrium potential is the membrane voltage at which its opposing electrochemical tendencies balance, giving no net passive flux of that ion. Individual ions still move in both directions. This is not the same as closing the channel. For a specified ion, the Nernst equation relates the equilibrium voltage to the concentration ratio and ionic charge.3
For the original examples here, use the supplied room-temperature approximation Eion = (58 mV/z)log10([ion]outside/[ion]inside), appropriate near 20°C. Concentrations approximate activities, z includes the sign of the ionic charge, and voltage is inside minus outside. The numerical factor depends on temperature; use a different factor if the passage supplies one.3
| Ion and concentrations | Ratio outside/inside | z | Predicted E |
|---|---|---|---|
| K⁺: 100 mM inside, 10 mM outside | 0.10 | +1 | −58 mV |
| K⁺: 100 mM inside, 1 mM outside | 0.01 | +1 | −116 mV |
| Cl⁻: 10 mM inside, 100 mM outside | 10 | −1 | −58 mV |
For the first potassium case, log10(0.10) = −1, so EK = −58 mV. Lowering outside potassium tenfold makes the ratio 0.01 and the logarithm −2, giving −116 mV. The voltage change is additive, −58 mV, rather than a tenfold change in voltage.
The chloride example has the opposite concentration ratio but also the opposite charge. Both changes matter: (58/−1) × 1 = −58 mV. Reusing a cation’s sign rule without retaining z would predict the wrong voltage.
Worked application: equilibrium voltage predicts direction
Return to potassium with EK = −58 mV. At Vm = −40 mV, the interior is not negative enough to balance the outward concentration tendency. Net potassium movement through an open selective channel is outward. At −80 mV, the electrical attraction is stronger than at equilibrium, so net potassium movement is inward. At −58 mV, the net passive flux is zero.2
A useful current convention is Iion = gion(Vm − Eion), with outward positive current and a positive conductance g. For potassium, positive current means outward K⁺ movement. For chloride, positive current corresponds to inward negative-charge movement. Keep electrical current and particle movement distinct when an anion is involved.3
Thus a chloride channel with ECl = −70 mV and Vm = −50 mV carries outward-positive current in this convention, while chloride ions move inward. Opening that channel tends to pull the voltage toward −70 mV. The expression describes the stated approximately ohmic channel model; the driving-force sign remains the central inference.
Rule: a resting membrane can carry balancing currents
One ion’s equilibrium voltage is not automatically the resting voltage of a membrane permeable to several ions. A steady voltage requires balanced total current, not zero current for every species. The following deliberately simple model has only constant potassium and sodium conductances, no applied current, and negligible pump current in the instantaneous balance. Its gradients are held fixed during the comparison.
Set gK(Vm − EK) + gNa(Vm − ENa) = 0. Rearranging gives Vm = (gKEK + gNaENa)/(gK + gNa). This is a conductance-weighted result under the stated assumptions, not permission to substitute permeability values into the same expression.
| gK | gNa | EK | ENa | Calculated Vm |
|---|---|---|---|---|
| 4 units | 1 unit | −90 mV | +60 mV | −60 mV |
| 9 units | 1 unit | −90 mV | +60 mV | −75 mV |
Initially, [4(−90) + 1(60)]/5 = −60 mV. Increasing gK gives [9(−90) + 1(60)]/10 = −75 mV, closer to EK. Neither equilibrium potential changed. At the initial voltage, the potassium current is proportional to 4(30) = +120 and sodium current to 1(−120) = −120. They cancel despite ongoing opposite ionic fluxes.
Changed case: inhibit a pump instead of opening a channel
The Na⁺/K⁺ ATPase uses ATP to move three sodium ions outward and two potassium ions inward per cycle. It maintains gradients against passive movement and also produces an electrogenic current. These are related but distinct effects.4
Immediately inhibiting the pump removes its current; that alone can shift voltage. It does not instantly equalize all ion concentrations or close every ion channel. During a short interval in which gradients remain nearly unchanged, passive channel currents can persist. Over longer intervals, continued leaks without adequate pumping change the gradients and their equilibrium potentials.
The exact trajectory depends on conductances, volumes, extracellular conditions, and other transport processes. Do not infer an instantaneous jump to zero or a permanent unchanged voltage merely from the phrase “pump inhibited.” First identify whether the question changes a channel’s conductance, an ion concentration, or gradient maintenance over time.
This separation of concentration, permeability, current, and voltage addresses the AAMC’s membrane-potential and nerve-cell scope. It also provides the foundation for interpreting the rapid conductance changes of an action potential.1
ACTIVE PRACTICE · CELL PHYSIOLOGY
Balance Opposing Ion Currents
Use the original two-ion teaching model: EK = −90 mV, ENa = +60 mV. Hold both equilibrium potentials fixed, ignore applied and pump currents, and set sodium conductance to 1 relative unit. At steady state, total current equals zero.
Predicted resting voltage: mV.
Outward-positive K⁺ current: relative units. Outward-positive Na⁺ current: relative units. Sum: .
Opposing ionic currents balance at the modeled steady voltage; neither current is individually zero.
Conductances are constant in this simplified instantaneous model, not permeabilities. Real resting membranes also involve pumps, multiple ionic pathways, and changing gradients.
Check your reasoning
Choose an answer before opening its feedback. Each option explains the reasoning behind that choice.
Check 1
For a monovalent cation at the stated temperature, use E = 58 mV × log₁₀([outside]/[inside]). Its concentration is 200 mM inside and 2 mM outside. What is its equilibrium potential, measured inside minus outside?
A. −116 mV.
Correct. The ratio is 0.01 and its logarithm is −2, giving 58 × (−2) = −116 mV.
B. −58 mV.
Review this choice. −58 mV would follow from a tenfold gradient; the supplied gradient is 100-fold.
C. +58 mV.
Review this choice. The outside/inside ratio is less than 1, so the logarithm is negative for this cation.
D. +116 mV.
Review this choice. This reverses the specified concentration ratio.
The principle to carry forward
Keep the outside/inside ratio and ionic charge explicit before taking the logarithm.
Check 2
A potassium-selective channel is open. The membrane is held at −85 mV, and the supplied potassium equilibrium potential is −65 mV. Which net passive movement is predicted?
A. Potassium moves outward because its concentration is higher inside.
Review this choice. The concentration tendency alone is insufficient. At a voltage more negative than equilibrium, the electrical tendency dominates.
B. Potassium moves inward because the membrane is more negative than EK.
Correct. Vm − EK = −20 mV gives inward positive-charge current in the stated convention.
C. There is no net movement because any negative membrane voltage balances potassium.
Review this choice. Only the equilibrium voltage balances the electrochemical tendencies for this ion.
D. The channel pumps potassium inward using ATP.
Review this choice. A passive channel does not couple transport to ATP hydrolysis.
The principle to carry forward
Compare the actual voltage with the ion-specific equilibrium voltage; concentration alone does not settle direction.
Check 3
A selective inhibitor stops Na⁺/K⁺ ATPase cycling. Ion channels remain open, and measurements show that ion concentrations change negligibly during the first brief recording interval. Which interpretation is most consistent with those conditions?
A. All passive sodium and potassium movement must immediately stop.
Review this choice. Existing electrochemical gradients and open channels still permit passive movement.
B. The membrane potential must immediately become exactly zero.
Review this choice. Removing pump current need not set total voltage to zero; other currents and gradients remain.
C. Passive currents can persist initially; longer-term gradient loss requires a separate time-course analysis.
Correct. The immediate loss of electrogenic pumping is distinct from subsequent changes in concentrations. The stated early interval cannot establish the later trajectory.
D. The unchanged initial concentrations establish that the pump never helped maintain them.
Review this choice. A maintained gradient can persist briefly after its maintenance process is interrupted.
The principle to carry forward
Separate immediate current changes from the later consequences of altered ion inventories.
Continue learning
From a membrane to a neural signal
Lesson 4 of 6.
- Previous in From a membrane to a neural signal: Neuron structure: locate a failure in input, conduction or output
- Next in From a membrane to a neural signal: Action potentials: distinguish firing, recovery and conduction speed
- Return to this path’s overview
See every step in From a membrane to a neural signal
From a membrane to a neural signal
Connect membrane properties and gradients to signal propagation and synaptic integration.
- Lipids and membrane properties
- Membrane transport and osmosis
- Neuron structure: locate a failure in input, conduction or output
- Resting membrane potential: balance forces, currents and gradients · You are here
- Action potentials: distinguish firing, recovery and conduction speed
- Synaptic transmission: locate the change and predict the response
Related lessons
- Action potentials: distinguish firing, recovery and conduction speed
- Synaptic transmission: locate the change and predict the response
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Sources
1. AAMC: B/B content category 3A
2. Purves et al.: The Forces that Create Membrane Potentials
3. Basic Neurochemistry: The Ionic Hypothesis and Rules of Ionic Electricity