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 another impulse.
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.
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