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Sleep Shrinks Synapses, Preps For New Learning


3D images of synapses that shrink during sleep. Credit: Wisconsin Center for Sleep and Consciousness

Sleep, is an enduring mystery, and scientists continue to study its forms and its functions, and some new research shows sleep helps make us smarter.

Researchers at the University of Wisconsin - Madison have found sleep helps improve brain function by shrinking synapses, the junctions between nerve cells.

VOA spoke with researcher Chiara Cirelli who said the research team started with the hypothesis that we sleep so that our brain can restore and recharge itself. She said the idea seems simple, elegant and logical, but testing it and discovering how it works has been incredibly difficult.

Cirelli and Giulio Tononi of the Wisconsin Center for Sleep and Consciousness have been trying to prove the "Synaptic Homeostasis Hypothesis" since they published a first version of it in 2003.

Giulio Tononi and Chiara Cirelli. Credit: John Maniaci/UW Health
Giulio Tononi and Chiara Cirelli. Credit: John Maniaci/UW Health

Cirelli said they began by "literally, measuring the size of the synapses" in the brain. "There are 100 billion synapses in our brain," she says and we know that "that stronger synapses are also bigger."

They knew that during sleep the brain "can sample all our synapses, and renormalize them in a smart way, comprehensive and balanced."

So they decided to see if that renormalization has a physical component, that is, are they bigger after being awake all day, and smaller after a good night's sleep.

How to measure a synapse

Synapses are only about 20-40 nanometers wide, and the team looked for changes in these already tiny gaps between nerve cells. They had to wait until advances in electron microscopy made it possible to see these tiny changes.

A university press release said it was "a massive undertaking, with many research specialists working for four years to photograph, reconstruct, and analyze two areas of cerebral cortex in the mouse brain. They were able to reconstruct 6,920 synapses and measure their size."

Cirelli says it is an incredibly painstaking process because "all the actual measurements of the synapses (what we call the “segmentation”) has to be done manually."

To make sure there was no bias, "the team deliberately did not know whether they were analyzing the brain cells of a well-rested mouse or one that had been awake."

The result proved the SHY hypothesis by finding a few hours of sleep led on average to an 18 percent decrease in the size of the synapses. "This shows," Cirelli says, "in unequivocal ultrastructural terms, the balance of synaptic size and strength is upset by wake and restored by sleep,"

"Sleep," the study concludes, "is the price we pay for brains that are plastic and able to keep learning new things."

Cirelli says what "happens with sleep, is that salient and novel information is integrated within our body of knowledge, irrelevant details are forgotten, and new space is created for new memories to be formed the next day."

She says our synapses shrink as our brain cleans house, and we wake up refreshed and ready to fill up those synapses with new information.

What can we do with this information

Cirelli says the work gets more complex from here on by researching the effect lack of sleep has on synapses. The preliminary data says without sleep those synapses never shrink, and the concern is that "if synapses continue to strengthen, they will saturate, and thus neurons, which use synapses to communicate, will start responding too often and too much, also to inappropriate stimuli."

In short, the noise in the brain will increase, at the expense of the real ”signal.”

The team is also interested in the possibility there could be other ways to help the brain sift through material, perhaps through meditation or other forms of quiet wakefulness. The research also holds out hope that could help people with chronic sleep disorders.

The team has already found one of possibly several molecules that make the synapses downsize. It's called Homer 1a, and is only present in the brain during sleep. If they can chart more of these molecules and discover how they get the synapses to shrink there could one day be a way to refresh the brain without the need for sleep.

The research findings are the culmination of more than a decade of work performed by researchers at the University of Wisconsin - Madison (UWM) and is published in the journal Science.

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