Which statement concerning local anesthetics is CORRECT?

Which statement concerning local anesthetics is CORRECT?

• A. The un-ionized form binds to the nerve membrane to actually block conduction
• B. If one node of Ranvier is blocked, conduction will be reliably interrupted
• C. The ability of a local anesthetic to block nerve conduction is directly proportional to the diameter of the fiber
• D. The presence of myelin enhances the ability of a local anesthetic to block nerve conduction

Answer: (D)

Local anesthetics block nerve conduction by inhibiting voltage-gated sodium channels, preventing the neuron from depolarizing and propagating an action potential. They reach the inside of the axon in their uncharged form, cross the membrane, and then become charged to bind from the intracellular side of the sodium channel, stopping sodium influx during depolarization. In myelinated fibers, conduction occurs in a saltatory manner with clusters of sodium channels concentrated at the nodes of Ranvier. Blocking sodium channels at these nodes is enough to interrupt the entire impulse because the insulated segments between nodes rely on rapid, node-to-node depolarization. Myelin thus makes blockade more effective: it concentrates the critical sites and reduces the amount of drug needed to stop transmission. Therefore, the statement that the presence of myelin enhances the ability of a local anesthetic to block nerve conduction is correct. The other statements are not accurate for this reason: the drug’s blocking action is performed by the charged form inside the neuron, not by the uncharged form at the membrane; blocking a single node usually cannot reliably interrupt conduction because other nodes can still carry the impulse; and smaller diameter fibers are blocked more easily than larger ones, so the relationship with fiber diameter is inverse to what that choice suggests.

Local anesthetics block nerve conduction by inhibiting voltage-gated sodium channels, preventing the neuron from depolarizing and propagating an action potential. They reach the inside of the axon in their uncharged form, cross the membrane, and then become charged to bind from the intracellular side of the sodium channel, stopping sodium influx during depolarization.

In myelinated fibers, conduction occurs in a saltatory manner with clusters of sodium channels concentrated at the nodes of Ranvier. Blocking sodium channels at these nodes is enough to interrupt the entire impulse because the insulated segments between nodes rely on rapid, node-to-node depolarization. Myelin thus makes blockade more effective: it concentrates the critical sites and reduces the amount of drug needed to stop transmission.

Therefore, the statement that the presence of myelin enhances the ability of a local anesthetic to block nerve conduction is correct.

The other statements are not accurate for this reason: the drug’s blocking action is performed by the charged form inside the neuron, not by the uncharged form at the membrane; blocking a single node usually cannot reliably interrupt conduction because other nodes can still carry the impulse; and smaller diameter fibers are blocked more easily than larger ones, so the relationship with fiber diameter is inverse to what that choice suggests.