Explain saltatory conduction in myelinated neurones
Relate myelin and nodes of Ranvier to rapid impulse transmission.
A Level; Cambridge International Biology 9700, 2028–2030. Written answers are self-checked; this is not practical-skills certification.
Before you begin
- A control pathway links a stimulus, receptor, coordination system and response.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Where are action potentials regenerated during saltatory conduction?
What you will learn
- Describe how myelin insulates axon membrane between nodes
- Explain how action potentials are regenerated at nodes of Ranvier
Saltatory conduction in a myelinated axon
| Feature | Meaning |
|---|---|
| Myelin | Insulates most of the axon membrane |
| Nodes | Exposed gaps where action potentials are regenerated |
| Between nodes | Local current spreads beneath the myelin |
| Effect | Faster impulse transmission than continuous regeneration along an unmyelinated axon |
Build the mechanism
Myelin forms an insulating sheath around much of an axon. Gaps between myelin segments are nodes of Ranvier, where voltage-gated channels are concentrated. Local current spreads rapidly beneath the myelin and depolarises the next node to threshold; a new action potential is generated there. This saltatory pattern is faster than regenerating an action potential at every adjacent point of an unmyelinated axon.
Keep the sequence precise
The action potential is regenerated at the nodes; it does not jump through a gap with no current. Myelin reduces ion exchange across covered regions and supports faster conduction.
Pause and explain
A student says an impulse jumps through the air between nodes. Correct the explanation.
Worked example
Why can a myelinated axon conduct an impulse faster than an unmyelinated axon?
Show the worked solution
- Myelin insulates regions between nodes.
- The action potential is regenerated at separated nodes rather than every adjacent point.
Answer Myelin insulates internodes, so local current rapidly depolarises the next node. Action potentials are regenerated at nodes of Ranvier, producing faster saltatory conduction.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Trace impulse transmission from one node of Ranvier to the next.
Give me a hint
Name the first event, then follow each link in order.
Compare my reasoning
- Current spreads under the insulating myelin.
- The next node reaches threshold and generates an action potential.
An action potential at one node creates local current that spreads beneath the myelin. It depolarises the next node to threshold, where a new action potential is generated.
Look for these in your work
- Mechanism stated
- Steps linked in order
A demyelinating condition exposes more internodal membrane. Predict how conduction may change.
Give me a hint
Use the terms in the question and state what changes.
Compare my reasoning
- Insulation is reduced.
- More membrane may need to be depolarised and conduction can slow or fail.
Reduced myelin insulation can slow conduction because local current is less effectively confined between nodes; the next node may take longer to reach threshold or may fail to do so.
Look for these in your work
- Evidence interpreted
- Biological direction explained
A student says an impulse jumps through the air between nodes. Correct the explanation.
Give me a hint
Apply the mechanism to the new context without adding unsupported claims.
Compare my reasoning
- There is no air gap in the axon.
- Local electrical current spreads beneath the myelin to the next node.
The impulse does not jump through air. Local current spreads within the axon beneath insulating myelin, and an action potential is regenerated at the next node of Ranvier.
Look for these in your work
- Mechanism applied
- Limits preserved
Common mistakes
- Saying myelin itself carries the electrical impulse
- Saying all ion movement stops along the axon
What can you explain now?
Where are action potentials regenerated during saltatory conduction?
Compare with the explanation
At nodes of Ranvier
The nodes are gaps between myelin segments with excitable membrane.
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Make it stick
What makes conduction saltatory, and why does that increase speed?
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