Muscle innervation is not a connectome

Since the word "genome" entered the lexicon, mainly on the heels of the Human Genome Project, there has been no shortage of people trying to cash in on the idea of "-omes." The idea is that an "-ome" is a complete catalogue of all... something. So we have budding fields like proteomics (all the proteins in an organism), and, in my own field, neuromics (all the neurons in an organism) and now, connectomics (all the neural connections in an organism).

Lu and colleagues purport to have a connectome. That's what the title says, after all. This is, unfortunately, misleading. This is innervation of one muscle. It's detailed, yes, but it's no more a "connectome" than sequencing one gene is a "genome." I suppose, though, that "Motor innervation patterns of the interscutularis muscle" is less likely to land you a spot in a high-profile journal than a title using, "connectome."

Putting my irritation with the unwarranted title aside... what's to learn here?

The authors used a genetically modified mouse that expressed a fluorescent protein in all its motor neurons. Then, they used confocal microscopy techniques to reconstruct the paths of multiple motor neurons leading to a small muscle in the head. Neither of these techniques are particularly groundbreaking in principle, but there's no doubt that achieving this level of high detail would have required a lot of patience. From there, there's lots of math and models, but it boils down just to looking closely at what they saw.

The authors summarize their main findings as these.

The more muscle fibres a neuron innervates, the bigger the twitch. Although this is presented as a conclusion, the authors didn't actually measure twitch generated by muscles in this paper, so there is still more physiology.

The bigger the neuron, the more muscle fibres it innervates. Not a huge surprise, considering that there will be more demands to create and transport chemicals to the tips of the nerves if there are more synapse.

Every motor neuron follows its own unique path, and it's often a path loaded with detours. This speaks to the development of muscle innervation being quite loosely controlled. Again, no big surprises, as there'd been a reasonable amount of evidence showing that axon guidance is something that tracks (say) chemical gradients, rather than any sort of precise pathfinding or lock and key mechanism.

Perhaps surprisingly, it's probably going to take a long time before these methods are successfully applied to many invertebrate species, particularly arthropods. Although invertebrates have fewer neurons, the way they connect to muscles is much more complicated.

Reference

Ju Lu, Juan Carlos Tapia, Olivia L. White, Jeff W. Lichtman (2009). The Interscutularis Muscle Connectome PLoS Biology, 7 (2) DOI: 10.1371/journal.pbio.1000032