How do local anesthetics block nerve impulses?

Local anesthetics block nerve impulses by preventing the transmission of electrical signals along nerve fibers. Under normal conditions, a nerve impulse is propagated when ion channels in the nerve membrane open, allowing an influx of sodium ions that creates small electrical currents.1

Because local anesthetic molecules are hydrophobic, they are able to penetrate the fatty membrane of the nerve fiber. Once inside, these molecules block the sodium channels from the interior of the cell. By preventing the movement of sodium ions, the anesthetic stops the flow of the electrical currents necessary to transport the impulse, effectively inducing a temporary loss of sensation.1

The effectiveness of this process is influenced by the diameter of the nerve fibers; thinner fibers, such as those conveying dull aching pain, are generally the most susceptible to being blocked first. The action of the anesthetic concludes as the drug is dispersed, metabolized, and excreted by the body.1