Which type of fiber do local anesthetics predominantly affect first?

Local anesthetics primarily affect unmyelinated C fibers first due to their functional and structural characteristics. C fibers are smaller in diameter and unmyelinated, which makes them more susceptible to the effects of local anesthetics. These agents work by blocking sodium channels, which is crucial for the conduction of nerve impulses. As C fibers transmit pain sensations, the ability of local anesthetics to block these fibers first results in the loss of pain sensation more rapidly than other types of fibers.

In contrast, myelinated A fibers, particularly A-delta and A-beta fibers, are larger and have a higher conduction velocity. They are less susceptible to local anesthetics compared to the smaller, unmyelinated C fibers, and may require a higher concentration of the anesthetic or a longer duration of exposure to produce significant conduction block. Myelinated B fibers are also affected by local anesthetics, but their influence is intermediate relative to the unmyelinated C fibers. Ultimately, while local anesthetics can impact various types of nerve fibers, the unmyelinated C fibers are the primary target, leading to a distinct sequence of sensory loss starting with pain.