Myelination and saltatory conductionSpec 8.5
In short
In a myelinated axon, the myelin sheath insulates the membrane so depolarisation can occur only at the nodes of Ranvier. Local circuits make the action potential jump from node to node, which is saltatory conduction. This is much faster than conduction along an unmyelinated axon.
In an unmyelinated axon the action potential must be regenerated at every point along the membrane, so conduction is slow (about 0.5 to 2 m s⁻¹). In a myelinated axon, the myelin sheath is an insulator, so ions can enter and leave only at the nodes of Ranvier. The voltage-gated Na⁺ and K⁺ channels are concentrated at the nodes.
- An action potential at one node makes Na⁺ enter and depolarise the node.
- Local circuits carry the positive charge along inside the axon, through the myelinated section, to the next node.
- The next node reaches threshold and an action potential forms there.
- The impulse appears to jump from node to node. This is saltatory conduction (Latin saltare, to leap).
Because depolarisation occurs only at the nodes, conduction can reach about 100 m s⁻¹. Fewer ion channels need to open and fewer ions move, so less ATP is needed for the sodium–potassium pumps to restore the resting potential.
The impulse does not travel through the myelin. The action potential is generated at the nodes, and local circuits carry the depolarisation between them.
Conduction velocity
An impulse travels 0.45 m along a myelinated motor axon in 5.0 ms. Calculate the speed of conduction in m s⁻¹.
- Convert time: 5.0 ms = 5.0 × 10⁻³ s
- speed = distance ÷ time = 0.45 ÷ 5.0 × 10⁻³
Answer: 90 m s⁻¹
Convert to SI units before dividing: milliseconds to seconds, millimetres to metres.
Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026