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Assuming that a longer distance between gaps in the myelin sheath is beneficial for an organism due to the increased propagation speed, what is the limiting factor in determining the maximum spacing between nodes? I have previously encountered the analogy of the Nodes of Ranvier as forming a circuit between themselves and the previous node in order to pass on the action potential, which leads me into thinking that it is simply electrical resistance that is the cause however I assume the analogy is an over simplification and would be very interested in any alternatives.

Also, is there a significant variation in node separation between organisms? Which organisms have the greatest gaps?

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This is best answered via something called cable theory. Basically, as the action potential (AP) propagates along the membrane of the axon, it's tripping voltage gated channels that "renew" the flux of ions into the stream at the nodes. There are no ion channels under the sheathing (or they are there anatomically and inactive, but I don't remember), so the current is able to zip through that area. There's no renewal of AP, but there's no loss due to leak of ions either. Since there's no influx of ions from outside of the membrane until the next node, the capacitance and resistance of the membrane is exponentially decaying the voltage. So, if the distance between the nodes was too long, the voltage would just die off and the AP wouldn't propagate.

I don't have any anatomical data about individual species, but know that only the smaller diameter axons require sheathing to compensate for the slower ion flow due to higher resistance (e.g., the squid giant axon(not to be confused with the "giant squid" axon) has no myelination due to its large diameter).

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Right on. The only thing I would add is to explicitly mention the distinction between axial resistance and membrane resistance in cable theory. The axial, or longitudinal, resistance is what is relevant here as it determines how much current flows down the center of the axon. As you say, this parameter is mostly affected by the axon diameter (larger axons have a lower axial resistance). Thus, the decay of voltage along a large diameter axon will be less severe than along a small diameter axon. –  yamad Dec 15 '11 at 20:09
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