A neurone spends its whole life holding a charge across its membrane, ready. An impulse is what happens when that charge flips for about a thousandth of a second — and then flips straight back.
📚 What you need to know
An impulse is not a current flowing along a wire. It is a brief reversal of the potential difference across the membrane.
At rest, the inside of the axon is negative relative to the outside. This resting potential is about −70 mV.
Sodium–potassium pumps use ATP to move 3 Na+ out for every 2 K+ in, setting up concentration gradients.
The membrane is less permeable to Na+ than to K+, so K+ diffuses out faster than Na+ diffuses back in.
Negatively charged proteins inside the axon add to the negative charge.
A membrane at resting potential is polarised. Reversing it is depolarisation.
An action potential takes the membrane from about −70 mV to about +40 mV and back again.
What an impulse really is
This is the idea to get straight before anything else. Neurones are not wires and the impulse is not electricity flowing along them.
Instead, there is a difference in electrical charge across the cell surface membrane — a potential difference, also called the membrane potential or simply the voltage across the membrane. An impulse is a brief, local reversal of that difference, which then triggers the same reversal in the patch of membrane next door, and so on down the axon. What travels is the reversal, not the ions.
A useful picture: a line of dominoes. Nothing travels the length of the line except the falling — each domino only moves a centimetre. The wave moves; the pieces barely do.
The resting potential
When an axon is not transmitting anything, the inside is negative compared with the outside, by about 70 millivolts. That is the resting potential, and a membrane in that state is described as polarised.
The minus sign in −70 mV is not decoration. It tells you the inside is the negative side, which is exactly what the reversal later undoes.
Two processes create it
The resting potential is not an accident. It is actively built and maintained by two things working together.
1. Active transport by the sodium–potassium pump
Carrier proteins called sodium–potassium pumps sit in the membrane.
They use ATP to actively transport sodium ions out of the axon and potassium ions in.
The rate is unequal: 3 Na+ out for every 2 K+ in.
This builds a concentration gradient for both ions — high Na+ outside, high K+ inside — and, because more positive charge leaves than enters, it directly makes the inside more negative.
2. Different rates of diffusion back
Both gradients make the ions want to diffuse back, through sodium ion channels and potassium ion channels, by facilitated diffusion.
But the membrane is much less permeable to sodium ions than to potassium ions.
So K+ leaks out faster than Na+ leaks back in.
Positive charge builds up outside, leaving the inside more negative still.
Two separate reasons the inside ends up negative: the pump’s unequal 3:2 ratio, and potassium leaking out faster than sodium leaks in.
Don’t forget the third contributor: large negatively charged proteins are stuck inside the axon and cannot cross the membrane. They add to the negative charge without moving anywhere.
The action potential
To send an impulse, the membrane has to be depolarised — the potential difference reversed. When that happens, an action potential is generated.
The reversal
about −70 mV → about +40 mV → back to about −70 mV
Read the y-axis carefully. The trace passes through zero on the way up, which means the inside is briefly positive relative to the outside — a complete reversal.
An action potential is triggered when a neurone is stimulated — for example when a receptor cell detects a change in the environment.
It involves the rapid movement of sodium and potassium ions across the axon membrane.
Depolarisation is the reversal itself: sodium ions rush in and the inside becomes positive.
Repolarisation restores the negative inside, and the membrane returns to resting potential.
Watch your vocabulary here, because three similar words do three different jobs. Polarised is the resting state. Depolarised is the reversed state. Repolarisation is the process of getting back.
Worked examples
WORKED EXAMPLE
Explain how the sodium–potassium pump helps to establish the resting potential. [3]
Step 1: name the process and its costactive transport, using ATPStep 2: give the direction and the ratio3 Na⁺ out for every 2 K⁺ inStep 3: state the two consequences
Concentration gradients are created for both ions, and more positive charge leaves than enters.
Unequal active transport makes the inside more negative and sets up the gradientsthe 3:2 ratio is worth a mark on its own — always quote it
WORKED EXAMPLE
The inside of a resting axon is at −70 mV. During an action potential it reaches +40 mV. Calculate the total change in membrane potential.
Step 1: subtract, keeping the signs+40 − (−70) = +40 + 70Step 2: evaluate= 110A change of 110 mVsubtracting a negative adds — answering “30 mV” means you dropped the sign
WORKED EXAMPLE
A neurone is treated with a drug that blocks ATP production. Predict the effect on the resting potential and explain why. [3]
Step 1: identify what needs ATP
The sodium–potassium pump uses ATP for active transport.
Step 2: what stops happening
Ions are no longer pumped, so the concentration gradients gradually break down.
Step 3: the consequence for the charge
Ions diffuse until the difference in charge disappears.
The resting potential is lost, so no impulses can be transmittednote the word “gradually” — the existing gradients take time to run down
💡 Exam tip
Quote the values: −70 mV resting and about +40 mV at the peak. Numbers make an answer specific.
Say potential difference across the membrane, not “electricity” or “current”.
Name the transport type each time: the pump is active transport; the channels are facilitated diffusion.
Always mention that the membrane is less permeable to sodium when explaining resting potential.
Keep the sign on the voltage. −70 mV describes the inside relative to the outside.
Use polarised, depolarised and repolarisation precisely. They are not interchangeable.
⚠ Common mix-up
Describing the impulse as electricity flowing along the axon. It is a travelling reversal of membrane charge.
Getting the pump ratio backwards. Three sodium out, two potassium in.
Saying the pump is diffusion. It is active transport and it costs ATP.
Forgetting the permeability difference. The pump alone does not fully explain the resting potential.
Writing that the outside is negative. The inside is the negative side at rest.
Mixing up depolarisation and repolarisation. “De” is the reversal; “re” is the recovery.
Thinking any stimulus produces an action potential. It has to reach the threshold first.
Up next: Nerve Impulses (Skills) — what makes one axon faster than another, saltatory conduction, and how to test a correlation properly.
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