Speed of nerve impulsesSpec C2.2.4, C2.2.11
In short
Impulse speed rises with axon diameter and myelination. Squid giant axons, correlation coefficients and R².
The speed of nerve impulses varies a lot between nerve fibres. Two factors matter most: axon diameter and myelination.
| Fibre | Diameter | Approximate speed |
|---|---|---|
| Small non-myelinated fibre | About 1 µm | About 1 m s⁻¹ |
| Squid giant axon (non-myelinated) | Up to about 0.5–1 mm | About 25 m s⁻¹ |
| Myelinated vertebrate fibre | About 10–20 µm | Up to about 100 m s⁻¹ |
- Squid giant axons vs smaller non-myelinated fibres: a wider axon has less resistance to the flow of ions inside it, so local currents spread further and faster. The giant axon allows a rapid escape response.
- Myelinated vs non-myelinated fibres: myelinated fibres conduct far faster, even when much narrower, so many fast fibres fit into a small nerve.
Correlation and the coefficient of determination
- A positive correlation: one variable rises as the other rises, e.g. conduction speed and axon diameter.
- A negative correlation: one variable falls as the other rises. The guide's example is conduction speed and animal size. Published data for this vary between groups of animals, so in an exam describe the trend that the data you are given actually show.
- The correlation coefficient, r, runs from −1 to +1. Values near ±1 show a strong correlation; values near 0 show little or none.
- The coefficient of determination, R², is r squared. It gives the proportion of the variation in the dependent variable that is explained by variation in the independent variable.
- Correlation does not by itself show causation.
Using r and R²
For a set of non-myelinated axons, the correlation coefficient between axon diameter and conduction speed is r = 0.90. Calculate R² and interpret it.
- R² = r² = 0.90²
- R² = 0.81
- 0.81 = 81%: 81% of the variation in conduction speed is explained by variation in axon diameter.
- The other 19% is due to other factors, such as temperature or measurement error.
Answer: R² = 0.81; a strong positive correlation, with 81% of the variation in speed explained by diameter
Maths skill:
State the direction (positive or negative) and strength of a correlation from r. Use R² only to say how much of the variation is explained, and never claim it proves cause.
Written and checked against the IB Biology SL specification · Updated October 2026