Man in REM sleep. Credit: Giphy.

Rapid eye movement sleep (REM) is a unique mammalian sleep phase during which the eyes move quickly in different directions. There’s much more going on when you slumber during this phase than just retina gymnastics, though. The brain is more active than in the non-REM phase which is evidenced by intense dreaming that may occur. REM sleep is also known to consolidate learning and memories. It was never clear until recently, however, how this mechanism that transforms temporary memories into permanent looks like in the brain.

Now, a paper published in Nature Neuroscience may have made REM sleep a lot less mysterious. According to experiments carried out by New York University School of Medicine researchers, some neural structures which help form connections get pruned during REM while others are strengthened. In other words, the brain selects which of these structures are allowed to support stronger connections in a timely manner, ultimately causing long-lasting memories to form.

These neural structures in questions are called dendritic spines. They’re small outgrowths found on a neuron’s dendrite — the branched extension of the neuron where impulses received from other cells at synapses are transmitted to the cell body.

The spines aren’t permanent structures. Instead, these will grow, shorten or entirely disappear in time as the importance of different connections changes. This shift in priorities is critical — otherwise, we would never be able to make sense of all the information we absorb on a daily basis. Previous research found up to 10 percent of all new synapses are formed daily but only a smaller number will be stably maintained over time.

A depiction of spine formation and elimination. Credit: Wikimedia Commons.

To investigate the functions and underlying mechanisms of REM sleep, the researchers subjected lab mice to motor learning tasks then examined postsynaptic dendritic spines of neurons in the mouse primary motor cortex. Some of the mice were either allowed to enjoy a full night’s rest or deprived of the REM phase.

Mice that had the chance to go through REM sleep cycles showed significantly higher pruning of new dendritic spines compared to the REM-deprived mice. This difference was observed only in the case of new dendritic spines. Previously existing spines were pruned or strengthened at the same rate, signaling that REM was a decisive factor.

The researchers also analyzed how dendritic pruning changes as the mouse ages. They found that neural pruning happens the most frequently during a mouse’s juvenile stage. Pruning occurred during REM sleep later in life too, at adulthood, but less frequently. Without REM sleep, the size of spines that are retained doesn’t grow.

These findings seem to agree with previous work on humans. REM sleep deprivation during development can have a detrimental effect on cognitive development which is why doctors will often tell teens sleep is very important. In adulthood, REM sleep deprivation can cause behavior changes and mood swings.

Calcium channels seem to play a role in the decision-making that leads to the pruning or strengthening of the dendritic spines. Observations suggest sudden changes in the amount of calcium seen during REM sleep can kick start the selection process. When calcium channels were blocked, no selection occurred.

“More study is needed to investigate whether REM sleep has similar or different roles in regulating synapse development and plasticity of other types of neurons in different cortical layers and brain regions, ” the researchers wrote in their paper.