Figure 2.
Local anesthetic action. An injected local anesthetic exists in equilibrium as a quaternary salt (BH+) and tertiary base (B). The proportion of each is determined by the pKa of the anesthetic and the pH of the tissue. The lipid-soluble base (B) is essential for penetration of both the epineurium and neuronal membrane. Once the molecule reaches the axoplasm of the neuron, the amine gains a hydrogen ion, and this ionized, quaternary form (BH+) is responsible for the actual blockade of the sodium channel. The equilibrium between (BH+) and (B) is determined by the pH of the tissues and the pKa of the anesthetic (pH/pKa).