I went to the dentist the other week for a chipped tooth. He used a local anesthetic to numb the nerves to my maxilla, or upper jaw. So I thought I’d write a few words about local anesthetics. When injected near a nerve, these drugs cause reversible loss of function of that nerve. So, if the nerve carries fibers for sensation, blocking it results in anesthesia (lack of sensation).
Nerves transmit information by conducting action potentials, or brief impulses. They do this by allowing sodium and potassium to rapidly flow in and out of the fiber, a process called depolarization and repolarization. If you can chemically block this process, you can block the transmission of action potentials, thus rendering the nerve incapable of sending information. And this is exactly how local anesthetics, such as Novocain work.
All nerve fibers are sensitive to local anesthetics, but generally smaller diameter fiber are more sensitive than larger fibers. Local anesthetics block conduction in the following order: small myelinated axons (e.g. those carrying pain impulses), non-myelinated axons, then large myelinated axons. Because pain fibers are the smallest diameter fibers, a differential block can be achieved (i.e. block pain sensation more readily than other senses, such as light touch).
Permanent nerve damage can occur after a peripheral nerve block but is rare. Symptoms are very likely to resolve within a few weeks. The vast majority of those affected recover within four to six weeks. 99% of these people have recovered within a year. Very few nerve blocks result in some degree of permanent persistent nerve damage.
Nerve fibers are not the only tissues that use potassium and sodium conduction as the basis for their action. So do muscle fibers. For this reason, the amount of anesthetic injected must be carefully monitored. This is also the reason that some local anesthetics are effective in treating some severe types of cardiac arrhythmias.
True allergies to local anesthetics is very rare.