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Your Cells Run on Electricity

How Ion Gradients Become Nerve Signals

Big Question: How do cells use unequal ion distributions and selective membrane permeability to create voltage, transmit information and power biological work?

Quick Answer

Cells create electrical potential differences by maintaining unequal concentrations of ions on opposite sides of a selectively permeable membrane. Pumps such as the sodium-potassium ATPase use energy from ATP to maintain ion gradients. Ion channels then allow selected ions to move down their electrochemical gradients. Because ions carry charge, their movement creates electrical current and changes membrane voltage.

In a neuron, a sufficiently large depolarisation can open voltage-gated sodium channels. Sodium ions move inward, depolarising the membrane further. Sodium channels then inactivate while potassium channels open, allowing potassium ions to move outward and repolarise the membrane. The result is an action potential: a brief, regenerative change in membrane voltage that can propagate along an axon.

ATP → ion gradients → selective channels → ion movement → voltage change → biological signal.

What You Will Learn

• Why a cell membrane can separate charge.
• Why concentration gradients are stored potential energy.
• How the sodium-potassium pump helps maintain ion gradients.
• Why potassium permeability strongly influences resting membrane potential in many animal cells.
• What an electrochemical gradient is.
• How voltage-gated channels create action potentials.
• Why action potentials are not electrons racing down a biological wire.
• How myelin changes signal propagation.
• How electrical signals connect to synapses, muscles and the heart.
• How Nernst and Goldman-type reasoning increase the resolution of the model.
• How classic squid-axon experiments connected measurement to mechanism.

Part 1 — A Membrane Is an Electrical Boundary

The plasma membrane is built mainly from a phospholipid bilayer containing proteins and other molecules. Its hydrophobic interior is a strong barrier to most ions. Sodium, potassium, calcium and chloride do not freely cross the lipid bilayer at useful rates.

That barrier matters because electrical potential requires separation. If every ion crossed instantly, concentration differences and voltage differences would rapidly disappear.

No barrier → no sustained gradient. No controlled gateway → no useful electrical signalling.

Cells therefore combine a relatively ion-resistant membrane with highly selective proteins that act as channels, pumps and transporters.

Part 2 — Concentration Gradients Store Potential

Imagine many potassium ions on one side of a membrane and fewer on the other. If potassium-selective channels open, random molecular motion produces a net movement from higher concentration toward lower concentration.

This is diffusion, but because potassium ions are charged, another force appears. As positive potassium ions move, charge separation develops. The resulting electrical field can oppose further movement.

The ion therefore responds to two influences:

• chemical gradient — arising from concentration difference;
• electrical gradient — arising from voltage and charge.

Together they form the electrochemical gradient.

An ion does not care only where there is more of itself. It also responds to electrical attraction and repulsion.

Part 3 — The Sodium-Potassium Pump Builds the Gradient

In animal cells, the sodium-potassium ATPase uses ATP to transport sodium and potassium against their concentration gradients. In its familiar cycle, three sodium ions are moved out for every two potassium ions moved in per ATP hydrolysed.

The pump is therefore both a transporter and electrogenic: each cycle moves a net positive charge outward. Its larger importance, however, is that it maintains the sodium and potassium gradients that many channels and secondary transporters later exploit.

The pump does not single-handedly set the resting voltage. Resting membrane potential emerges from ion gradients plus membrane permeability, especially the permeability of resting membranes to potassium in many animal cells.

Pump builds the hill. Channels let selected ions run down it.

Explore NCBI on active transporters and ion gradients in a new tab →

Part 4 — Why the Resting Cell Is Usually Negative Inside

Many animal cells contain a high potassium concentration inside and a high sodium concentration outside. At rest, membranes are often much more permeable to potassium than sodium because of potassium-selective leak channels.

Potassium therefore tends to diffuse outward. As positive charge leaves, the interior becomes more negative relative to the exterior. That electrical difference increasingly attracts potassium back inward and opposes further outward diffusion.

An equilibrium can be approached where chemical and electrical driving forces balance for that ion. The voltage associated with that balance can be calculated using the Nernst equation.

Real cells are permeable to multiple ions, so the actual resting membrane potential depends on several concentration gradients and relative permeabilities. The Goldman-Hodgkin-Katz framework provides a higher-resolution model.

Part 5 — Voltage Is a Difference, Not a Substance

Students sometimes imagine “voltage” as a material stored in a neuron. It is not. Voltage is an electrical potential difference between two locations.

When a neuronal resting potential is described as about −70 millivolts, it means the electrical potential inside is lower than the chosen outside reference by about 70 millivolts. The exact value varies with cell type, species, conditions and measurement.

A crucial subtlety is that the bulk solutions inside and outside a cell remain nearly electrically neutral. Only a tiny fraction of ions needs to redistribute close to the membrane to create the measured voltage difference.

Part 6 — Ion Channels Turn Stored Gradients Into Current

Ion channels are membrane proteins that form selective pathways through which particular ions can move rapidly down electrochemical gradients.

Channels can be controlled in different ways. Some respond to membrane voltage. Others open after a chemical messenger binds. Others respond to stretch, temperature or intracellular signals.

When a channel opens, ions do not receive energy from the channel. The energy is already stored in the electrochemical gradient. The channel lowers the barrier to movement.

A channel is not a pump. A pump can push uphill using energy; a channel provides a downhill route.

Read NCBI on ion channels and electrical membrane properties in a new tab →

Part 7 — The Action Potential: A Regenerative Voltage Wave

An action potential begins when membrane depolarisation reaches conditions that activate enough voltage-gated sodium channels. Sodium ions move inward down their electrochemical gradient, making the inside less negative and then briefly positive relative to the outside.

This depolarisation opens additional nearby sodium channels. The response therefore contains positive feedback.

Sodium channels then inactivate, while voltage-gated potassium channels open more strongly. Potassium moves outward, helping repolarise the membrane. Continued potassium conductance can briefly make the membrane more negative than its resting level before resting permeability patterns are restored.

1. resting membrane potential;
2. depolarising stimulus;
3. threshold region reached;
4. rapid sodium conductance increase;
5. strong depolarisation;
6. sodium-channel inactivation;
7. potassium conductance rises;
8. repolarisation;
9. possible after-hyperpolarisation;
10. return toward resting state.

The exact timing and ion-channel mix differ among neurons and other excitable cells. The school diagram is a model, not a universal waveform.

Part 8 — Why the Signal Travels

Local depolarisation produces electrical currents in neighbouring membrane regions. If those regions reach the activation range of voltage-gated channels, they generate their own action potentials.

The action potential therefore does not travel because one group of sodium ions races from the spinal cord to a fingertip. Instead, each local membrane region triggers the next.

Local voltage change → nearby channel opening → new local voltage change → propagation.

Part 9 — Myelin Changes the Geometry of Signalling

Myelin is an insulating wrapping around many axons. It reduces current leakage across covered membrane and changes the membrane’s electrical properties. Voltage-gated sodium channels are concentrated at gaps called nodes of Ranvier.

Depolarisation spreads rapidly beneath myelinated regions and action potentials are regenerated at nodes. This is called saltatory conduction.

“The impulse jumps” is a useful shorthand, but the deeper model is continuous electrical spread under myelin plus regenerative ion-channel activity at nodes.

Part 10 — Electrical Signal Becomes Chemical Signal at Many Synapses

At many chemical synapses, an arriving action potential changes voltage at the presynaptic terminal. Voltage-gated calcium channels open. Calcium entry promotes vesicle fusion and neurotransmitter release. The neurotransmitter crosses a narrow synaptic cleft and binds receptors on the next cell.

The signal therefore changes form:

electrical → calcium signal → chemical messenger → receptor → new electrical or biochemical response.

This conversion helps explain why nervous systems can amplify, inhibit, integrate and modify information rather than merely relay it.

Part 11 — Your Muscles Also Use Voltage

Skeletal muscle fibres are excitable cells. Electrical activity at the muscle membrane and transverse tubules triggers calcium release inside the fibre. Calcium then permits interactions between contractile proteins that generate force.

Again, voltage is not the final work. It is a control signal linking one event to another.

Part 12 — Your Heart Is an Electrical Organ Too

Cardiac cells generate and respond to membrane potentials using several ion channels with characteristic timing. Pacemaker cells can depolarise rhythmically, while electrical coupling coordinates contraction across heart tissue.

Different cardiac cell types produce different action-potential shapes. Calcium currents are especially important in cardiac excitation-contraction coupling.

This is why disturbances of ion channels or ion concentrations can alter heart rhythm.

Someone Looked Closely Enough: Hodgkin, Huxley and the Squid Giant Axon

One reason the electrical basis of nerve signals became measurable was a biological accident of scale. Squid possess unusually large axons controlling rapid escape behaviour. Their diameter made them experimentally accessible with the technology of the twentieth century.

Alan Hodgkin and Andrew Huxley measured how membrane current changed with voltage and time. Their work showed that separate voltage- and time-dependent conductances could explain major features of the action potential. The mathematics did not merely fit a curve; it connected measurable electrical behaviour to membrane mechanisms that were later identified with ion channels.

animal behaviour → giant axon → electrode → current → equation → mechanism.

The lesson is larger than neuroscience: a useful model can begin with careful measurement before every molecular component is known.

Think Like a Scientist: How Do We Know Cells Have Voltage?

Scientists can insert microelectrodes into cells or use patch-clamp techniques to measure tiny electrical currents through membranes and even individual channels.

• Microelectrodes measure voltage differences between cell interior and exterior.
• Voltage clamp holds membrane voltage while measuring resulting currents.
• Patch clamp can measure current through tiny membrane patches and individual channels.
• Ion substitution changes external concentrations to test which ions carry current.
• Channel blockers help isolate contributions from particular channel classes.
• Genetics and structural biology connect electrical behaviour to specific proteins.

Good evidence joins multiple scales: voltage trace, ion movement, protein structure and whole-organism function.

Observation vs Inference

• Observation: membrane potential rapidly becomes less negative after stimulation.
• Observation: lowering external sodium reduces the action-potential overshoot.
• Inference: inward sodium current contributes strongly to the rising phase.
• Further test: block sodium channels selectively and measure the changed waveform.

Science becomes stronger when a proposed mechanism predicts what should happen after a controlled change.

Common Misconceptions and How to Repair Them

Misconception	Better model
Nerves are biological copper wires.	Signals depend on local ion movement across membranes and regenerative channel opening.
The sodium-potassium pump creates each action potential directly.	The pump maintains gradients; rapid action-potential currents mainly flow through ion channels.
All ions move because of concentration differences only.	Charged particles respond to electrochemical gradients.
Resting means inactive.	A resting cell continuously maintains gradients and membrane permeability.
Voltage is stored liquid electricity.	Voltage is an electrical potential difference.
The same sodium ions travel down the whole axon.	Local currents trigger new channel opening in adjacent membrane regions.
Every neuron rests at exactly −70 mV.	Resting potentials vary among cell types and conditions.
Myelin makes electricity move instantly.	Myelin reduces leakage and changes cable properties, increasing propagation speed while nodes regenerate the signal.

Secondary Science Bridge

• particle movement and diffusion;
• cell membrane structure;
• active transport;
• nervous coordination;
• energy from respiration;
• electrical current as movement of charge;
• cause-and-effect explanations using changes in gradients and permeability.

Junior College Biology Window

At JC resolution, the membrane-potential model can be expressed quantitatively. The Nernst equation predicts the equilibrium potential for one ion from its concentration ratio, temperature and charge. The Goldman-Hodgkin-Katz approach extends the idea to multiple ions and relative membrane permeabilities.

Action-potential shape then becomes a changing conductance problem. Sodium, potassium and sometimes calcium conductances vary with voltage and time. Channel activation and inactivation introduce feedback, refractory periods and firing behaviour.

Modern neuroscience adds another layer: where channels are placed in a neuron matters as much as which channels exist. A 2025 review in Nature Reviews Neuroscience examines the highly regulated delivery and distribution of voltage-gated ion channels across neuronal compartments.

Read the 2025 Nature Reviews Neuroscience review in a new tab →

Deep Science Window — A Gradient Is Stored Free Energy

When pumps maintain ions far from electrochemical equilibrium, the system stores free energy. Opening a channel allows some of that energy to dissipate as ions move downhill.

Cells couple these gradients to other processes. Sodium gradients can help drive glucose or amino-acid uptake. Proton gradients power ATP synthesis in mitochondria and chloroplasts by a different but deeply related principle.

Life repeatedly spends energy to create a gradient, then uses the gradient to make something else happen.

Deep Science Window — Channel Structure Explains Selectivity

A potassium channel can allow potassium ions to pass rapidly while strongly rejecting smaller sodium ions. Size alone does not explain this. Selectivity filters coordinate dehydrated ions with precise chemical geometry, making passage energetically favourable for one ion and unfavourable for another.

Recent structural biology continues to reveal how voltage sensors, pores and drug-binding regions work. This connects atomic-scale structure to whole-body physiology.

Explore a 2024 Nature Reviews Molecular Cell Biology review on voltage-gated ion channels →

Evidence Boundaries

• −70 mV ≠ universal resting potential. Values vary by cell type and condition.
• Na⁺/K⁺ pump ≠ sole cause of resting voltage. Ion gradients and selective permeabilities are central.
• Action potential ≠ one universal waveform. Channel combinations differ among neurons, muscle and cardiac cells.
• Electricity in cells ≠ electron flow in copper. Biological currents are usually carried by ions in solution.
• One channel ≠ one behaviour. Channel density, distribution, regulation and interacting proteins matter.
• Model equation ≠ organism. Mathematical models simplify real membranes but can still make powerful predictions.

Teach → Learn → Memorize → Test

1. TEACH — Start With the Battery Problem

Ask: “If a neuron has no battery, where does its voltage come from?”

Build the answer in this order: membrane barrier → unequal ion concentrations → pump maintains gradients → channels provide selective routes → charge moves → voltage changes.

2. LEARN — Change One Variable

• What if external potassium concentration rises?
• What if ATP production collapses for a long period?
• What if voltage-gated sodium channels are blocked?
• What if myelin is lost?
• What if membrane permeability to sodium suddenly increases?

3. MEMORIZE — Load-Bearing Facts

Idea	Minimum fact worth retaining
Membrane potential	Electrical potential difference across the membrane.
Electrochemical gradient	Combined chemical and electrical driving force on an ion.
Na⁺/K⁺ ATPase	Uses ATP to maintain Na⁺ and K⁺ gradients.
Ion channel	Selective pathway for passive ion movement.
Depolarisation	Membrane potential becomes less negative or more positive.
Repolarisation	Membrane returns toward a more negative resting range.
Action potential	Regenerative transient change in membrane voltage.
Myelin	Insulates axon segments and increases propagation efficiency and speed.

4. TEST — Retrieve → Explain → Predict → Transfer

1. Retrieve: define membrane potential.
2. Explain: connect ion gradients and permeability.
3. Predict: change an ion concentration or channel state.
4. Transfer: apply the model to heart, muscle, sensory receptors or another excitable cell.

Checkpoint Questions

1. Why can ions not simply cross the lipid bilayer freely?
2. What two components form an electrochemical gradient?
3. What does the sodium-potassium pump do?
4. Why is the resting membrane often influenced strongly by potassium permeability?
5. What is membrane voltage?
6. Why does opening a sodium channel not require the channel to supply the ion’s energy?
7. What creates positive feedback during the rising phase of many neuronal action potentials?
8. Why does an action potential propagate without the same ions travelling the full axon?
9. How does myelin improve propagation?
10. Why can blocking ion channels affect nervous-system function?
11. What is the difference between a pump and a channel?
12. Why is ATP indirectly essential for sustained electrical signalling?

Answer Key

Open after attempting the questions

1. The hydrophobic membrane interior presents a high barrier to charged ions.
2. A chemical concentration difference and an electrical potential difference.
3. It uses ATP to maintain sodium and potassium concentration gradients.
4. Resting membranes in many animal cells are relatively permeable to potassium through leak channels.
5. A difference in electrical potential across the membrane.
6. The electrochemical gradient already stores the driving energy; the channel provides a pathway.
7. Depolarisation opens sodium channels, sodium entry causes more depolarisation, and that opens more channels.
8. Each local membrane region regenerates the signal by opening its own channels.
9. It reduces current leakage and allows rapid spread between nodes where action potentials are regenerated.
10. Channels control ion movement that creates membrane currents and voltage changes.
11. Pumps can move substances uphill using energy; channels permit passive downhill movement.
12. ATP-powered transport maintains the gradients that signalling consumes and depends upon.

Can You Explain WHY?

• Why does a neuron need both pumps and channels?
• Why can a tiny redistribution of ions create measurable voltage?
• Why does raising extracellular potassium change excitability?
• Why does a sodium-channel blocker affect signal generation so strongly?
• Why can a cell be electrically active while remaining almost electrically neutral overall?
• Why does electrical signalling still depend on cellular respiration?

Manual Summary — KNOW → CONNECT → EXPLAIN → APPLY → CHECK

KNOW: membrane, ion gradient, pump, channel, electrochemical gradient, resting potential and action potential.

CONNECT: ATP to pumps, pumps to gradients, gradients to ion flow, ion flow to voltage, voltage to biological signalling.

EXPLAIN: cells become electrically excitable because membranes separate ions and proteins control their movement.

APPLY: predict what happens when ion concentrations, channels, pumps or myelin change.

CHECK: distinguish biological ion currents from electron flow in metal wires and distinguish pumps from channels.

Research Sources and Further Reading

• NCBI Bookshelf — Physiology, Action Potential
• NCBI Bookshelf — Ion Channels and the Electrical Properties of Membranes
• NCBI Bookshelf — Active Transporters Create and Maintain Ion Gradients
• OpenStax — Principles of Bioelectricity
• Nature Reviews Molecular Cell Biology (2024) — Structural biology and molecular pharmacology of voltage-gated ion channels
• Nature Reviews Neuroscience (2025) — Sculpting excitable membranes

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