How does myelin affect nerve conduction?

• A. It decreases conduction velocity by increasing membrane resistance.
• B. It converts electrical impulses into chemical signals.
• C. It inhibits the flow of ions at the nodes of Ranvier.
• D. It insulates axons and increases conduction velocity via saltatory conduction.

Answer: (D)

Myelin acts as an electrical insulator around the axon, produced by glial cells. This insulation raises the membrane resistance and lowers the membrane capacitance of the wrapped segments, so the depolarizing current leaks less while traveling along the axon. Because the myelin-covered stretches don’t have many ion channels, the signal doesn’t need to be regenerated there. Instead, voltage-gated Na+ channels are concentrated at the gaps between myelin segments, the nodes of Ranvier, where the action potential is re-initiated. This setup lets the electrical impulse jump from node to node, a process called saltatory conduction, which greatly speeds up conduction velocity compared with continuous conduction on unmyelinated fibers.

Converting electrical signals to chemical signals happens at synapses, not through myelin. The nodes of Ranvier are precisely where ion flow is re-energized, not inhibited. And myelin actually increases conduction speed rather than decreases it by reducing current leakage along the axon.