4-2. MECHANISM OF ACTION OF THE LOCAL ANESTHETICS
a. The nerve fiber is a long cylinder surrounded by a semipermeable (allows
only some substances to pass) membrane. This membrane is made up of proteins and
lipids (fats). Some of the proteins apparently act as channels, or pores, for the passage
of sodium and potassium ions through the membrane.
b. The movement of nerve impulses along a nerve fiber is associated with a
change in the permeability of the membrane. The pores widen, and sodium ions (Na+)
move to the inside of the fiber. At the same time, potassium ions (K+) diffuse out
through other pores (see Figure 4-1). The entire process is called depolarization.
Immediately after the nerve impulse has passed, the pores again become smaller.
Sodium ions (Na+) are now "pumped" out of the fiber. At the same time, potassium ions
are actively transported into the fiber. The nerve membrane is then ready to conduct
Figure 4-1. Mechanism of nerve impulse transmission.
c. Local anesthetics block depolarization of the nerve membrane. That is, to
make the conduction of the nerve impulse impossible.
d. The local anesthetic effect lasts as long as the agent maintains a certain
critical concentration in the nerve membrane. There is a potential problem: the local
concentration needed to prevent conduction of the nerve impulse is much greater than
the tolerable blood level. TO AVOID A SYSTEMIC TOXIC REACTION TO THE LOCAL
ANESTHETIC, THE SMALLEST AMOUNT OF THE MOST DILUTE SOLUTION THAT
WILL EFFECTIVELY BLOCK THE PAIN SHOULD BE ADMINISTERED.
4-3. THE USE OF VASOCONSTRICTORS IN CONJUNCTION NITH LOCAL
a. Indications. Vasoconstrictors (like epinephrine) are sometimes used in
conjunction with local anesthetics. Vasoconstrictors are used to prolong the duration of
action of local anesthetics. Vasoconstrictors also help to control bleeding.
Furthermore, the vasoconstrictor delays the absorption of the local anesthetic by
reducing the blood flow to the affected area. This results in a reduction of the toxic
effects of the local anesthetic, since the rate of absorption keeps pace with the rate the