Researchers at New York University have found new evidence that sleep after learning strengthens connections between brain cells and helps consolidate new memories.
The study, published last month in the journal Science, shows that sleep after learning encourages the growth of dendritic spines, the tiny protrusions from brain cells that connect to other brain cells and facilitates the passage of information across synapses, the junctions at which brain cells meet. Moreover, the activity of brain cells during deep sleep, or 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,” said senior investigator Dr. Wen-Biao Gan, professor of neuroscience and physiology and a member of the Skirball Institute of Biomolecular Medicine at NYU Langone Medical Center. “If you don’t sleep well you won’t learn well. 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.”
On the cellular level, sleep is anything but restful: Brain cells that spark as we digest new information during waking hours replay during deep sleep, also known as slow-wave sleep, when brain waves slow down and rapid-eye movement, as well as dreaming, stops. Scientists have long believed that this nocturnal replay helps us form and recall new memories, yet the structural changes underpinning this process have remained poorly understood.
Researchers trained two sets of mice: one trained on a spinning rod for an hour and then slept for seven hours; the second trained for the same period of time on the rod but stayed awake for seven hours. The scientists 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 on 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.
Finally, the scientists showed that brain cells in the motor cortex that activate when mice learn a task reactivate during slow-wave deep sleep. Disrupting this process, they found, prevents dendritic spine growth.
“Our data suggest that neuronal reactivation during sleep is quite important for growing specific connections within the motor cortex,” said Gan.