In nerve fibers, what mechanism allows myelin to increase conduction velocity?

• A. By enabling saltatory conduction where action potentials jump between nodes of Ranvier.
• B. By increasing the length constant.
• C. By decreasing the threshold of voltage-gated channels along the axon.
• D. By increasing neurotransmitter release at synapses.

Answer: (A)

Saltatory conduction is the mechanism by which myelin speeds up nerve impulses. Myelin wraps the axon with insulating layers, which markedly increases membrane resistance and lowers membrane capacitance of the insulated segments. This makes the passive spread of current along the axon more efficient, allowing the depolarizing current to reach the next node of Ranvier quickly. At these nodes, there is a high density of voltage-gated Na+ channels, so the action potential is regenerated there. As a result, the impulse appears to jump from node to node, rather than traveling continuously along every millimeter of the axon, greatly increasing conduction velocity.

The increased length constant is a related effect of myelin, but the key feature that explains the faster speed is the saltatory mechanism—signal regeneration only at nodes. Transmission at synapses, on the other hand, is about neurotransmitter release and not the speed of conduction along the axon.