Variation in the speed of nerve impulses
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Flip to reveal answersWhat three factors mainly determine the speed of a nerve impulse?
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Question
What three factors mainly determine the speed of a nerve impulse?
Answer
**Myelination**, **saltatory conduction** (nodes of Ranvier), and **axon diameter**.
Question
What is the myelin sheath?
Answer
A **fatty insulating layer** that wraps around the axon of some neurons.
Question
What is a node of Ranvier?
Answer
A **gap in the myelin sheath** where the axon membrane is exposed and the action potential is regenerated.
Question
What is saltatory conduction?
Answer
Conduction in which the impulse **'jumps' from one node of Ranvier to the next**, instead of moving continuously along the membrane.
Question
Why does myelination speed up conduction?
Answer
The myelin **insulates** the axon, so the impulse only forms at the nodes and **jumps** between them — much faster than continuous conduction.
Question
Where does depolarization occur on a myelinated axon?
Answer
**Only at the nodes of Ranvier** — the gaps in the myelin; the sheath insulates the rest of the axon.
Question
How does axon diameter affect conduction speed?
Answer
A **wider** axon conducts **faster** because it has **less internal resistance** to the flow of charge.
Question
Which conducts faster: a myelinated or an unmyelinated axon?
Answer
A **myelinated** axon — it uses fast saltatory conduction; an unmyelinated axon conducts slowly and continuously.
Question
Of four axons, which conducts a nerve impulse most slowly?
Answer
The **thin, unmyelinated** one — no saltatory conduction and high internal resistance.
Question
Of four axons, which conducts a nerve impulse fastest?
Answer
The **wide, myelinated** one — saltatory conduction plus low internal resistance.
Question
What does the word 'saltatory' mean, and why is it apt?
Answer
It comes from the Latin for **'to jump'** — the impulse leaps from node to node.
Question
Why is a myelinated axon described as 'insulated'?
Answer
The fatty myelin sheath acts like the plastic coating on a wire, **insulating** the axon so the impulse only forms at the bare nodes.
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