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Chapter 3. Histology of the Peripheral Nerves and Light Microscopy

Which of the following nerve fibers would be the first to be blocked by local anesthetics?

A. Aβ fibers

B. Aα fibers

C. Aγ fibers

D. C fibers

C is correct. Aγ myelinated fibers supply the muscle spindles. They have a smaller diameter than other myelinated fibers. Myelinated nerve fibers with a smaller diameter are more rapidly blocked because the “critical length” of the smaller nerve fiber contains a larger number of Ranvier nodes than a larger myelinated nerve fiber.

A and B are incorrect. Proprioceptive afferent (Aα) and motor efferent (Aβ) nerve fibers have the same diameter and are therefore equally sensitive to local anesthetics. The sensitivity of nerve fibers to local anesthetics is not determined by whether they are sensory or motor, but rather by their diameter.

D is incorrect. C fibers have the smallest diameter. However, they are unmyelinated. Because myelinated fibers are more easily blocked than unmyelinated fibers, C fibers are more resistant to the action of local anesthetics.

Which of the following local anesthetics would you choose for effective sciatic nerve block using Labat’s posterior approach?

A. 30 mL of 1% mepivacaine

B. 20 mL of 1.5% mepivacaine

C. 20 mL of 0.75% bupivacaine

D. 30 mL of 0.2% ropivacaine

B is correct. The dose and concentration of local anesthetic should be optimized for different nerve blocks. The larger the nerve, the more concentrated the local anesthetic must be to obtain effective neural blockade.

A is incorrect. A high volume with a low concentration of local anesthetic solution is associated with a lower success rate and a delayed onset time compared to a low volume with a high concentration of the same local anesthetic.

C is incorrect. Bupivacaine provides longer duration of nerve blockade compared to other commonly used local anesthetics. However, it also has the worst cardiotoxic profile. Bupivacaine carries a significantly higher risk of cardiac arrest and difficulties in resuscitation. This is especially important because currently there is no monitoring that could prevent systemic toxicity of local anesthetics. Bupivacaine has fallen out of favor in many centers due not only to its potential for serious toxicity, but also the availability of ropivacaine, a local anesthetic characterized by a slightly decreased duration of action and an improved safety profile.

D is incorrect. Ropivacaine 0.2% is usually sufficient to provide excellent sensory analgesia but ...

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