I know that the the bigger the neuron's diameter is, the faster the neuron signal is transmitted. This makes sense according to the proportionality of resistance to the inverse of area and thus, in physics, there is lower resistance in wires that have bigger diameters (cross-sectional areas). I can imagine that this is the same logic for our neurons as well because they are like wires. I also know that the more myelinated a neuron is, the faster the neuron signal travels too because myelination acts as an insulation that reduces resistance in signal transmission.

But then, my question is: does the number of nodes matter and affect the rate at which a neural signal is transmitted? Can you please help explain why or why not? Let's say for example, that two neurons have same amount of diameter and myelinated sheaths hypothetically, but one of them has more nodes. Does that number of nodes of ranvier make the saltatory conduction faster?


The number of nodes is a trade-off between speed of conduction and reliability. The fastest transmission would be where nodes are spaced at roughly the maximum distance to trigger sufficient depolarization at the next node - however, small changes in ionic concentrations or channel densities could lead to failures, so in practice the nodes are typically a bit closer.

The slowest transmission would be to have no saltatory conduction at all.

(I mean these to be general principles; one could probably design a hypothetical system with very close nodes that is actually worse than having no saltatory conduction, and there is probably a perfect edge case where nodes that are spaced at the maximum distance are slower because of membrane time constants)

The reasons for this are: that travel between nodes is very fast, regeneration at nodes is comparatively slow. However, there is a maximum distance for reliable transmission, so we can't simply have one long stretch containing no nodes in between.

edit: references were requested. This was off the top of my head so I would suggest any introductory neuroscience textbook, Purves is a preferred one of mine, but this will run you significant $ if you are not/have not been a student in a neuroscience course. The Wikipedia articles for https://en.wikipedia.org/wiki/Node_of_Ranvier and also for Saltatory Conduction both look good to me, though they do not address the specific issue in detail. This brief Neuron review is more appropriate to the specific question asked, though I am not sure people without an academic library subscription will have free access: http://www.sciencedirect.com/science/article/pii/S0896627300803232

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    $\begingroup$ can you provide some references to your answer? we always appreciate outside literature/references $\endgroup$ – Vance L Albaugh Oct 18 '16 at 22:01

Saltatory conduction is faster, because between nodes, the action potential skips the intervening distance. There, passive spread of charge takes over, which is much faster than channel-mediated conductance which is restricted by protein gating mechanics as passive currents can travel with speeds approaching the speed of light (through metal wires that is).

The optimal internode distance for speed of action potential transmission is that distance where the depolarization currents just do not die out before reaching the next node through passive charge conduction. A bit further, and the depolarization current of one node doesn't reach the next due to current leakage. A bit shorter and more nodes are necessary per unit of axon length, slowing down saltatory conduction as more (slow) ionic channel conductance will replace passive current spread.

In other words, your notion that more nodes speed up transmission is incorrect. On the contrary, less nodes speed up transmission. The extreme case is no nodes at all, where a continuous channel density lines the axon (an approximately infinite number of nodes). This would boil down to to normal, slow action potential transmission.

Further reading
- Saltatory conduction of nerve impulses
- Why is saltatory conduction faster than continuous conduction?

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  • $\begingroup$ Your answer is the same as the question you cited biology.stackexchange.com/questions/8282/… which has been negated by the answer to it. $\endgroup$ – Hans Sep 16 '16 at 20:29
  • $\begingroup$ @Hans - what do you mean? My answer is redundant; my answer is inferior; the question is a dupe? Answer above does contain additional info. $\endgroup$ – AliceD Sep 16 '16 at 20:51
  • $\begingroup$ I am saying your statement is confusing and possibly self-contradictory.You say "on the contrary, less nodes speed up transmission." That is incorrect as is answered by the cited answer. There is an optimal positive, non-unit density of nodes, less or more of it slows the transmission speed. The statement "the extreme case is no nodes at all, where a continuous channel density lines the axon (an approximately infinite number of nodes)" contradicts itself: no nodes at all vs continuous, infinite number (imprecise, do you take the size of a channel into account?). $\endgroup$ – Hans Sep 16 '16 at 21:10
  • $\begingroup$ I say that a continuous array of channels is slower $\endgroup$ – AliceD Sep 16 '16 at 22:17
  • $\begingroup$ You are not responding to all my points which point out all the confusion/contradictions in your answer. Are you going to respond to those point by point? As for your last comment "I say that a continuous array of channels is slower", it is again unclear: slower than what? $\endgroup$ – Hans Sep 16 '16 at 22:49

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