Altering a single neuron causes a surprisingly sweeping change in the rat brain.

The frequency of brain waves sets two states of the brain apart.

Maybe the flapping of a butterfly's wings in Brazil can't really cause a tornado in Texas, but this whimsical idea may be an apt analogy for how the brain works. Perturbing one single neuron, HHMI researchers have discovered, can change the electrical landscape of the whole brain.

Yang Dan, an HHMI investigator at the University of California, Berkeley, didn't set out to show this. She wanted to probe how the strength of connections between neurons changes over time. So she and her colleagues set up an experiment: they would activate single neurons one at a time in a rat brain and then measure local connections from nearby neurons.

They found that instead of just tweaking how it interacted with other nearby neurons, activating any one neuron flipped the anaesthetized rat's entire brain between two states. In one state, which resembles deep sleep, brain waves are slow and synchronized throughout the brain. In the other state, which resembled rapid eye movement (REM) sleep—a less deep state of sleep—neurons were less synchronized and fired faster and more often. The flip worked in both directions.

“This is surprising,” says Dan, “because it shows that the weight of a single neuron is so much greater than we thought. It's the power of the individual.” There are 100 billion neurons in the human brain, she points out, with only weak connections between them. But with each neuron connected—albeit weakly—to thousands of other neurons, a signal can spread like wildfire.

Why activating a single neuron would cause such a drastic change throughout the entire brain, Dan can't fully explain. “So far what we have is a fascinating observation,” she says, “but in terms of explanations we only have speculation.”

Dan's research, published in the May 1, 2009, issue of Science, suggests that the cortex—the part of the brain where her team was perturbing neurons—plays an important role in controlling the state of the brain. Previously, scientists had focused mostly on other areas—the hypothalamus and brain stem. To further flesh out the issue, Dan hopes to study rats that are awake, rather than anaesthetized.

Illustration: PHANE / Photo Researchers, Inc.