A researcher applies a depolarizing current to a node of Ranvier on a myelinated axon. The action potential jumps to the next node 1 mm away in 0.02 ms. Which property of myelin is most directly responsible for this saltatory conduction?

• A Myelin increases the density of voltage-gated Na⁺ channels along the internodal membrane
• B Myelin provides metabolic support to the axon that speeds active ion pumping
• C Myelin prevents K⁺ efflux during repolarization, sustaining the depolarized state along the axon
• D Myelin reduces membrane capacitance and increases membrane resistance of the internodal segment, allowing rapid electrotonic spread of current to the next node ✓

Explanation

The myelin sheath insulates the internodal membrane, dramatically decreasing its capacitance (less charge needed to change voltage) and increasing its resistance (minimal ionic leak). This allows the depolarizing current generated at one node to spread electrotonically—almost instantaneously—to the next excitable node of Ranvier, where a new action potential is regenerated. Voltage-gated Na⁺ channels are concentrated at nodes, not in the internodal myelin-covered segments.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.