8.4 : Local Anesthetics: Differential Sensitivity of Nerve Fibers

Local anesthetics (LAs) block the sodium channels of nerve trunks, sensory nerve endings, and neuromuscular junctions. Although LAs can block all kinds of nerves, the sensitivity of nerve fibers differs according to nerve types and structures. LAs are known to block myelinated fibers faster than unmyelinated ones. Also, they block pain or sensory neurons at low concentrations without affecting the motor neurons involved in muscle contractions. This helps relieve labor pain without affecting the muscular contraction required to deliver the baby.

However, LAs fail to control pain effectively in inflamed tissues. During inflammation, the tissue pH becomes low. At such an acidic pH, LAs ionize, and their movement across the cell membrane gets disturbed. As a result, LAs stay outside the cell in their ionized form. Furthermore, inflamed tissues have higher vascularization and permeability, which allows the rapid removal of LAs into systemic circulation. Vasoconstrictors like adrenaline are usually given with LAs to reduce the blood flow and prolong the effect of LA. But its effectiveness is diminished in inflamed tissues.