IB Biology HLNerves & SignallingPaper 1 & 2~10 min read
Nerve Conduction Velocity
Myelin does not make ions move faster. It does something cleverer — it stops most of the membrane from bothering to depolarise at all, so the impulse can skip whole stretches of axon.
📚 What you need to know
The myelin sheath is formed from Schwann cells and insulates the axon membrane.
Where the axon is wrapped in myelin, depolarisation cannot happen, because the sheath stops sodium and potassium ions diffusing across.
The small uninsulated gaps, the nodes of Ranvier, contain clusters of ion pumps and channels, so action potentials can only occur there.
The action potential therefore jumps from one node to the next — saltatory conduction.
The local circuits of current that depolarise the next section stretch between the nodes.
Saltatory conduction lets an impulse travel up to 50 times faster than in an unmyelinated axon of the same diameter.
Conduction velocity is calculated as distance ÷ time.
Saltatory conduction
Think about what has to happen in an unmyelinated axon. Every single section of membrane, one after another, has to open its sodium channels, depolarise, then repolarise. That takes time, and there are a great many sections.
A myelinated axon takes a shortcut:
Along most of the axon the membrane is covered in myelin. Ions cannot cross it, so no depolarisation and no action potentials can happen there.
Only at the nodes of Ranvier is the membrane exposed. These are where the ion pumps and channels are clustered.
When a node depolarises, the local currents inside the axon spread all the way to the next node rather than to the next patch of membrane.
That next node reaches threshold and fires. The action potential has effectively jumped the myelinated stretch.
The red dots are the nodes — the only places with enough ion channels to fire. Everything in between is skipped, which is where the speed comes from.
🧠
Where the word comes from
Saltare is Latin for "to jump". Saltatory conduction is jumping conduction. If you remember the word you remember the mechanism.
A second benefit. Because only the nodes depolarise, far fewer sodium and potassium ions cross the membrane in total. That means less work for the sodium–potassium pumps afterwards, so a myelinated neurone is also more energy efficient. Worth a mark on an "explain the advantages" question.
How much faster?
Saltatory conduction allows an impulse to travel much faster — up to 50 times faster — than in an unmyelinated axon of the same diameter. That last phrase matters: the comparison is fair only if diameter is controlled.
The times shown are illustrative, but the ratio is real. Skipping the insulated stretches is what turns a slow crawl into a fast hop.
Calculating conduction velocity
This is a straightforward speed calculation, but the units are where marks disappear.
Conduction velocity
velocity (m s−1) = distance travelled (m) ÷ time taken (s)
Distance is usually given in mm or cm — convert to metres.
Time is usually given in ms — convert to seconds.
Do both conversions before dividing, and write them down.
In a real experiment you stimulate the nerve at one point and record at another, so the distance is the gap between stimulating and recording electrode. If a question gives you two recording sites, the distance is between them, not from the stimulus.
Worked examples
WE 1
Calculate conduction velocity
A nerve is stimulated and the impulse is recorded 8.0 cm further along the axon after 1.6 ms. Calculate the conduction velocity in m s−1. (3 marks)
Step 1: convert the distance
8.0 cm ÷ 100 = 0.080 mStep 2: convert the time
1.6 ms ÷ 1000 = 0.0016 sStep 3: divide
velocity = 0.080 ÷ 0.0016
50 m s⁻¹show both conversions on separate lines. If your final answer is wrong you can still pick up the conversion marks
WE 2
Explain why myelination increases speed
Explain why an action potential travels faster along a myelinated axon than along an unmyelinated axon of the same diameter. (4 marks)
Point 1: what myelin does
Myelin is a lipid insulator, so ions cannot diffuse across the membrane where it is present.
Point 2: so where can it fire?
Depolarisation and action potentials can only happen at the nodes of Ranvier, where there are clusters of ion channels and pumps.
Point 3: the mechanism
Local circuits of current stretch between one node and the next, bringing the next node to threshold, so the action potential jumps — saltatory conduction.
Point 4: why that is faster
Fewer patches of membrane have to depolarise and repolarise, so the impulse covers the same distance in far less time — up to 50 times faster.
Fewer depolarisations needed, so less time takenthe phrase "of the same diameter" in the question is a hint that diameter is controlled, so do not mention it as a reason
💡 Exam tips
Use the word saltatory conduction and then explain it. The word alone is rarely enough.
Say action potentials happen only at the nodes of Ranvier — that is the key marking point.
Quote up to 50 times faster when comparing.
In any velocity calculation, convert to metres and seconds first.
If an advantage question has spare marks, add energy efficiency: fewer ions move, so less ATP is needed to restore the gradients.
⚠ Common mistakes
Saying the impulse jumps along the outside of the myelin. It is the depolarisation that skips, through local currents inside the axon.
Saying myelin makes the ions move faster. It stops them moving at all where it is present.
Forgetting the nodes have clusters of channels. Without them, nothing could fire there either.
Comparing a myelinated axon with a wider unmyelinated one and calling it evidence for myelination. Two variables have changed.
Leaving the answer in cm per ms. Convert.
Writing "nodes of Ranvia" or "Ranvier nodes". Spell it properly — nodes of Ranvier.
Up next: Synaptic Transmission — what happens when chemicals from outside the body get into the synapse, and how neurones add signals together.
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