REM sleep is known to help solidify memories, but the mechanism for making memories more permanent is not well-understood. A recent study published in Nature Neuroscience shows that, during REM sleep, some of the structures neurons use to make connections with each other are pruned, while others are maintained and strengthened. The findings indicate that sleep's role in solidifying memories comes through allowing the brain time to selectively eliminate or maintain newly formed neural connections.

Dendritic spines are small outgrowths on a neuron’s dendrite, which is the portion of the neuron that receives chemical signals from other neurons. These spines enhance the strength of connections between neurons so they can play an important role in strengthening new neural circuits and solidifying new memories. These spines aren't permanent structures; instead, nerve cells can create new ones or get rid of existing ones (a process called pruning) as the importance of different connections shifts.

The new memories in this case were formed in mice, which were trained to complete a treadmill-like motor task. Then, the mice were either deprived of REM sleep or allowed to experience this form of sleep. The mice that were allowed REM showed significantly higher pruning of new dendritic spines compared to the mice that were REM sleep deprived. This difference in pruning was only seen for new dendritic spines, and previously existing dendritic spines were pruned at the same rate.

The researchers looked at how REM sleep influenced dendritic spine pruning at various points throughout the mice’s lives. They found that this neural pruning occurred while the mice were in REM sleep during their development (during the equivalent of mouse adolescence) but could also occur when the mice experienced REM sleep later in life after motor learning tasks. REM sleep increased the size of the spines that were retained, both during development and after motor learning tasks—these unpruned new spines were strengthened, reinforcing the developing neural circuitry.

In other words, during REM sleep, the brain selects which portions of new neural circuitry it wants to eliminate and which portions it wants to strengthen and enhance for future use.

The researchers then looked at the role calcium channels, which let calcium ions across membranes, may play in these decisions, as changes in the levels of calcium in cells is a normal part of brain activity. They found that sudden changes in the amount of calcium seen during REM sleep were critical for selective pruning and strengthening. When these calcium channels were blocked, the previously seen changes in dendritic spines no longer occurred.

Too little REM sleep during development is known to have detrimental effects on brain maturation, and this recent study provides new insight regarding the mechanisms that may be at play here. Without sufficient REM sleep during development, juvenile and adolescent brains may not be able to adjust the connections among their neurons to hold on to what they've learned. Similarly, REM sleep is known to help with learning during all stages of development, including adulthood. In both cases, lack of REM sleep prevents the brain from eliminating unneeded spines generated during learning and prevents the strengthening of critical new spines that make newly learned tasks stick.

Nature Neuroscience, 2017. DOI: 10.1038/nn.4479(About DOIs)