Afferent nerve fibre affected by local anesthesia first
**Core Concept**
Local anesthesia works by blocking nerve conduction, and the order in which afferent nerve fibers are affected is crucial for understanding the mechanism of action. The afferent nerve fibers are responsible for transmitting sensory information from the periphery to the central nervous system.
**Why the Correct Answer is Right**
The afferent nerve fibers are the first to be affected by local anesthesia because they are the smallest in diameter. This is due to the fact that the local anesthetic agent, such as lidocaine, enters the nerve fiber through the nodes of Ranvier and binds to the sodium channels, blocking the influx of sodium ions. The smaller afferent nerve fibers have a higher surface-to-volume ratio, making them more susceptible to the effects of the local anesthetic.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the efferent nerve fibers, responsible for transmitting motor signals, are not affected first by local anesthesia. This is because they have a larger diameter and are less permeable to the local anesthetic agent.
**Option B:** This option is incorrect because the preganglionic autonomic nerve fibers, responsible for transmitting sympathetic and parasympathetic signals, are not the first to be affected by local anesthesia. This is because they are not the smallest in diameter and have a lower surface-to-volume ratio.
**Clinical Pearl / High-Yield Fact**
The order in which afferent nerve fibers are affected by local anesthesia is a classic exam trap, and students should remember that the smaller afferent nerve fibers are the first to be affected.
**Correct Answer: B. Afferent nerve fibers are the smallest in diameter and are first affected by local anesthesia.**