Question
Which of the following statements correctly describes the mechanism of action of local anesthetics?
| A. |
Local anesthetics preferentially block activated and inactivated voltage-gated sodium channels.
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| B. |
Local anesthetics block the sodium–potassium (Na⁺/K⁺)-ATPase pump.
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| C. |
Large myelinated motor fibers are blocked before pain fibers.
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| D. |
Local anesthetics preferentially bind to resting sodium channels.
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Show Answer
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Correct Answer » A
Explanation
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Local anesthetics produce reversible nerve conduction blockade by blocking voltage-gated sodium (Na⁺) channels from the intracellular side of the nerve membrane.
They have a greater affinity for activated (open) and inactivated sodium channels than for resting channels. This property is known as use-dependent (frequency-dependent) blockade, making rapidly firing nerves more susceptible to blockade.
Why Other Options Are Incorrect
B. Local anesthetics block the sodium–potassium (Na⁺/K⁺)-ATPase pump.
- Incorrect because local anesthetics do not inhibit the Na⁺/K⁺-ATPase pump.
- Their action is specifically on voltage-gated sodium channels.
C. Large myelinated motor fibers are blocked before pain fibers.
- Incorrect because small, lightly myelinated and unmyelinated pain fibers (Aδ and C fibers) are blocked before large motor fibers (Aα).
D. Local anesthetics preferentially bind to resting sodium channels.
- Incorrect because local anesthetics bind preferentially to activated and inactivated sodium channels, not resting channels.
