Numerous studies published over the past decade have shown that a good night’s sleep is essential for brain health as it enhances the consolidation of newly formed memories in people. But exactly how these observations were related was unclear. A new study discovered the mechanism by which a good night’s sleep improves learning and memory.
In the study published in the journal Science on June 6th, researchers at New York University School of Medicine and Peking University Shenzhen Graduate School show for the first time that sleep after learning encourages the growth of dendritic spines, the tiny protrusions from brain cells that connect to other brain cells and facilitate the passage of information across synapses, the junctions at which brain cells meet. In addition, the activity of brain cells during deep sleep, or slow-wave sleep, after learning is critical for such growth.
The findings, in mice, provide important physical evidence in support of the hypothesis that sleep helps consolidate and strengthen new memories, and show for the first time how learning and sleep cause physical changes in the motor cortex, a brain region responsible for voluntary movements.
“We’ve known for a long time that sleep plays an important role in learning and memory. If you don’t sleep well you won’t learn well,” said senior investigator Wen-Biao Gan, PhD, professor of neuroscience and physiology and a member of the Skirball Institute of Biomolecular Medicine at NYU Langone Medical Center. “But what’s the underlying physical mechanism responsible for this phenomenon? Here we’ve shown how sleep helps neurons form very specific connections on dendritic branches that may facilitate long-term memory. We also show how different types of learning form synapses on different branches of the same neurons, suggesting that learning causes very specific structural changes in the brain.”
To find out the mechanism by which a good night’s sleep improves learning and memory, researchers trained 15 mice to run backwards or forwards on a rotating rod. They allowed some of them to fall asleep afterwards for 7 hours, while the rest were kept awake.
The team monitored the activity and microscopic structure of the mice’s motor cortex, the part of the brain that controls movement, through a small transparent “window” in their skulls. This allowed them to watch in real time how the brain responded to learning the different tasks.
They found that learning a new task led to the formation of new dendritic spines – tiny structures that project from the end of nerve cells and help pass electric signals from one neuron to another – but only in the mice left to sleep.
This happened during the non-rapid eye movement stage of sleep. Each task caused a different pattern of spines to sprout along the branches of the same motor cortex neurons.
At the same time, the neurons that were active during the initial task were re-activated, seemingly to stabilize the newly formed spines.
This growth spurt continued after the mice woke up. About 5 per cent of spines in the motor cortex were formed anew in the 8 to 24 hour period after the mice woke up, said co-author Guang Yang, also at the Skirball Institute. “Our previous studies suggest that about 10 per cent of these new spines should be maintained over subsequent weeks to months,” he said.
“Now we know that when we learn something new, a neuron will grow new connections on a specific branch,” said Dr. Gan. “Imagine a tree that grows leaves (spines) on one branch but not another branch. When we learn something new, it’s like we’re sprouting leaves on a specific branch.”
Dr. Gan’s team is now trying to answer these questions. “We would like to see how brain activity during sleep affects signaling within specific sets of branches and ultimately causes the formation of new spines,” he said.
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