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I have not been able to locate any research that indicates whether a single axon of a neuron or nerve cell can conduct multiple simultaneous (i.e. spatially separate) action potentials. I am aware that in many neurons refractory periods would preclude this possibility, however in some cases delays along axons are large enough.

Is this the case? Can you point me to any citations supporting your answer?

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  • $\begingroup$ Yes, but there is an upper bound to the maximum amount of energy per axon. There is no empirical evidence of this about what is the exact energy spectrum which each axon can hold. This would also show you which action potentials an axon can propagate or hold. There is no sufficient imaging technology there openly which could be used to do some of the research. $\endgroup$
    – Léo Léopold Hertz 준영
    Oct 10, 2014 at 5:28

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When you say multiple simultaneous action potentials I assume that the stimuli for all of them are temporally overlapping. In such case a neuron can integrate the different stimuli and launch an action potential. Of the multiple stimuli some can be excitatory while others can be inhibitory. The net response would be an integration of all the signals [ref].

However, if there is a gap in the stimuli then the cell may not react as it would be in its refractory period (which results until the ions levels are back to their original state and the membrane potential is restored).

There can be long delays, which depends on various factors such as ion deficiency, ATP deficiency (ATP is needed by Na+/K+ ATPase) or neuropathological conditions. In case of nervous signaling over many neurons then synapses can suffer from synaptic fatigue, which results because of depletion of neurotransmitters.

EDIT:

Maximum AP propagation speed/Nerve conduction velocity = ~120m/s (Can't find original source. From wikipedia)

Longest myelinated axon (sciatic nerve in humans) = ~ 1m

Time required to reach synapse = ~8ms

Time required for repolarization = ~5µs

So multiple action potentials (~160), are possible (lets call it situation-X).

The squid giant axon is ~500µm. I believe it is also non-myelinated. In this case if NCV is less than 100m/s then situation-X can happen. NCV can go as low as 0.5m/s (again the reference is wikipedia as I cannot find the original source).

Having said that, situation-X can also cause synaptic fatigue if the firing rate (AP frequency) goes beyond a certain value. This value would depend on how much neurotransmitter is released per AP, how much is in storage and the uptake rate.

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  • $\begingroup$ "stimuli for all of them are temporally overlapping" ... this is not the right idea. the question is not about summation of stimuli but production of APs in short succession which are simultaneously propagated in the same cell (obviously with some spatial distance). Also refractory periods are discussed in the question. $\endgroup$
    – watsonic
    Nov 12, 2014 at 11:52
  • $\begingroup$ @watsonic Ahh. You mean what if in a long axon the first AP has not reached the synapse but the soma/dendritic region is already repolarized? $\endgroup$
    – WYSIWYG
    Nov 12, 2014 at 11:58
  • $\begingroup$ yes, for example. or one AP on the axon proximal to the soma and one distal. $\endgroup$
    – watsonic
    Dec 17, 2014 at 9:05
  • $\begingroup$ @watsonic see the edit. I hope it answers your question. $\endgroup$
    – WYSIWYG
    Dec 17, 2014 at 10:12
  • $\begingroup$ Let us continue this discussion in chat. $\endgroup$
    – WYSIWYG
    Dec 17, 2014 at 10:56
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I don't see why not, once the sodium and potassium channels along the axon have returned to their resting state. But I am not aware of a hard citation. I would suggest looking in the recent optogenetic literature, where researchers are exciting axons and axonal terminals using channelrhodopsin. People have most likely quantified action potentials for varying illumination durations and intensities, and you might be able to judge by the maximum firing rate how feasible multiple simultaneous spikes would be.

Also, it's most likely possible to trigger an antidromic spike while the neuron is firing an orthodromic spike. Not quite what you asked, but close.

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  • $\begingroup$ Antidromic electrically evoked potentials plus normal physiological responses :) - Good thinking ! $\endgroup$
    – AliceD ♦
    Dec 17, 2014 at 10:16

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