International A Level Biology / The nervous system and neurones
How an impulse travels: local circuits and saltatory conduction
Edexcel International A Level Biology (YBI11), chapter 23: The nervous system and neurones, specification point 8.4, 8.5. Part of the full course.
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An impulse travels along an axon because local currents bring the next section of membrane to threshold, causing it to fire its own action potential. Myelin makes conduction faster by allowing the impulse to jump between nodes of Ranvier.