Nerve impulse propagation
CIE A-Level Biology· Unit 16: Control and Coordination· 15 min read
1. Propagation along unmyelinated axons★★☆☆☆⏱ 4 min
When an action potential occurs at a point on an axon membrane, the membrane becomes depolarised: sodium ion channels open, and the membrane potential changes from around -70mV to +40mV. This depolarisation creates local currents in the cytoplasm of the axon.
Local currents
Small movements of ions along the axon, triggered by the difference in charge between the depolarised active region and the adjacent resting region, that stimulate depolarisation of the next section of membrane.
The flow of positive sodium ions from the active region towards the adjacent resting region lowers the potential difference of the resting membrane, causing it to reach threshold potential. This triggers opening of voltage-gated sodium channels in the next section, generating a new action potential. This process repeats sequentially along the axon.
Explain how an action potential propagates from point A to point B along an unmyelinated axon, where B is immediately distal to A.
- 1
At point A, the action potential is fully generated, so the membrane is depolarised (
+40mV), while point B is still at resting potential (-70mV) - 2
The difference in charge between A and B creates a local current: positively charged sodium ions diffuse from A towards B through the axon cytoplasm
- 3
The arrival of positive ions at B depolarises the B membrane, bringing it from resting potential to threshold potential
- 4
Voltage-gated sodium ion channels open at B, causing sodium influx and a full action potential to be generated at B
- 5
Point A enters its refractory period, preventing the action potential from moving back towards the cell body
Exam tip:
Always mention the refractory period when explaining why propagation is unidirectional - this is a common marking point.
2. Saltatory conduction in myelinated axons★★★☆☆⏱ 5 min
Myelinated axons are wrapped in layers of myelin, an insulating fatty layer produced by Schwann cells. Myelin prevents ion movement across the membrane where it is present, so action potentials can only form at the gaps between myelin segments called nodes of Ranvier.
Saltatory conduction
The process where an action potential 'jumps' from one node of Ranvier to the next along a myelinated axon, rather than being generated sequentially along the entire membrane.
Example:
Conduction speed in a large myelinated mammal axon can be up to 120 m s⁻¹, compared to ~1 m s⁻¹ in an unmyelinated axon of the same diameter.
Local currents formed during an action potential at one node travel through the cytoplasm of the insulated axon segment to the next node. This depolarises the next node to threshold, triggering a new action potential. The action potential effectively jumps between nodes, which is much faster than continuous conduction along unmyelinated axons.
Compare how an action potential travels along a 1cm myelinated axon versus an unmyelinated axon of the same diameter.
- 1
Unmyelinated axon: Action potentials are generated sequentially at every point along the entire 1cm length of the axon membrane.
- 2
Myelinated axon: Myelin insulates most of the membrane, so action potentials only form at nodes of Ranvier (around 1-2mm apart).
- 3
Local currents travel through the insulated axon segment between nodes, so the action potential jumps from node to node.
- 4
This reduces the total number of action potentials that need to be generated, leading to a much faster conduction speed in the myelinated axon.
Exam tip:
When describing saltatory conduction, never say that the action potential 'jumps over the myelin' - say it jumps between nodes of Ranvier, and explain that it is carried by local currents between nodes.
3. Factors affecting conduction speed★★★☆☆⏱ 4 min
Three main factors affect how fast a nerve impulse propagates along an axon: myelin presence, axon diameter, and temperature:
Myelination: Myelin enables saltatory conduction, drastically increasing speed by reducing the number of action potentials needed along the axon.
Axon diameter: A larger diameter axon has less resistance to the flow of ions in local currents, so the current travels further and faster. This increases conduction speed.
Temperature: Higher temperature increases the rate of diffusion of ions, so local currents spread faster, and ion channels open and close more quickly. Speed increases with temperature up to ~40°C, after which proteins denature and speed drops.
A biologist measures conduction speed in three axons of the same species: Axon 1 (10µm diameter, unmyelinated), Axon 2 (20µm diameter, unmyelinated), Axon 3 (10µm diameter, myelinated). Predict the order of conduction speed from slowest to fastest and explain your answer.
- 1
Order from slowest to fastest: Axon 1 < Axon 2 < Axon 3
- 2
Axon 1 is slower than Axon 2: both are unmyelinated, but Axon 2 has a larger diameter, which reduces resistance to ion flow in local currents, leading to faster propagation.
- 3
Axon 3 is faster than both: even though it has the same diameter as Axon 1, it is myelinated. Myelination allows saltatory conduction, which increases speed far more than an increase in diameter for the same size axon.
Check your understanding:
Which of the following changes will decrease the speed of impulse propagation?
A: Increasing axon diameter
B: Myelination of the axon
C: A temperature drop from 37°C to 20°C
D: Increased number of nodes of Ranvier
Reveal answer
C —Correct: lower temperature reduces ion diffusion rate, decreasing speed. Incorrect: A increases speed, B increases speed, D has minimal effect on speed for a given axon length.
4. Unidirectional propagation and the refractory period★★☆☆☆⏱ 2 min
A key feature of nerve impulse propagation is that it always travels in one direction, from the cell body towards the axon terminal. This is entirely due to the refractory period of voltage-gated sodium channels.
Refractory period
A short period after an action potential has passed where sodium channels cannot reopen, preventing another action potential from being generated immediately in that section of membrane.
Because the region behind the active action potential is refractory, the only direction the action potential can propagate is forwards, into the resting, excitable region of the axon ahead. The refractory period also limits the frequency of action potentials that can pass along an axon, which allows the nervous system to encode stimulus intensity by frequency rather than size of action potential.
5. Common Pitfalls
Wrong move:
Saying that myelin speeds up conduction by increasing axon diameter
Why:
Myelin is an insulating layer that does not change axon diameter; the speed increase comes from saltatory conduction, not diameter change
Correct move:
State that myelin insulates the axon, allowing action potentials to only form at nodes of Ranvier, leading to saltatory conduction which is much faster than continuous conduction
Wrong move:
Claiming that the action potential jumps over the myelin sheath between nodes
Why:
This incorrect phrasing loses marks; the action potential is regenerated at each node, carried by local currents between nodes
Correct move:
State that the action potential propagates by jumping from one node of Ranvier to the next along the axon
Wrong move:
Forgetting to link the refractory period to unidirectional propagation
Why:
CIE examiners almost always award a marking point for mentioning the refractory period when explaining unidirectional propagation
Correct move:
Always include that the region behind the active action potential is refractory (sodium channels cannot open), so propagation can only go forwards
Wrong move:
Claiming that higher temperature always increases conduction speed
Why:
Above ~40°C, protein ion channels and membrane structure denature, so conduction speed drops rather than increases
Correct move:
State that speed increases with temperature up to the optimum for the organism, after which denaturation causes speed to decrease
6. Quick Reference Cheatsheet
Feature | Unmyelinated axon | Myelinated axon |
|---|---|---|
Conduction type | Continuous | Saltatory |
Action potential location | Entire axon membrane | Only at nodes of Ranvier |
Relative speed | Slow | Fast |
Energy use for propagation | Higher (more ion movement) | Lower (less ion movement) |
Effect of diameter on speed | Large speed increase | Moderate speed increase |
7. Frequently Asked
Why does impulse propagation only go one way?
After an action potential passes through a section of axon, sodium channels enter a refractory period where they cannot open again. This prevents the action potential from moving backwards towards the cell body, ensuring unidirectional propagation.
How does myelin increase conduction speed?
Myelin insulates the axon membrane, so action potentials can only form at uninsulated nodes of Ranvier. Local currents carry the signal between nodes, reducing the number of action potentials that need to be generated, leading to much faster propagation.
When this came up on past exams
AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2022 · 1
Saltatory conduction speed question
- 2023 · 2
Describe axon propagation
- 2021 · 3
Factors affecting impulse speed
What's Next
Nerve impulse propagation is the precursor to synaptic transmission, where the signal is passed between neurons or from neurons to effector cells. Understanding how impulses travel along axons helps explain how signal timing is controlled in the nervous system, and how conditions like multiple sclerosis (which damages myelin) disrupt nerve function. Next, you can build on this core knowledge to learn how synapses transmit, modulate and integrate signals between neurons in neural pathways.