If offered a delicious smell, a roundworm will usually stop its wandering to investigate the source, but sometimes it won't. Just as with humans, the same stimulus does not always provoke the same response, even from the same individual. New research at Rockefeller University, published online on March 12, 2015 in Cell, offers a new neurological explanation for this variability, derived by studying a simple three-cell network within the roundworm brain. "We found that the collective state of the three neurons at the exact moment an odor arrives determines the likelihood that the worm will move toward the smell. So, in essence, what the worm is thinking about at the time determines how it responds," says study author Dr. Cori Bargmann, Torsten N. Wiesel Professor, Head of the Lulu and Anthony Wang Laboratory of Neural Circuits and Behavior at Rockefeller University. "It goes to show that nervous systems aren't passively waiting for signals from outside, they have their own internal patterns of activity that are as important as any external signal when it comes to generating a behavior." The researchers went a step deeper to tease out the dynamics within the network. By changing the activity of the neurons individually and in combination, first author Andrew Gordus, a research associate in the lab, and his colleagues could pinpoint each neuron's role in generating variability in both brain activity and the behavior associated with it. The human brain has 86 billion neurons and 100 trillion synapses, or connections, among them. The brain of the microscopic roundworm Caenorhabditis elegans, by comparison, has 302 neurons and 7,000 synapses. So while the worm's brain cannot replicate the complexity of the human brain, scientists can use it to address tricky neurological questions that would be nearly impossible to broach in our own brains.
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