Sophisticated worms
One cell does it all: Sensory input to motor output in extraordinary neuron

It’s one of the basic tenets of biological research — by studying simple “model” systems, researchers hope to gain insight into the workings of more complex organisms.

Caenorhabditis elegans — a tiny, translucent worm with just 302 neurons — has long been studied to understand how a nervous system is capable of translating sensory input into motion and behavior.

New research by the laboratory of Professor Aravi Samuel in the Harvard Physics Department and the Center for Brain Sciences, however, is uncovering surprising sophistication in the individual neurons of the worm’s “simple” nervous system.

Quan Wen, a postdoctoral fellow in the Samuel lab who spearheaded the research, has shown that a single type of neuron in the C. elegans nerve cord (the worm equivalent of the spinal cord) packs both sensory and motor capabilities. The locomotory systems of most creatures, including humans, use different neurons to gather sensory information about animal movement or to send signals to muscle cells. C. elegans encodes an entire sensorimotor loop into one particularly sophisticated type of motor neuron. The work is described in the journal Neuron.

“This type of circuit is completely new — this is not the way people think about any motor circuit,” Samuel said.

The discovery arose from researchers asking a simple question: How does C. elegans organize its movements?

“What sent us down this road was a phenomenon that we’ve observed in the lab,” Samuel explained. “If we place the worms in a wet environment, they will swim. On surfaces, however, they crawl. The question was how the animal ‘knew’ to do each. The answer had to be feedback: Something is telling the worm that it’s in a low-viscous environment here, and a high-viscous environment there.

“The general name for this is ‘proprioceptive feedback,’ ” Samuel continued. “It’s that process that allows your brain to understand what each of your legs is doing and coordinate your ability to walk, it gives you an awareness of your body posture. The real puzzle in this case, however, was that C. elegans has so few neurons … we didn’t understand how proprioceptive feedback could come back into the system.”

(Image credit: snickclunk)