Impulse speed increases with axon diameter and myelination because insulation and wider axons reduce resistance to action-potential propagation, linking cellular mechanisms to rapid communication in nervous systems.
Coverage 2025–2025 · Updated 15 Jul 2026
Impulse conduction speed increases with axon diameter and myelination because both allow local currents to influence the next excitable membrane region more rapidly.
| Comparison | Faster case | Mechanistic reason |
|---|---|---|
| Giant squid axon vs smaller unmyelinated fibre | Giant squid axon | Larger diameter lowers internal resistance |
| Myelinated vs unmyelinated fibre | Myelinated fibre | Insulation limits leakage and impulses regenerate mainly at nodes |
Describe a positive correlation when conduction speed rises as axon diameter rises, and a negative correlation when one variable falls as the other rises. The correlation coefficient r gives direction and strength; R² estimates the proportion of variation in conduction speed explained by the fitted relationship.
If r = 0.90 for diameter and speed, the association is strong and positive; R² = 0.81 means about 81% of the observed variation in speed is explained by the fitted relationship, not that diameter is proven to be the sole cause.
Correlation does not by itself prove causation. Faster impulses do not have larger action-potential amplitudes; speed and signal strength are different properties.
This objective is assessed through multiple choice, commonly using Describe / Determine / Calculate.
Describe / Determine / Calculate / Suggest / Outline
Build the answer around this relationship: Larger axon diameter is associated with faster impulse transmission.
Representative question
The diagrams represent sections through different axons. Which axon has the slowest speed of impulse?
Neurons carry electrical impulses in the nervous system; motor, sensory, and relay neurons differ in axon, dendrite, and cell body arrangement. Sodium-potassium pumps use ATP to move 3 Na+ out and 2 K+ in; ion gradients make the resting axon polarized at about -70 mV. A nerve impulse is a propagated action potential along a nerve fibre; stimulus-triggered sodium influx reverses membrane polarity. Larger axon diameter lowers resistance and increases impulse speed; myelin sheaths and nodes of Ranvier enable faster saltatory conduction. Synapses connect neurons to neurons, muscles, or glands; chemical synapses transmit one way across a narrow synaptic cleft. Action potentials open voltage-gated Ca2+ channels in presynaptic terminals; Ca2+ causes vesicle fusion and neurotransmitter exocytosis into the cleft. Neurotransmitters diffuse and bind receptors on the postsynaptic membrane; EPSPs depolarize the membrane and make threshold more likely.
