AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
The passage of an action potential along non-myelinated and myelinated…
Full specification wording
Nerve impulses (A-level only): The passage of an action potential along non-myelinated and myelinated axons, resulting in nerve impulses.
An action potential is not a substance that travels; it is regenerated over and over along the membrane.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 4 of 6
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
An action potential is not a substance that travels; it is regenerated over and over along the membrane. In a non-myelinated axon, the sodium ions that enter at the depolarised point spread sideways inside the axon as local currents, depolarising the adjacent patch of membrane to threshold, which opens its sodium ion channels and produces a fresh action potential there. This repeats continuously along the whole length of the membrane, so conduction is relatively slow. In a myelinated axon the sheath insulates most of the membrane, so ions cannot cross except at the nodes of Ranvier. Local currents therefore run from one node to the next and the action potential is regenerated only at the nodes, which is saltatory conduction, the impulse appearing to jump.
What earns the marks
- Local currents, meaning sodium ions spreading along the inside of the axon to the next region of membrane.
- The next region being depolarised to threshold so that a new action potential is generated there.
- Depolarisation occurring along the whole length of the membrane in a non-myelinated axon.
- Myelin preventing ion movement across the membrane except at the nodes of Ranvier.
- Saltatory conduction, the action potential jumping from node to node, giving faster transmission.
Common mistakes and how to put them right
Saying the same action potential physically travels along the axon rather than being regenerated at each point.
Claiming the impulse jumps over the axon in a myelinated neurone, instead of between nodes on the same axon.
Forgetting that a non-myelinated axon still conducts, just more slowly.
Attributing one-way travel to the myelin sheath rather than to the refractory period behind the wave.
Describing local currents as an electric current in a wire rather than as ion movement.
Exam tips
- Use the word regenerated; it separates a describing answer from an explaining answer.
- When comparing the two axon types, make the comparison explicit in both directions rather than describing each separately.
- Link speed back to how much of the membrane has to be depolarised, not simply to the presence of myelin.
Try the lesson
The start of the Nerve impulses (A-level only): The passage of an action potential along non-myelinated and myelinated axons, resulting in nerve impulses. lesson, made from checked teaching notes for this specification point.
Have a go first
In a non-myelinated axon, how does an action potential pass along the axon?
Listen
An action potential is not a substance that travels; it is regenerated over and over along the membrane. In a non-myelinated axon, the sodium ions that enter at the depolarised point spread sideways inside the axon as local currents, depolarising the adjacent patch of membrane to threshold, which opens its sodium ion channels and produces a fresh action potential there. This repeats continuously along the whole length of the membrane, so conduction is relatively slow. In a myelinated axon the sheath insulates most of the membrane, so ions cannot cross except at the nodes of Ranvier. Local currents therefore run from one node to the next and the action potential is regenerated only at the nodes, which is saltatory conduction, the impulse appearing to jump. A strong answer earns marks for these points. local currents, meaning sodium ions spreading along the inside of the axon to the next region of membrane. the next region being depolarised to threshold so that a new action potential is generated there. depolarisation occurring along the whole length of the membrane in a non-myelinated axon. myelin preventing ion movement across the membrane except at the nodes of Ranvier. Exam tip: Use the word regenerated; it separates a describing answer from an explaining answer.
Key words
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local currents
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saltatory conduction
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nodes of Ranvier
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regeneration of the action potential
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See every definition
- local currents
- Sodium ions spreading sideways inside the axon from the depolarised point, depolarising the adjacent patch of membrane to threshold.
- saltatory conduction
- The action potential being regenerated only at the nodes, so the impulse appears to jump from node to node.
- nodes of Ranvier
- The points where ions can cross the membrane, because the sheath insulates most of the membrane elsewhere.
- regeneration of the action potential
- A fresh action potential produced at the next region of membrane once local currents have depolarised it to threshold.
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