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What is Maximum number of Na+ channels per square micrometer is present in: and explain in detail?
Maximum number of Na+ channels: Nodes of Ranvier >> Initial segment.
The spatial distribution of ion channels along the axon plays a key role in the initiation and regulation of the action potential. Voltage-gated Na+ channels are highly concentrated in the nodes of Ranvier and the initial segment in myelinated neurons. 
The number of Na+ channels per square micrometer of 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.​
Along the axons of unmyelinated neurons, the number is about 110.