Question

Rank the sensitivity of each nerve fiber type to the physiologic effects of LA in vivo. Most sensitive to least sensitive.

a) A delta, A alpha, B and C fibers
b) B, C fibers, A delta, A alpha
c) A alpha, A delta, B and C fibers
d) B, C fibers, A alpha, A delta

Answer 1

Local anesthetics are more effective on nerves with smaller diameters and those without myelination. C fibers and B fibers are the most sensitive, followed by A delta fibers and A alpha fibers, which are the least sensitive. The correct sensitivity ranking is B, C fibers, A delta, A alpha.

Step-by-step explanation:

The question pertains to the sensitivity of nerve fiber types to local anesthetics (LAs) in a physiological in vivo context. In general, nerve fibers can be classified by diameter and myelination, which affect their conduction velocity and sensitivity to LAs. The nerve fibers are categorized as follows:

• A alpha fibers are thick, myelinated axons that are primarily involved in motor function and the most rapid proprioception.
• A delta fibers are thin, myelinated axons that carry pain and temperature sensations.
• B fibers are myelinated preganglionic autonomic fibers.
• C fibers are thin, unmyelinated axons that carry slow pain and temperature sensations, as well as postganglionic sympathetic fibers.