Final answer:
Nodes of Ranvier have a high concentration of sodium (Na+) channels, which help facilitate the rapid and efficient transmission of action potentials along myelinated axons through saltatory conduction.
Step-by-step explanation:
Nodes of Ranvier contain an abundance of sodium and potassium channels. These are voltage-gated channels that are crucial for the transmission of electrical signals along an axon. In myelinated neurons, which have a myelin sheath insulating the axon, ion flows occur exclusively at the Nodes of Ranvier. This clustering of Na+ (sodium) and K+ (potassium) ion channels allows the action potential to "jump" from node to node in a process known as saltatory conduction. The speed of signal propagation is increased due to this mechanism as compared to unmyelinated axons, where ion channels are distributed over the entire membrane surface and the action potentials spread smoothly but more slowly. Hence, the correct answer to the question is A) Sodium.