Even as textbooks, and almost all web pages I've seen so far, explain electrotonic spread/conduction as the passive current flow along an axon, they do so with continuous conduction only. Apart from myelin sheaths which enwrap axons leaving only the Nodes of Ranvier exposed, and of course, the energy requirement and speed of conduction differences, I can't see how else continuous conduction differs from saltatory. My question then is why electrotonic spread is never mentioned in saltatory conduction.


Generally textbooks take the following pedagogical flow in basic neurophysiology:

Ions flow, so voltage changes propagate

This is the "electrotonic part". The key concept is that if you add some ions or change the voltage of one part of a neuron, adjacent areas will also change in voltage as current flows 'passively'. The further away you go, the longer it takes the signal to arrive and the smaller it will be as the charge spreads out.

A more advanced course might also talk about the sources of those charges, like sensory receptors or neurotransmitter action, and might talk about the ways that polarizing pulses integrate over space and time. Or these may be saved for later.

(side note: in reality, neurons aren't all that passive, there are lots of conductance changes occurring even in this supposedly "electrotonic" scheme, but the electrotonic equations tend to work pretty well, and in a simplified experimental system they are sufficient. Biology is almost always too complex to try to include everything at once)

Action potentials

The key concepts here are that if the voltage changes enough, it stops being possible to think about just the passive flow, you now have to think closely about voltage-gated channels that create a positive feedback loop response to voltage. Above threshold, voltage-gated channels cause sufficient depolarization that adjacent areas of the membrane are also depolarized beyond threshold, and we call this propagating signal an action potential.

At this point, you are supposed to remember and understand that what is driving this active response is still a somewhat "passive" flow of charge that you understood from the "electrotonic" part of the course. That part is the physics and is always present, you can't get rid of it. However, you're in a new scheme where you can't use the electrotonic equations to understand what happens anymore.

Saltatory conduction

Myelination and saltatory conduction comes next. In this section, you're supposed to be thinking about the way action potentials spread, but to add one extra little wrinkle. What if, instead of transmitting piecemeal to the next adjacent segment of membrane, axons were more insulated? In this scenario, the flow of charges will extend much further. You should still be applying the concepts you learned in the "electrotonic" part, though: if you add some ions/change the voltage of one part of the neuron, adjacent areas will also change in voltage as current flows. The further away you go, the longer it takes the signal to arrive and the smaller it will be as the charge spreads out.

Therefore, even though insulation lets the signal travel further, it still decreases in amplitude over long distance, and you need to "boost" the signal again. That's where nodes of Ranvier come in.

Back to your specific question...

"Does electrotonic spread/conduction occur in saltatory conduction?" - No, but not because the "physics" part is different: saltatory conduction is necessarily active (involves voltage-gated channels), not electrotonic, even though all the electrotonic principles still apply.

The "electrotonic" part of the lesson contains important concepts that you are supposed to remember and carry through the other sections, even if it's not discussed explicitly. I think making that active/passive distinction is possibly a bit misleading, but the concepts you are supposed to grab from the electrotonic section in terms of the physics of how charge/voltage moves around in a neuron applies to everything.

Additionally, you should gather that the nodes of Ranvier are spaced in order to conduct action potentials. These are big voltage changes. Smaller voltage changes that are subthreshold will of course also travel down the axon (this is the physics part - nothing stops physics!), but if they aren't strong enough to open voltage gated channels at the next node of Ranvier then the voltage change isn't of any consequence and just decays with distance. If it was strong enough, then by definition it is superthreshold rather than subthreshold, and we are talking about an action potential.

All the content in this answer is material like you would find in a basic undergraduate neuroscience textbook. The ones I usually recommend are Purves or Kandel.

  • $\begingroup$ Ok. Since the passive current actually flows, why don't we say electrotonic potential is a propagated response? Because of its reduction in amplitude as it moves? $\endgroup$ – Chemo-Mike Sep 18 '20 at 13:30
  • $\begingroup$ @Chemo-Mike Yes, it's "propagated" locally in the cell (so you will observe spatial summation of potentials at different nearby dendrites), but not propagated down the axon so cannot typically influence other cells. $\endgroup$ – Bryan Krause Sep 18 '20 at 14:00

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