This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
[{'content': 'Maximum number of Na+ channels per square micrometer is present in: \nSelect and explain. \nA. Cell body, B. Axon terminal, C. Surface of myelin, D. Nodes of Ranvier', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Maximum number of Na+ channels: Nodes of Ranvier >> Initial segment.\nThe\xa0spatial\xa0distribution\xa0of\xa0ion\xa0channels\xa0along\xa0the\xa0axon plays a key role in the initiation and regulation of the action potential. Voltage-gated Na+\xa0channels are highly concentrated in the nodes of Ranvier and the initial segment in myelinated neurons.\xa0\nThe number of Na+\xa0channels per square micrometer\xa0of membrane in myelinated mammalian neurons has been estimated to be 50–75 in the cell body, 350–500 in the initial segment, less than 25 on the surface of the myelin, 2000–12,000 at the nodes of Ranvier, and 20–75 at the axon terminals.\u200b\nAlong the\xa0axons of unmyelinated neurons, the number is about 110.', 'role': 'assistant'}]