Brain cells take a break

As you fall into deep sleep, some neurons pause their electrical activity

Scientists have long wanted to know what happens inside the human brain when deep asleep. You may be unconscious, but your brain cells are busy with activity. Neurons, brain cells that conduct electricity, keep your mind humming even while your body is resting.

In a new study, a team of scientists found that neurons take breaks periodically as a person heads into deep sleep. These pauses in neuron activity help keep people asleep, even if they hear noises or are touched. Sydney Cash, a neurologist at Massachusetts General Hospital and Harvard Medical School in Boston, and his team found a way to study electricity in the brain, inside and out.

Scientists use different tools to study electrical currents in the brain. One of the most useful is the EEG, or electroencephalogram. An EEG represents the brain’s activity as a graph that looks like a long series of differently shaped waves. The height, width and closeness of those waves give scientists a peek at what’s happening in a person’s head. Even though they can study the patterns, however, scientists don’t know what causes the waves to form.

In the study led by Cash, the researchers were interested in a particular type of EEG squiggle called a K-complex. To people who don’t understand EEG patterns, a K-complex just looks like a squiggle that’s larger than the lines around it. To a trained scientist, a K-complex shows a significant change in the electrical activity in the brain.

A K-complex may show up on an EEG when the sleeping person hears a noise or has his or her sleep disturbed. Or these squiggles may show up for other reasons.

EEGs can’t see everything, however. They only measure electric signals — including K-complexes — on the outside of the brain. In the new study, the scientists found a way to see even deeper into the brain. They studied patients with epilepsy, a medical condition that can cause a person to suffer from serious seizures. Epilepsy is believed to be caused by overactive neurons.

In previous surgeries, the people with epilepsy had had tiny electrodes implanted deeper inside their brains. Electrodes are also used to study electrical currents, and doctors had hoped that these devices would help them identify the source of the epileptic seizures.

Cash, who studies epilepsy, realized that those same electrodes could be used to study electrical activity deeper inside the brain while at the same time an EEG told the scientists what was happening on the surface. By comparing the two sets of information, the scientists thought they could better understand brain activity.

They were right. While the patients slept, Cash and his team collected data from both the EEG and from the electrodes. They found that whenever the EEG showed a K-complex, there was a dip in activity inside the brain. In other words, the K-complex was a sign on the outside that neurons on the inside were taking a break. These breaks help keep people asleep. Cash’s research also shows that K-complexes don’t spread to the entire brain, which means that only some neurons take a break at any given time.

The brain has long been one of the most mysterious parts of the human body. Studies like this one help scientists open a window onto the inner workings of our heads — and possibly figure out how the circuitry works. Understanding how neurons behave is important, but scientists also need to know what these cells do when they’re not at work. As this study shows, neurons take a break so you can too.

Stephen Ornes lives in Nashville, Tenn., and his family has two rabbits, six chickens and a cat. He has written for Science News Explores since 2008 on topics including lightning, feral pigs, big bubbles and space junk.

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