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The model organism C. elegans is about 1 mm in length. This is quite small. In fact, some C. elegans neurons span >25% of the length of its body (ref.).

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This observation leads me to the following question: Which neuron in C. elegans has the largest span, and why is it this large?

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I am a member of OpenWorm.org, an open-source project that aims to simulate a C. elegans cell-by-cell. I didn't know the answer to this myself off the top of my head, but I have access to a number of folks who have been working hard on C. elegans anatomy for the last couple of years so I went ahead and asked them this question on our discuss mailing list. Thanks to Chris Grove and Tim Busbice for providing the answer that I am reporting below:

Chris Grove:

Finding the neuron with the largest span partly depends on your definition of "length" in this context. In terms of largest amount of total axon/dendrite length, this would most probably be the PVDL/PVDR sensory neurons. They produce a large number of branches of dendrites that act to perceive mechano and thermo-sensation. See the WormAtlas page here.

Tim Busbice:

If by "length" one means, absolute longest extension from head to tail, that's a tougher call, as there are several that extend from the head to the tail. An example of these are the AVDL/AVDR command interneurons, that function as touch modulators for backward locomotion induced by head-touch, and also drive backward movement of the animal along with driver cell AVA neuron, AVE and A-type motor neurons.

You can find a lot of info on C. elegans anatomy on WormAtlas and on an OpenWorm data spreadsheet in which we are consolidating detailed anatomy data from different sources.

Also I would advise checking out the OpenWorm Browser - a web-based 3D viewer that you can use to explore the C. elegans anatomy cell-by-cell (the 3D model was developed by Chris Grove, see VirtualWorm for details). For instance you can easily search for those "longest" neurons mentioned above! Screenshot of one of them selected (AVDL) below:

AVDL neuron

Hope this helped!

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    $\begingroup$ Awesome answer! I meant length in Tim Busbice's sense, since these types of neurons cause havoc for my network analysis. $\endgroup$
    – Douglas S. Stones
    Commented Mar 5, 2013 at 12:45
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    $\begingroup$ Glad to hear it helped - keep in mind AVDL/R are among the longest in that sense, but there are many neurons that span from head to tail, such as CANL/R [ wormatlas.org/neurons/Individual%20Neurons/CANframeset.html ]. $\endgroup$
    – JohnIdol
    Commented Mar 6, 2013 at 1:18

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