Electron microscope images of the inside of the brains of mice have given credence to the hypothesis that sleep plays an integral role in our ability to learn new things. The images show that the junctions between neurons, known as synapses, strengthen and grow during waking hours, then shrink by almost 20 percent during sleep, which opens up more room for them to grow, and learning to take place, when waking the next day.
In an effort to test their "synaptic homeostatis hypothesis (SHY)," which proposes that sleep is the price we pay for the plasticity of our brains, Drs. Chiara Cirelli and Giulio Tononi from the Wisconsin Center for Sleep and Consciousness used serial scanning 3D electron microscopy to capture images of the cerebral cortex of the mouse brain with extremely high spatial resolution.
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The research project took four years and involved photographing, reconstructing and analyzing two areas of a mouse brain's cerebral cortex and ultimately resulted in the research team reconstructing 6,920 synapses and measuring their size so as to provide some visual proof of the SHY.
When the researchers correlated the measurements of the synapses with the amount of sleep the mice had in the six to eight hours before the images were taken, they found that, on average, the size of the synapses decreased 18 percent after a few hours' sleep.
"This shows, in unequivocal ultrastructural terms, that the balance of synaptic size and strength is upset by wake and restored by sleep," Cirelli says. "It is remarkable that the vast majority of synapses in the cortex undergo such a large change in size over just a few hours of wake and sleep."
Additionally, such decreases were found in both areas of the cerebral cortex analyzed and were proportional to the size of the synapses. The researchers say that around 80 percent of the synapses exhibited this scaling, with the largest ones remaining unchanged – they suspect this is because those synapses may be associated with the most stable memory traces.
"Extrapolating from mice to humans, our findings mean that every night trillions of synapses in our cortex could get slimmer by nearly 20 percent," says Tononi.
The team's research appears in the journal Science.