PreACT Exam Prep

Category - PLAN High-School Assessment Practice Test Questions

What is the function of the myelin sheath that surrounds neurons?
  1. Increase speed of impulses
  2. Maintain polarization
  3. Decrease friction
  4. Control growth of neurons
  5. Oxygen exchange
Explanation
Answer: A, Increase speed of impulses. The myelin sheath serves exclusively to increase the speed at which impulses travel across neurons.

Key Takeaway: The myelin sheath increases electrical resistance across a cell membrane by a factor of 5, 000 and decreases capacitance by a factor of 50. Combined, these two factors prevent the electrical current from escaping the axon, thus improving the speed at which impulses travel.
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