Which statement concerning peripheral nerve structure and function is FALSE?

Which statement concerning peripheral nerve structure and function is FALSE?

• A. Both nonmyelinated and myelinated nerves are surrounded by Schwann cells
• B. The speed of propagation of an action potential along a nerve axon is greatly enhanced by myelin
• C. Generation of an action potential is an "all-or-nothing" phenomenon
• D. Myelination renders nerves less sensitive to local anesthetic blockade

Answer: (D)

Understanding how myelination affects nerve conduction and drug blockade helps explain why that statement isn’t true. Myelin speeds conduction by insulating the axon and enabling saltatory conduction between nodes of Ranvier. In the peripheral nervous system, Schwann cells wrap many axons; large-diameter axons receive a full myelin sheath, while smaller unmyelinated fibers are still ensheathed by Schwann cells but lack a continuous myelin layer. The action potential is an all-or-nothing event: once threshold is reached, it propagates with a consistent amplitude. Local anesthetics block conduction by inhibiting voltage-gated sodium channels. In myelinated fibers, these channels are highly concentrated at the nodes of Ranvier, and conduction depends on depolarization at those nodes. Blocking a node disrupts the entire conduction process, so myelinated fibers can be blocked effectively by local anesthetics. Therefore, myelination does not render nerves less sensitive to local anesthetic blockade; it tends to increase their susceptibility. The other statements are true: Schwann cells surround both myelinated and nonmyelinated fibers; myelin greatly increases conduction velocity; and AP generation remains an all-or-nothing phenomenon.

Understanding how myelination affects nerve conduction and drug blockade helps explain why that statement isn’t true. Myelin speeds conduction by insulating the axon and enabling saltatory conduction between nodes of Ranvier. In the peripheral nervous system, Schwann cells wrap many axons; large-diameter axons receive a full myelin sheath, while smaller unmyelinated fibers are still ensheathed by Schwann cells but lack a continuous myelin layer. The action potential is an all-or-nothing event: once threshold is reached, it propagates with a consistent amplitude.

Local anesthetics block conduction by inhibiting voltage-gated sodium channels. In myelinated fibers, these channels are highly concentrated at the nodes of Ranvier, and conduction depends on depolarization at those nodes. Blocking a node disrupts the entire conduction process, so myelinated fibers can be blocked effectively by local anesthetics. Therefore, myelination does not render nerves less sensitive to local anesthetic blockade; it tends to increase their susceptibility.

The other statements are true: Schwann cells surround both myelinated and nonmyelinated fibers; myelin greatly increases conduction velocity; and AP generation remains an all-or-nothing phenomenon.