The Brain Has Been Asleep While You’re Still Awake?

We all know that staying up late will cause people confused and unresponsive. Why the lack of sleep can bring about such a terrible experience? MIT scientists have recently solved this puzzle, and relevant paper is published in the journal eLife on October 13.

Slow oscillating brain waves are a major feature of deep sleep, and these slow oscillations also appeared during coma and general anesthesia, as well as related with the loss of consciousness. The researchers found that brain circuits, which originates in thalamic reticular nucleus (TRN), are capable of inducing this slow oscillation, and triggering small regions of the brain to fall asleep or become less alert, while the rest of the brain remains awake. If TRN activity is strong enough, the brain waves will even take over the entire brain.

Current sleep research is mostly concerned with the overall control of sleep, which occurs when the entire brain is full of slow waves. This study shows that sleep-deprived animals will show slow waves in certain areas of the their brains while they are awake, indicating that the brain can also exists alertness control at a local level.

Through optogenetics technology, researchers undertook weak stimulation the TRN in awake mice, and slow waves appeared in a small portion of the cerebral cortex. When they gave the mice more stimulation, the entire cortex have shown 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," said Laura Lewis, a research affiliate in MIT's Department of Brain and Cognitive Sciences and one of the lead authors of the new study.

The researchers noted that, TRN is responsible for fine-tuning the brain’s control of the local areas, and promoting exchanges between them through the coordination of different areas of slow waves. This mechanism could help the brain consolidate new memories, and the TRN also explains why sleep-deprived people easily distracted.

"I'm inclined to think that happens because the brain begins to transition into sleep, and some local brain regions become drowsy even if you force yourself to stay awake," Lewis says.

The researchers believe the TRN may help the brain consolidate new memories by coordinating slow waves between different parts of the brain, allowing them to share information more easily. Further studies showed that TRN stimulation can induce deep non-REM sleep. This research can help people design new sleeping pills and anesthetics, closer to natural sleep state of unconsciousness.

"The TRN is rich in synapses -- connections in the brain -- that release the inhibitory neurotransmitter GABA. Therefore, the TRN is almost certainly a site of action of many anesthetic drugs, given that a large classes of them act at these synapses and produce slow waves as one of their characteristic features." explained Emery Brown, professor of MIT.

Reference:

Thalamic reticular nucleus induces fast and local modulation of arousal state. eLife