This is the fourth of seven pages which constitute a course in local anesthetics.  Each page stands on its own, however for a thorough understanding of dental local anesthetics the reader is advised to read the pages in order.

<==Toxicity and dosage                 Anesthesia Problems==>

How nerves conduct an impulse

How a nerve fiber transmits an impulse

Nerves are NOT like electrical wires with electrons traveling their length to transfer information from one end to the other.  They are actually complex electro-chemical structures which utilize the electrical potential difference between the fluid inside of the axon, and the fluid that surrounds the axon.  The fluid inside the axon (called cytoplasm) contains a high concentration of potassium ions, while the fluid outside contains a high concentration of sodium ions.  There is no real difference in electrical potential between a potassium ion and a sodium ion, however, the fact that they exist in different concentrations on either side of the cell membrane sets up an electrochemical pressure gradient between the two.  Sodium ions want to flow into the nerve cytoplasm, while the potassium ions want to flow out, but both are prevented from doing so by the presence of the nerve cell membrane.

When a nerve is stimulated, this sets up a chain reaction in which sodium ions begin to penetrate through the nerve cell membrane and flow into the axon, while potassium ions begin to flow out.  This activity happens at the nodes of Ranvier.  This process is called depolarization of the nerve membrane.  The imbalance in the chemical makeup of the extracellular fluid then causes an imbalance in the concentration of sodium ions at the adjacent node which stimulates an identical depolarization at this node as well.  This process proceeds from node to node until the impulse reaches the cell body of another nerve in the ganglion where it stimulates a similar cascade in a network of other neurons which make contact with it.  

You might think that once all the potassium and sodium ions have exchanged places, the nerve would no longer be able to conduct impulses.  The  nerve, however, is a living entity and can regenerate the original concentrations of ions using energy from the food you eat in almost the same way that muscle cells use that same energy to cause muscle movement.  It does this using proteins embedded in the cell membrane which act as "ion pumps".

The image above is a false color preparation of neurons within a ganglion.  It gives a good representation of a mixture of neuron cell bodies, dendrites and axons complete with Schwann cells and their intervening nodes of Ranvier.  Compliments of Scientific American Magazine. 

How local anesthesia interrupts this process

Local anesthetics work to block nerve conduction by reducing the influx of sodium ions into the nerve cytoplasm.  If the sodium ions cannot flow into the neuron, then the potassium ions cannot flow out, thus inhibiting the depolarization of the nerve.  If this process can be inhibited for just a few nodes of Ranvier along the way, then nerve impulses generated downstream from the blocked nodes cannot propagate to the ganglion.  In order to accomplish this feat, the anesthetic molecules must actually enter through the cell membrane of the nerve.  Herein lies the differences in the potency, time of onset and duration of the various local anesthetics.  

The structure of local anesthetics

The diagrams above show the essential structures of the two major types of local anesthetic agent; the molecule shown in the left diagram represents the structure of procaine (Novocain).  The chain that connects the benzene ring on the left with the amide tail on the right is an "ester linkage".  The diagram to the right represents lidocaine and its analogs.  The connecting chain in this case is called an "amide linkage".   The amide linkage contains an extra nitrogen to the left of the C=O (carboxyl) group. 

All local anesthetics are weak bases.  They all contain:

• An aromatic group (the benzene ring seen on the left side of both structures above)

• An intermediate chain, either an ester or an amide; and

• An amine group seen on the right side of both molecular structures above. 

The characteristics of any given anesthetic is determined by the exact structure and relationship of each of these three components.  The aromatic ring structure is soluble in lipids (The nerve cell membrane is made of a lipid bilayer and thus the aromatic ring is important in making it possible for the anesthetic molecule to penetrate through the nerve membrane.  The amino structure (seen on the right side of the molecules diagramed above) is soluble in water which is what makes it possible for the anesthetic molecule to dissolve in the water in which it is delivered from the dentist's syringe into the patient's tissue.  It is also responsible for allowing it to remain in solution on either side of the nerve membrane.  The trick that the anesthetic molecule must play is getting from one side of the membrane to the other.  

The structure of the cell membrane

Every cell in the body has a membrane which separates that cell from other cells, and from the extracellular fluids that surround it.  The membrane has a definite chemical structure which creates a stable two dimensional sheet which naturally retains its structure in aqueous (water based) solution.  It is composed of a bilayer of phospholipid molecules arranged as shown in the diagram above.  Each phospholipid molecule is composed of two components; a phosphate radical (shown as a blue ball) which  carries an electrical charge, and therefore likes to associate with water molecules, and two long hydrocarbon chains (green) which do not carry a charge and therefore associate with each other in order to avoid contact with the surrounding water molecules. (Not unlike oil, which does not mix with water either).  The stability of this structure is based on the fact that the phosphate radicals face outward into the surrounding medium.   They are soluble in water and mix well with it. On the other hand,  the lipid tails are hydrophobic and avoid contact with the water relying on the phosphate radicals to "protect" them.  The lipid tails mingle with each other in the same way that the pioneers used to "circles the wagons" in order to protect themselves.  This maintains the structural integrity of the cell membrane.  This super stable micro structure is perhaps one of the most important chemical structures in all of creation because it enables the formation of discrete biological elements separated into cellular components.  

While the phospholipid bilayer defines an essentially two dimensional sheet, it actually has a third dimension meaning that it has thickness.   In addition, the bilayer is essentially a non aqueous liquid with other structures such as proteins embedded within it.  Thus, a cell membrane can be thought of as a sheetlike "ocean" of oily liquid with protein molecules floating around in it.  The proteins can have complex shapes and functions depending upon the structure programmed for them by the genetic machinery of the cell.  The ionic channels that the allow the influx of sodium ions, as well as the efflux of potassium ions during depolarization of the membrane are actually complex protein structures embedded in the neuron membrane.

 

The phospholipid bilayer not only exists as a chemical entity.  It can actually be seen on electron micrographs.  The thumbnail on the left shows HIV virons budding off a natural human T cell .  The lipid bilayer is clearly visible on both the mother cell and in the budding virons.  Click on the thumbnail to view the image at full resolution.

PH, PKa, Acids and Bases---and why they are the key to the effectiveness and longevity of an injectable local anesthetic

This section is quite conceptually difficult because it involves some essential chemistry, but it makes for very rewarding reading because it will enable the reader to understand the differences between the common local anesthetic solutions.  It will help to explain the reasons that some anesthetics take longer to set than others, and why some cause more prolonged anesthesia than others.  

Synthetic anesthetics are prepared as weak bases and during manufacture, precipitate as powdered solids.  These solids are unstable in air and poorly soluble in water.  They are therefore combined with an acid to form a salt which can be combined with sterile water or saline .  The salt dissolves to produce a stable solution which is injectable.  The PH (the acid/base balance) of the solution is adjusted to complement the specific molecular structure of the anesthesia in question, however all anesthetic solutions are acidic prior to injection.  Remember that the lower the PH, the more acidic the solution is, and the higher the PH, the more alkaline (basic) it is.  

After injection of the anesthetic, the solution quickly takes on the ph of the surrounding tissue, and the molecular structure shifts between two forms; an uncharged base molecule (RN) and a positively charged cation (RNH+).  These two forms of the anesthetic molecule exist in an equilibrium dependent upon the exact PH of the solution:

If the surrounding medium becomes more acidic (lower PH) due to infection or other metabolic conditions, by definition the concentration of hydrogen ions increases.  These positively charged ions combine with the uncharged anesthetic radical (molecule) shifting the above equation to the left, and producing a higher proportion of charged cationic structures.  If the PH rises, (ie. the solution becomes more alkaline) there are fewer positively charged hydrogen ions.  Thus the charged radicals release their hydrogen ions into solution and the equation shifts to the right producing more of the uncharged base.

The definition of PKa and how it affects diffusion

The PH that produces an equal number of uncharged basic molecules (RN) and charged cationic forms (RNH+) is called the PKa (also called the dissociation constant).   This is important because the molecular form of the anesthetic that is able to diffuse through the lipid membrane of the nerve cell is the uncharged (RN) form, while once inside the neuron, the active form that inhibits sodium influx is the charged cationic (RNH+) form.  As more and more of the uncharged base diffuses through the membrane, the concentration of the uncharged base outside the membrane decreases and the formula re-equilibrates forming more of the uncharged base from the newly higher concentration of positive cations.  This continues until eventually nearly all the base diffuses from the outside of the cell membrane to the inside. 

Once inside the cell membrane, the formula shifts to the left (see diagram below) in an attempt to recreate the original concentrations of cations and neutral base molecules.  But the positively charged base molecules inside the cell now tend to bind to sodium channel proteins and are removed from the dynamic balance.  This creates a sort of "vacuum" which keeps drawing more and more neutral base molecules from the outside of the cell.  It is the binding of the base cations to cellular sodium channel proteins which is the mechanism that limits nerve conduction and creates numbness.

Since the PH of normal body tissue is 7.4, the ideal PKa of an anesthetic would also be 7.4.  This would mean that 50% of all the molecular structures outside the nerve cell bodies would be in the form of the uncharged base, and quick diffusion of the anesthetic into the cell bodies would occur.  Unfortunately, all local anesthetics have PKa values above 7.4.  The higher the PKa, the lower the concentration of uncharged base, and the slower the diffusion into the nerve cells.  Thus, the higher the PKa,  the longer it will take for that anesthetic to set. 

The PH of normal body tissue is 7.4.  In situations in which there is an active infection present, the tissue PH can be considerably lower, in the vicinity of 5 or 6.  On the other hand, even in inflamed tissue, the cytoplasmic PH inside the neuron generally remains at the normal 7.4.  Very reduced tissue PH shifts the equation (outside of the nerve cell) to the left reducing the number of neutral (RN) radicals available to diffuse through the nerve cell membrane.  This accounts for the difficulty in anesthetizing such an area.  The relative difference between the PKa of the anesthetic and the PH of the body tissue can make quite a large difference in the percentage of anesthetic that is available to diffuse immediately through the nerve membrane, and thus on the amount of time it takes for the anesthetic effect to be felt.   The table below shows the PKa and other vital statistics of the seven most commonly used dental anesthetics:

When an anesthetic solution is injected into healthy tissue, it quickly takes on the PH of the surrounding tissue which is 7.4 in normal tissue (without inflammation).  This is why the third column labeled "% RN at PH 7.4" is important.  Remember that only the uncharged basic RN radical can penetrate the lipid membrane components. The higher this percentage is, the quicker the anesthetic penetrates the membrane.   

Just because only, say, 18% of an anesthetic solution is available to diffuse through the cell membrane at any one time, this does not mean that all the anesthetic molecules cannot eventually diffuse into the nerve cells.  As the number of RN radicals decreases outside of the nerve cell because of absorption, more of the cationic form (RNH+) converts to the RN form to maintain the dynamic balance between the two forms.  A low tissue PH simply delays the process.  Unfortunately, as the time of onset increases, the chances of the unused anesthetic being absorbed into the blood stream also increases, which is why procaine was abandoned as soon as lidocaine became available.  It simply "wore off" before it had a chance to enter the nerve and take effect.

Once the molecules diffuse through the membrane, the neutral base (RN) is once again subject to the PH dependent equation above, and many neutral RN radicals shift back to their cationic form (RNH+) to maintain the dynamic balance inside the neuron.  Once inside the nerve cell, the active component that combines with the sodium ion channels is the acidic cation form (RNH+). 

The irony of this situation is that once inside the nerve cell,  the slowest diffusing anesthetic (Bupivicaine with only 18% available to diffuse through the membrane) has the distinct advantage of making more of the absorbed anesthetic available.  82% (100% minus 18%) of the absorbed base shifts into its cationic form and actively binds with the sodium channel proteins to block their activity!  Bupivicaine has the added advantage of binding strongly with these cellular proteins in its cationic form causing it to be a very long acting anesthetic.  Bupivicaine (Marcaine®) is used today for prolonged surgical operations as a way of maintaining numbness for many hours after the procedure to help reduce postoperative pain.  (Note that procaine takes so long to diffuse through the nerve membrane that most of it has been reabsorbed by the blood vessels before it ever has a chance to penetrate the nerve membrane.  In addition, procaine does not bind especially strongly with cellular proteins, thus reducing its length of action.) 

<==Toxicity and dosage                 Anesthesia Problems==>

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