Throughout the day, the human brain absorbs a torrent of knowledge, processes the information, and learns, before “switching off” over night. New research explains this process in action. Microscopic images of synapses – the junction between two nerve cells – have been shown to expand with daytime stimulation and shrink with sleep, thus resetting the brain ready for the next day.

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Microscopic images showed that synapses shrink with sleep and grow when awake.
Image credit: Wisconsin Center for Sleep and Consciousness

Scientists at the University of Wisconsin School of Medicine and Public Health conducted a 4-year study that demonstrates the “synaptic homeostasis hypothesis” (SHY) functioning.

Results of the study were published in the journal Science.

SHY is a theory proposed by Dr. Chiara Cirelli and Dr. Giulio Tononi, both of the Wisconsin Center for Sleep and Consciousness. Cirelli and Tononi speculate that sleep is the price we must pay for brains that are pliable and able to learn new things repeatedly.

Constant stimulation of the synapse during wakeful hours results in it growing stronger and larger, and it is thought that this expansion plays a key role in memory and learning. To ensure that the synapse does not become saturated and neural signaling and memories obliterated, SHY indicates that this growth must be counterbalanced.

Sleep is thought to be the perfect counterbalance and ideal time for the process of renormalization. During sleep, people are less aware of the “here and now” and can block out the external world, which may balance and restore the synapse.

As synapses become stronger and more efficient they get bigger. In contrast, as they weaken, the synapses shrink in size.

Cirelli and Tononi aimed to test their theory by analyzing whether the size of the synapses alters between sleeping and waking hours.

Analysis involved 4 years of research and many specialists. The team used technology with high spatial resolution (called serial scanning 3-D electron microscopy) to photograph, reconstruct, and investigate two areas of cerebral cortex in the brain of a mouse. The technology allowed them to reconstruct 6,920 synapses and take measurements of their size.

The team was unaware of whether the mouse they were examining was a well-rested mouse or a mouse that had been awake, in order to ensure that the results were not influenced.

Findings of the research show that the size of the synapses correlated with the amount of sleep the mouse had had previous to the image being captured. A few hours of sleep were found to reduce the size of the synapses by 18 percent in both areas of the cerebral cortex. The changes were also confirmed to be proportional to the synapses’ size.

While the scaling took place in around 80 percent of the synapses, the scaling was selective and the largest synapses did not follow the same pattern. Scaling may not have happened in the larger synapses since they might be linked with the most stable memory traces.

This shows, in unequivocal ultrastructural terms, that the balance of synaptic size and strength is upset by wake and restored by sleep. 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.”

Dr. Chiara Cirelli

“Extrapolating from mice to humans, our findings mean that every night trillions of synapses in our cortex could get slimmer by nearly 20 percent,” concludes Tononi.

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