Saltatory conduction at Higher Level: An AHL statement: why myelinated fibres conduct so fast.
Practise this as you read
- Outline saltatory conduction.
- Explain the role of the nodes of Ranvier.
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The big idea: In a myelinated fibre the myelin insulates the axon, and the sodium channels and pumps are clustered at the nodes of Ranvier. So an action potential can only form at the nodes: local currents carry the signal to the next node, which fires in turn. The impulse jumps from node to node: saltatory conduction, much faster.
The points to remember
- Myelin insulates the axon between the nodes.
- Channels and pumps are clustered at the nodes of Ranvier.
- Action potentials form only at the nodes.
- Local currents reach the next node, which fires.
- The impulse jumps node to node: saltatory, much faster.
Remember it as: Insulate, cluster, jump.
Real example: multiple sclerosis. In multiple sclerosis the immune system damages myelin in the brain and spinal cord. Impulses then slow down or fail where the myelin is lost, which causes problems with movement and sight.
Trap: The impulse does not travel through the myelin: myelin insulates; the action potentials happen at the nodes.
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How it is asked: Expect an Outline of how myelin and nodes speed up an impulse, an Explain of saltatory conduction, or an MCQ on where depolarization occurs on a myelinated neuron.
IB-style questionOutline[2 marks]
Outline how saltatory conduction makes impulses faster in myelinated fibres.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.