Hello Seth

Action potentials are a very common form of communication between neurons, especially since it is fast and can propagate far distances, in axons for example. Therefore any animal that need to react quickly and is larger than some small value will use APs. For sure all vertebrates use it. I know of only one animal that doesnt use APs, and that is the C. Elegans, which is a small nematode.

Graded potentials occur practically in all neurons, regardless of which species. They are normally initiated at the synapse but can be caused by other biological and artificial means. So they are common, present in every neuron. Action potentials occur in neuronal axons and in muscle cells, i.e. in electrically excitable cells. They are also very common and necessary for the transmission of signals for any significant distance in a multicellular organism. If you are asking if there are axons that transmit via a graded potential, probably by definition such neuronal processes would not be called axons.

Hello Seth,

I'd say it is hard to quantify what is more common but the question is also kind of misleading in my eyes. Most action potentials are actually generated because of graded potentials that can be summed up and may lead to a sufficient membrane depolarization to initialize an action potential. This is important since the all-or-nothing feature of action potential is essential for reliable and fast signal conduction within the nervous system. Graded potentials however are depended on the strength and sort of the stimulus and in the end determine whether an signal in form of an action potential needs to be initiated. So at least in higher animals both sorts of electric signals work together.

Best regards, Martin

I agree with Martin that the question and any answer to it is misleading unless one specifies which function is considered. Formally, an action potential is always initiated by a graded potential. Plus, there are numerous occasions when the information is transmitted/processed with graded potentials only - for instance, in majority of retinal neurons in vertebrates. Thus in a sense graded potentials are more common than action potentials even in human. Yet this is hardly the answer to the question you meant, right?

Thank you for these answers.  I should restate my question to be about the prevalence of animals without action potentials  Similar to Rune, it is also the case for me that C. Elegans is the only animal I know of whose neurons use only graded potentials, and not APs, to communicate. (I don't know that that disqualifies its axons from being called "axons," what do you think Nikolaos?)  But it can't be that C. Elegans is the only one.  Learning about the worm made me wonder if animals without AP's might be more common than we usually suppose, particularly considering that the vast majority of animals are very small.

Your answers got me googling again and I found this:

http://www.quora.com/No-spikes-in-c-elegans-neurons-spikes-in-Aplysia-neurons-How-and-when-did-spikes-originate

The answerer argues that C Elegans IS exceptional (and the asker mentions that C. Elegans motor neurons DO spike).

More links I found:

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3951993/

http://neurodojo.blogspot.fi/2009/07/no-spikes-in-this-worm-motor-neurons.html

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2225687/

And should we think of C. Elegans as only being capable of

Learning about C. Elegans made me wonder if it might be the case   that fact made me realize  (but it's neurons do have axons, correct?).  Learning that fact got me wondering if this feature is secretly common. 

Hello,

If you want to know about the so-called "graded action potentials" (excitable but not all-or-none) you can find such example in Paramecium those electrophysiological properties have been studied mostly in 1970's. The membrane of Paramecium is excitable, that is, there are voltage-gated Ca2+ ion channels that can exhibit regenerative depolarizing transient potential generation similar to the "all-or-none" Na+ action potential in neurons in the mammalian system. The difference would be caused by their slow activation compare to their inactivation processes (that is Ca2+-dependent, not voltage) and activation of delayed rectifier K channels. I think you can find similar examples in excitable membranes that use voltage-dependent Ca2+ channels.

So perhaps your question can be re-formulated as: are there multicellular organisms with a nervous system that do well without spikes altogether (besides possibly C. elegans)? Then I would say that there are many species in Nature besides human, mouse, and C. elegans. So I would not dare making any general statement.

An interesting question that maybe could not be answered generally, as Victor points out. However, a matter aside is that also graded potential neurons may have axons, like in the insect compound eyes (photoreceptors and 1st order interneurons sometimes have 1-2 mm long processes) and in the graded motor interneurons in arthropod abdominal ganglia. A good starting point for those is an old book:

Roberts, A., & Bush, B. M. (1981). Neurones without impulses: their significance for vertebrate and invertebrate nervous systems (Vol. 6). Cambridge University Press.

I agree with Matti and would just add that in vertebrate retina photoreceptors, bipolar cells, horizontal cells and majority of amacrines are spike-free. On the other hand, photoreceptors in Balanus (an arthropod) do spike.

Yes, that is true. However, the distinction between "graded" and "AP-neurons" is also otherwise somewhat blurred, because there are many documented instances, where either Na- or Ca-based action potentials are being used in addition to the graded potentials (see e.g. Baden, et al, 2013, Trends in neurosciences, 36, 480-488, which contains several citations to that effect). This blurring further complicates possibilities for any general answer to the original question.