Practice question · Multiple choice
In multiple sclerosis the immune system strips myelin from axons. Signals slow dramatically and often fail entirely. Why does losing the insulation stop the signal rather than merely weakening it?
Hints
- Saltatory conduction means the spike jumps between nodes. Ask what has to survive the jump.
- The signal decays as it travels passively. Myelin decides how far it must travel unaided.
Show the answer
A. Because the spike is regenerated only at the gaps in the sheath
Why
Myelin does not carry the signal, it forces the spike to leap between nodes, and each landing must still be strong enough to fire the next. Strip it and the current bleeds out across the bare membrane, so the relay fails rather than merely slows. Insulation was doing load-bearing work.
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