Which structure hosts high densities of voltage-gated Na+ channels to restart the action potential in myelinated axons?

• A. Nodes of Ranvier
• B. Axon hillock
• C. Synaptic cleft
• D. Dendritic spines

Answer: (A)

High density of voltage-gated Na+ channels at nodes of Ranvier allows the action potential to be regenerated as it travels along a myelinated axon. The myelin sheath increases membrane resistance and decreases capacitance between nodes, so the depolarizing current travels quickly to the next node. When it arrives at a node, the abundant Na+ channels open, re-depolarize the membrane, and the signal continues—the essence of saltatory conduction.

The axon hillock is the starting point of the action potential due to a high concentration of Na+ channels there, not along the myelinated segments. The synaptic cleft is the space between neurons for neurotransmitter transfer, and dendritic spines are postsynaptic structures on dendrites—neither is involved in re-launching the action potential along the axon.