Scientists have found important physical evidence that sleep helps consolidate and strengthen new memories.
Researchers from the NYU Langone Medical Center have shown for the first time that sleep after learning encourages the growth of dendritic spines in mice.
Dendritic spines are 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.
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. If you don't sleep well you won't learn well," said senior investigator Wen-Biao Gan, 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," said Gan.
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 believe that this nocturnal replay helps us form and recall new memories, yet the structural changes underpinning this process have remained poorly understood.
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, the scientists were 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.
Over time mice learned how to balance on the rod as it gradually spun faster.
After documenting that mice, in fact, sprout new spines along dendritic branches, within six hours after training on the spinning rod, the researchers set out to understand how sleep would impact this physical growth.
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.