What underlies saltatory conduction in myelinated axons?

• A. Continuous action potentials along entire axon due to uniform channels.
• B. Action potentials occur at nodes of Ranvier with high Na+ channel density; myelin insulates internodes.
• C. Synaptic cleft conduction.
• D. Potassium channels at nodes cause conduction.

Answer: (B)

Saltatory conduction happens because the axon is insulated by myelin, with action potential-generating machinery focused at the gaps called nodes of Ranvier. The myelin sheath electrically isolates the internodal membrane, which greatly increases membrane resistance and decreases membrane capacitance. As a result, the depolarizing current that starts at a node can travel quickly through the insulated internode to the next node without losing much charge. At the next node, abundant voltage-gated Na+ channels open, renewing the action potential there and continuing the signal.

This arrangement is why the impulse “jumps” from one node to the next rather than propagating smoothly along every point of the membrane. The high density of Na+ channels at the nodes is essential for re-initiating the action potential, while the insulating myelin around the internodes blocks current leakage. If the channels were uniformly distributed along the axon, the conduction would be continuous and slower. Synaptic cleft conduction is unrelated to propagation along the axon, and while potassium channels help reset the membrane, they aren’t what enable the fast saltatory jump—the key is the high Na+ channel density at the nodes paired with myelin insulation.