Smallest brain

January 26, 2008 9:40 PM

What is the animal with the smallest number of neurons (but more than one cell)? More specifically, is there a multicellular organism that has about 10-50 neurons, thus making its "brain" significantly smaller than that of the nematode C. elegans?
posted by wyzewoman to science & nature (10 comments total)

I know that jelly fish larvae have no nervous system what so ever (at least that's what I've read) so that might be a place to start...
posted by wfrgms at 10:31 PM on January 26

Well, by definition if it's got more than one neuron it must have more than one cell - neurons are a specific type of cell.

(Was it Paul Keating who, when baiting an Opposition colleague, made the quote "if he had another neuron, he'd have a synapse"?)

As you're probably aware, though, it gets murky at the bottom of that particular pool. Cnidarians - sponges, jellyfish, anenomes, etc - have a nerve net, but it differs from that in higher animals in that the nerve cells don't have specific axons, dendrites, & the synapses between them. Wherever the cells cross over and touch each other, the action potential can & does propagate between cells (which is why, in part, when you touch one side of a jellyfish or anenome then whole thing reacts).

So, you might need to refine your question / thinking a bit further if you want a specific answer. Is a neuron just a nerve cell that can pass impulses between themselves and other cells? If so, I'd be looking at some of the small hydra, etc, for an answer. Or is it a something more specific; a nerve cell which passes one signals in one direction only and has specific input (dendrites) and output (axon) sections? In which case the nematodes would be as good a place as any to start looking.

Either way, it comes down to drawing a dividing line between colonies of organisms (and when/where cells start to specialise their functions) and multicellular organisms. I think there's still quite some ongoing discussion about that.

(Interesting C. elegans facts: the development path from fertilised ovum to every somatic cell in the body has been mapped out, which is partly why they're so interesting to developmental biologists. Adults of the 2 sexes (males & hermaphrodites) have different numbers of cells; males have 950-odd, hermaphrodites 1000-odd.)
posted by Pinback at 11:13 PM on January 26 [3 favorites]

When does a ganglion become a brain?

I'll have to dig out the fulltext of this one - looks interesting.
posted by ikkyu2 at 11:25 PM on January 26

Thank you very much.

I do appreciate the fact that a neuron is a cell, but wanted to avoid the possible "technicality" of defining a unicellular organism as the smallest brain. As for refining the question, both options are interesting. My original intention concerned cells that have specific input and output (even if not through separate neurites), like in the case of nematodes. I was wondering if there is a nematode out there with far fewer neurons than C. elegans, or perhaps some other organism altogether. (BTW, I think that planaria has more neurons than C. elegans). I guess that Cnidarians are also interesting in their own right. Is there a sponge/jellyfish/anenome out there that is considered a model "simplest" case in terms of the size and the structure of its network of non-specific nerve cells?

Again, I do appreciate your effort and thank you for your time!
posted by wyzewoman at 8:03 AM on January 27

Most rotifers have around 100 or so neurons. There may be some that have a lot less.

I've known some co-workers that might have had even fewer...
posted by metabeing at 10:27 AM on January 27 [1 favorite]

What about tardigrades? Some of them are extremely tiny.
posted by metabeing at 10:28 AM on January 27 [1 favorite]

Again, many thanks!

I found the following quote suggesting that the brains of tardigrades are more complex than those of C. elegans: "When Sydney Brenner was looking for a new model organism for applying genetics to study development and neurobiology, he stopped briefly at tardigrades, but decided they had too many neurons, before moving on to nematodes and eventually choosing the then little-known C. elegans." [Goldstein and Blaxter (2002) Curr Biol 12(14), R475].
As for Rotifers, a quick search suggests that they have at least 150-200 neurons, which is a bit less than the C. elegans 302 but still in the same ball park.

This makes me all the more curious about whether there exists a known example of animals that do have neurons (loosely defined as above), but only a total of 10-50 of them. Roughly speaking, these numbers constitute a "missing order of magnitude" in brain size between the nematodes/Rotifers (~200-300 neurons) and the single cell (technically not a neuron). Is there a fundamental reason for suspecting that, say, 30 neurons would just not be beneficial for survival? Any plausible evolutionary reason for this gap? Alternatively, is it reasonable to expect that such animals do exist but were just not noticed by science, or perhaps were not given enough attention to become a familiar house hold name for the non-expert?
posted by wyzewoman at 12:11 PM on January 27

It's actually been over a decade since I did neuroscience, but as I recall the sea slug Aplysia has well under 100.
posted by neuron at 1:57 PM on January 27

According to some references I found, neuron, Aplysia has "thousands" of neurons. There are only a few neurons in the loop that mediates retraction of the mantle, though, and they've been studied extensively.
posted by ikkyu2 at 8:11 AM on January 29

G.W. Bush.

Ha!
posted by Gooberoo at 12:35 PM on February 6 [1 favorite]