How does anesthetic drugs which block the Na+ channels in neurons prevent pain during tooth extraction?

How does novacaine work?

By:
Kim Loos
Question :

Would you please explain, in detail, how novacaine causes numbness. I have been unable to find information on the physiologic mechnism of action.
Answer :

First, let me explain that there are two basic types of local anesthetic used in dentistry: 1) novacaine, which is an ester-type anesthetic; and 2) lidocaine, which is an amide-type anesthetic. Novacaine was discovered first, but due to allergic reactions, lidocaine and related anesthetics were developed in the 1940s. With the amide anesthetics, allergies are rare.

Here are the steps involved in achieving local anesthesia as adapted from Malamed, Stanley F., Handbook of Local Anesthesia, 2nd Edition, C.V. Mosby Company (1986) p. 13:

1. Displacement of calcium ions from the nerve receptor site
2. Binding of local anesthetic molecule to this receptor site
3. Blockade of the sodium channel
4. Decrease in sodium conductance
5. Depression of rate of electrical depolarization
6. Failure to achieve threshold potential level
7. Lack of development of propagated action potential
8. Conduction blockade

These steps cause impulses that arrive in the blocked area to become stalled, thus preventing the “pain” impulses from reaching the brain.

The most commonly used local anesthetics are lidocaine or xylocaine. For most people, the anesthesia lasts about two hours. The duration of numbness depends on how long the anesthetic takes to diffuse out of the tissues.

To understand this, you need to think of the normal mechanism for the transmission of nervous signals. It depends on the existence of a different concentration of electric charges (ions) at either sides of the cell membrane, known as membrane potential. When a nervous signal reaches the cell, the flux of ions through the membrane channels changes, causing despolarization and the action potential.

Blocking the Na+ channels means that sodium cannot go back into the cell. Thus, the action potential cannot be generated and the nervous signal (in this case, the sensation of pain) can’t be transmitted.