How does the thickness of a nerve fiber affect local anesthetic blocking?

The thickness of a nerve fibre is a primary factor in the selective ability of local anesthetics to block conduction. In general, thinner nerve fibres are more susceptible to these drugs and are blocked first.1

This sensitivity is clinically significant because the fibres responsible for transmitting dull aching pain are among the thinnest, allowing them to be targeted while often sparing motor function at normal dosages. Conversely, thicker fibres require more extensive exposure or higher concentrations of the anesthetic to achieve a similar block. The overall effectiveness of the block also depends on the length of the fibre that must be affected to successfully interrupt the transmission of nerve impulses.1