Myelin myth busted

The Neurophilosophy blog has an excellent post on a recent article about sensory hairs involved in crayfish escape responses.

I did see an opportunity to correct a common error. What follows is a repost of my comment from the blog.

"Neuro myth busting" may have to become a blog label. Previous examples include this one on ascidian brains and one on reptilian brains.

Just a quick reminder for those who aren't biologists: Myelin is a covering around many neurons that makes signals travel along neurons faster.

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The crayfish is an invertebrate, and therefore does not produce myelin(.)

Many invertebrates produce myelin (Hartline & Colman 2007). In fact, many decapod crustaceans (shrimps and prawns) have myelinated giant interneurons that are the core of the escape system described here. These crustaceans have the fastest known conduction velocity in the animal kingdom, about 200 m/s. This is about double the typical textbook value given for myelinated mammalian neurons.

It's not clear why crayfish lack myelin. Shrimps and prawns are more basal taxa, and the distribution of myelin suggests that myelination was the ancestral condition. Many decapods (including crayfish) seem to have lost myelin, rather than the shrimps and prawns gaining it (Faulkes 2008).

References

Faulkes Z. 2008. Turning loss into opportunity: The key deletion of an escape circuit in decapod crustaceans. Brain, Behavior and Evolution 72(4): 351-361. doi: 10.1159/000171488

Hartline DK & Colman DR. 2007. Rapid conduction and the evolution of giant axons and myelinated fibers. Current Biology 17(1): R29-R35. 10.1016/j.cub.2006.11.042