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U.S. researchers said Thursday they have found important physical evidence that sleep helps consolidate and strengthen new memories.
In a study published in the U.S. journal Science, researchers at New York University showed 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.
Moreover, the activity of brain cells during deep sleep called slow-wave sleep after learning is critical for such growth.
"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," senior investigator Wen-Biao Gan, professor of neuroscience and physiology of the university, said.
"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," Gan said.
Gan and colleagues employed mice genetically engineered to express a fluorescent protein in neurons.
Using a special laser-scanning microscope that illuminates the glowing fluorescent proteins in the motor cortex, they were then able to track and image the growth of dendritic spines along individual branches of dendrites before and after mice learned to balance on a spin rod.
They trained two sets of mice: one trained on the spinning rod for an hour and then slept for 7 hours; the second trained for the same period of time on the rod but stayed awake for 7 hours.
The researchers found that the sleep-deprived mice experienced significantly less dendritic spine growth than the well-rested mice.
Furthermore, they found that the type of task learned determined which dendritic branches spines would grow. Running forward on the spinning rod, for instance, produced spine growth on different dendritic branches than running backward on the rod, suggesting that learning specific tasks causes specific structural changes in the brain.
"Now we know that when we learn something new, a neuron will grow new connections on a specific branch," said 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."
The researchers also showed that brain cells in a brain region called motor cortex that activate when mice learn a task reactivate during slow-wave deep sleep.
Disrupting this process prevents dendritic spine growth, Gan said. Their findings offer an important insight into the functional role of neuronal replay -- the process by which the sleeping brain rehearses tasks learned during the day -- observed in the motor cortex.
"Our data suggest that neuronal reactivation during sleep is quite important for growing specific connections within the motor cortex," Gan said.
"The findings are especially important for children," he said. "Sacrificing sleep to study only causes significantly less dendritic spine, thus you will not be able to remember what you have learned."
