IB Biology HLNerves & SignallingPaper 1 & 2~11 min read
Nerve Impulses
A neurone doing nothing is not really doing nothing. It is spending energy every second to hold a charge across its membrane, like keeping a battery charged so it is ready the instant you need it.
📚 What you need to know
An impulse is not a current flowing along a wire. It is a momentary reversal of the potential difference across the neurone membrane.
In a resting axon the inside is negative compared with the outside. This is the resting potential, about −70 mV.
Resting potential is set up by two things: the active transport of sodium and potassium ions, and a difference in how fast they diffuse back.
Sodium–potassium pumps use ATP to move 3 Na+ out for every 2 K+ in.
The membrane is much less permeable to Na+ than to K+, so K+ leaks out faster than Na+ leaks back in.
Negatively charged proteins trapped inside the axon add to the negative charge.
A resting membrane is polarised. To send an impulse it must be depolarised, which reverses the potential from about −70 mV to about +40 mV — an action potential.
What an impulse actually is
It is worth clearing this up straight away, because the picture most people start with is wrong.
An impulse is not electricity running down the neurone the way current runs down a copper wire. Nothing is flowing from one end to the other. What actually moves is a change in charge across the membrane — a small patch of membrane flips its charge, that flip triggers the next patch, and so on down the axon. What travels is the flipping, not the charge.
DefinitionMembrane potential = the difference in electrical charge between the inside and the outside of the membrane. You will also see it called the potential difference or the voltage across the membrane.
Think of a line of dominoes rather than a hosepipe. No single domino travels the length of the line, but the falling does. That is why an impulse does not get weaker as it goes — every section is knocked over just as hard as the one before it.
Building the resting potential
In an axon that is not carrying an impulse, the inside always has a negative electrical potential compared with the outside. That difference is the resting potential, and it is usually about −70 mV. That number means the inside is 70 millivolts more negative than the outside.
Two processes build it, and one extra factor helps.
1. Active transport by the sodium–potassium pump
Carrier proteins called sodium–potassium pumps sit in the cell surface membrane of the neurone.
They use ATP to actively transport sodium ions (Na+) out of the axon and potassium ions (K+) in.
The two ions are not swapped evenly. For every 3 sodium ions pumped out, only 2 potassium ions are pumped in.
This builds a concentration gradient for both ions: high Na+ outside, high K+ inside.
2. Different rates of diffusion back
Because of those gradients, both ions want to diffuse back the other way through ion channels, by facilitated diffusion.
But the membrane is much less permeable to sodium ions than to potassium ions. There are simply more open potassium channels in a resting neurone.
So K+ leaks out faster than Na+ leaks in. Positive charge drains out of the cell faster than it comes back.
The result: far more positive ions on the outside than the inside, which leaves the inside negative.
3. Proteins that cannot leave
Large negatively charged proteins are stuck inside the axon. They cannot cross the membrane, so they sit there adding to the negative charge on the inside. This is a supporting factor rather than a main one, but it is worth a mark if the question asks for everything contributing to resting potential.
The pump does two jobs at once: it builds the concentration gradients, and because it moves three positives out for two in, it directly removes positive charge from the inside.
The charges only build up in a thin layer right against the membrane. The bulk of the cytoplasm is not negative — it is the imbalance across the membrane that counts.
From resting to firing
Once resting potential is reached, the membrane is said to be polarised. To start an impulse, the membrane has to be depolarised.
Depolarisation is the reversal of the potential difference across the membrane.
It happens when an action potential is generated.
An action potential takes the membrane from about −70 mV to about +40 mV — the inside briefly becomes positive compared to the outside.
It happens because of the rapid movement of sodium and potassium ions across the axon membrane.
Something has to trigger it — usually a stimulus picked up by a receptor cell, or a signal arriving from another neurone.
Where the energy goes. The gradients built by the pump are stored energy, like water held behind a dam. Depolarisation does not need ATP — it just opens the gates and lets the ions rush down gradients that were paid for earlier. That is why an impulse can be so fast.
Worked examples
WE 1
Explain how a resting potential is established
Explain how a resting potential of −70 mV is established and maintained across the membrane of an axon. (4 marks)
Point 1: the pump
Sodium–potassium pumps use ATP to actively transport 3 Na⁺ out for every 2 K⁺ in.
Point 2: the gradients
This creates a concentration gradient for both ions — high sodium outside, high potassium inside.
Point 3: unequal diffusion
The membrane is more permeable to potassium than sodium, so K⁺ diffuses back out faster than Na⁺ diffuses back in.
Point 4: the outcome
More positive ions end up outside than inside; negatively charged proteins inside add to this, leaving the inside about 70 mV more negative.
Pump out unevenly, leak back unevenly, inside ends up negativeboth processes must appear — a full answer about the pump alone usually caps at 2 marks
WE 2
Predict the effect of a respiratory inhibitor
A neurone is treated with a chemical that stops mitochondria producing ATP. Predict and explain the effect on the resting potential. (3 marks)
Step 1: what needs ATP
Only the sodium–potassium pump needs ATP; diffusion through channels does not.
Step 2: what stops
Without ATP the pump stops, so no more ions are actively transported and the gradients are no longer topped up.
Step 3: what happens next
Ions keep diffusing down the existing gradients until those gradients disappear, so the charge difference across the membrane falls towards zero.
Resting potential is gradually lost — the neurone can no longer firesay gradually. The gradients already there take time to run down, so the effect is not instant
💡 Exam tips
Learn the two numbers cold: resting potential −70 mV, peak of an action potential +40 mV.
Learn the pump ratio as a phrase: three sodium out, two potassium in.
Always name the transport type. Pump = active transport. Channels = facilitated diffusion.
If the question says "explain how resting potential is maintained", give both the pumping and the permeability difference.
Use the words polarised and depolarised — they are marking-point vocabulary.
⚠ Common mistakes
Describing the impulse as electricity flowing along the neurone. It is a travelling reversal of membrane potential.
Saying the pump moves ions down a concentration gradient. It moves them against the gradient, which is why it needs ATP.
Swapping the ions. Sodium out, potassium in. Get this backwards and the whole answer collapses.
Saying the inside of the axon is negatively charged overall. Only the layer next to the membrane is; the charge difference is what matters.
Forgetting the permeability difference. The pump alone does not explain −70 mV.
Writing −70 V instead of −70 mV. Millivolts. A neurone is not a mains socket.
Up next: Nerve Impulses: Skills — what makes one neurone faster than another, and how to handle the correlation maths the IB attaches to it.
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