Factors Affecting The Speed of Conductance
Laura Armstrong
Teacher
Recall Questions
This topic requires prior knowledge of generating an action potential. You can test your knowledge on this below.
What is depolarisation in a neurone?
It is the phase when Na⁺ ions enter the axon, making the inside more positive.
What is the refractory period?
A short time after an action potential during which another impulse cannot be generated.
What is saltatory conduction?
The jumping of action potentials between nodes of Ranvier in a myelinated axon, which speeds up transmission.
Topic Explainer Video
Check out this @LauraDoesBiology video that explains factors affecting the speed of conductance or read the full notes below. Once you've gone through the whole note, try out the practice questions!
What Determines the Speed of Nerve Impulse Transmission?
The speed of conduction of an action potential is affected by three key biological factors:
Myelination
- The myelin sheath insulates the axon.
- It is made up of Schwann cells which wrap around the axon.
- It forces the action potential to jump between the nodes of Ranvier (gaps in the myelin).
- This is called saltatory conduction.
- Much faster than in non-myelinated neurones, where depolarisation must occur along the whole length of the axon membrane.
More myelin = faster conduction
Axon Diameter
- A wider axon has less internal resistance to the flow of ions.
- More space for ions to move = faster diffusion of ions = faster depolarisation.
- Also, less ion leakage in wider axons helps maintain the action potential.
Larger diameter = faster conduction
Temperature
- Affects the rate of diffusion of ions.
- Higher temperature = more kinetic energy and faster ion movement, so depolarisation and repolarisation happen more quickly.
- However, if temperature gets too high, proteins (e.g., ion channels) denature, stopping conduction.
Moderate temperature increases speed, but very high temps can halt transmission.
Summary Table
| Factor | Effect on Speed | Explanation |
| Myelination | Increases speed. | Saltatory conduction between nodes of Ranvier. |
| Axon diameter | Larger diameter = faster speed. | Less resistance to ion flow, less ion leakage. |
| Temperature | Increases speed (up to a point). | Faster diffusion of Ions. |
Key Terms
- Saltatory conduction: Rapid transmission of action potentials in myelinated axons as action potential jumps from node to node.
- Schwann cell: A cell that forms the myelin sheath in myelinated neurones.
- Axon diameter: Width of the neurone’s axon; larger diameter = less resistance to flow of ions = faster conduction.
- Denature: A process where proteins lose their tertiary structure and are unable to function due to high temperature.
No answer provided.
Exam Tips
Don’t just say “faster” – explain why!
In exam questions, always state the biological reason for increased speed:
- “Myelin sheath enables saltatory conduction”.
- “Larger axon diameter reduces resistance to flow of ions”.
- “Higher temperature increases rate of ion diffusion”.
No answer provided.
Explain how the speed of conduction of an action potential is affected by temperature, axon diameter, and the presence of a myelin sheath. (5 marks)
- The Myelin Sheath increases speed by enabling saltatory conduction.
- Where the action potential jumps between nodes of Ranvier / depolarisation only occurs at the nodes.
- In non-myelinated neurones, depolarisation occurs along the entire length of the axon membrane, which is slower.
- A larger axon diameter reduces resistance to the flow of ions, increasing speed.
- Increased temperature raises the rate of ion diffusion speeding up transmission.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!