This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General physiology, Subject name: Physiology
Maximum number of Na+ channels per square micrometer is present in 
Select and explain. 
A. Cell body, B. Axon terminal, C. Surface of myelin, D. Nodes of Ranvier
D: Nodes of Ranvier
Answer: D: Nodes of Ranvier 
Explanation: "Saltatory" Conduction in Myelinated Fibers from Node to Node: Even though almost no ions can flow through the thick myelin sheaths of myelinated nerves, they can flow with ease through the nodes of Ranvier.Therefore, action potentials occur only at the nodes. Yet the action potentials are conducted from node to node, this is called saltatory conduction.That is, electrical current flows through the surrounding extracellular fluid outside the myelin sheath, as well as through the axoplasm inside the axon from node to node, exciting successive nodes one after another. Thus, the nerve impulse jumps along the fiber, which is the origin of the term "saltatory."Ref: Guyton; 13th edition; Chapter 5; Membrane Potentials and Action Potentials