Lasers prove that areas of the brain remain resting even while we’re awake

The act of sleeping is often associated with an all or nothing brain state: entirely sleeping or completely awake. But now, MIT neuroscientists have recently discovered a brain circuit that can trigger small parts of the cranium to fall asleep or become less alert, while the rest of it stays awake.

The circuit originates in a structure known as the thalamic reticular nucleus (TRN), which relays signals to the thalamus and the brain’s cortex, inducing characteristics of brain waves in a deep sleep. Slow fluctuations can also occur during coma and anesthesia and is also are associated with decreased arousal. With enough TRN activity, these waves can take over the entire brain.

Experts at MIT believe TRN may help the brain create new memories by coordinating slow waves between different parts, which would allow them to share information easier. If you’ve ever zoned out without completely falling asleep, TRN may also be responsible for that.

“During sleep, maybe specific brain regions have slow waves at the same time because they need to exchange information with each other, whereas other ones don’t,” said Laura Lewis, a research affiliate in MIT’s Department of Brain and Cognitive Sciences.

The MIT team began its investigation of control in alertness or drowsiness with the TRN, because its physical location makes it ideally positioned to play a role in sleep, Lewis said. The TRN surrounds the thalamus like a shell and can act as a gatekeeper for sensory data entering, which then sends information to the cortex for further processing.

Using optogenetics, a method that allows scientists to stimulate or silence neurons with light, the researchers were able to prove their theory on mice. When they weakly stimulated the TRN in mice that were awake, slow waves appeared in a tiny part of the cortex, but when the stimulation was increased, the entire cortex showed slow waves.

“We also found that when you induce these slow waves across the cortex, animals start to behaviorally act like they’re drowsy. They’ll stop moving around, their muscle tone will go down,” Lewis said.

Their findings suggest that the TRN modifies the brain’s control over local brain regions, either enhancing or reducing slow waves in specific parts so those areas can communicate with each other. This may explain why sleep-deprived people zone out momentarily without actually falling asleep.

“The strength of this paper is that it’s the first to use optogenetics to try to dissect the role of part of the thalamo-cortical circuitry in generating slow waves in the cortex,” said Mark Opp, a professor of anesthesiology and pain medicine at the University of Washington who was not part of the research team.

Understanding how the brain controls rest can help the MIT neuroscientists figure out a way to produce different types of snoozing in subjects or result in anesthetic drugs that produce more of a sleep-like state.

Via MIT