360 — The C. elegans AWA Olfactory Neuron Fires Calcium-Mediated All-or-None Action Potentials

C. elegans neurons have action potentials. There. We said it. https://t.co/oNrjhKFR6n

— Cori Bargmann (@betenoire1) June 30, 2018

Some commonly-held neuroscience knowledge is that the neurons of the C. elegans worm do not spike. That is, in contrast with most other organisms’ brains, they do not produce membrane-based action potentials. While graded responses do exist in the rest of the animal kingdom (vertebrate retinas, for example), they’re best at transmitting over short distances. Nematodes were thought to be one of the few organisms with only graded potentials. Substantiating this claim is the absence of of voltage-gated calcium channels in the worm genome.

But — of course — nothing is that simple.

C. elegans is often used as “the simplest possible model” for connectomics research, but this attitude ignores a lot of very important nuance. For example, even individual neurons are more complex than most models appreciate.

In this paper, the researchers find that the AWA neuron — responsible for olfaction (the worm relies on this neuron to perform chemosensation gradient-climbing) — exhibits spikes that look quite like action potentials (APs) both in terms of membrane potential difference and in terms of data transmission. These all-or-nothing APs are distinct from graded responses both physiologically and chemically.

This is not the first time that calcium-dependent action potentials have been identified in the worm: It has been long known that the muscles of the worm produce spikes. But this is a sign that we perhaps need to better understand our “model organism” before drawing conclusions from the complete “non-spiking” connectome.