A new study has revealed an important way in which sleep helps the brain process emotions for the next day, and while the findings were discovered in mice, they could also help us solve some of the mysteries of human sleep.
The role of sleep in brain function is still very much an enigma, but there is overwhelming evidence that rapid eye movement (REM) sleep helps humans consolidate their emotional memories.
But how that actually plays out in the brain is something scientists are still investigating.
The prefrontal cortex is a part of the brain that is heavily involved in emotional processing, and yet during REM sleep, some of its neurons, known as pyramidal neurons, are strangely quiet .
That might sound paradoxical at first. After all, how is this part of the brain helping us control our emotions during sleep if it’s not being active when we’re actually getting some shut-eye?
As it turns out, silence is also a powerful tool. Research on sleeping and awake mice suggests the quieting of the prefrontal cortex during REM sleep helps the whole system reset.
The findings are consistent with other recent studies that suggest sleep keeps neuronal activity under control.
Without proper REM sleep, networks in the brain can become ‘oversaturated’ with emotional messages, like fear, making it harder to determine important signals from background noise. When awake, this can lead a mouse to act overly fearful or not fearful enough.
When active and awake, neurons in the brain receive messages from their ‘arms’ (aka their dendrites). These messages are then conveyed to the body of the neuron (aka the soma), which is responsible for propagating messages to other neurons.
During REM sleep, however, neurons in the prefrontal cortex of mice appear to behave differently. The dendrites show increased activity, but the soma shows decreased activity.
“This means a decoupling of the two cellular compartments, in other words soma wide asleep and dendrites wide awake,” explains neurologist Antoine Adamantidis from the University of Bern in Switzerland.
In simple terms, this decoupling means that neurons are processing information they have already received, but not sending messages on.
With the body of the neuron no longer sending off as many messages, the arms of the neuron have time to consolidate the information they have already received, essentially ‘learning’ which incoming messages should be sent off and which should not.
This allows the brain to better respond to environmental changes the next day, allowing animals to discriminate between danger and safety with greater effect.
When the activity of dendrites were inhibited during REM sleep, mice in the study lost their ability to discriminate between audio cues associated with danger and safety.
Meanwhile, when the soma was not effectively silenced during REM sleep, mice became more attuned to danger signals overall.
“This may result in overconsolidation of emotional memories observed in post-traumatic stress disorders and other affective psychiatric and mood disorders often associated with REM sleep disturbances,” the authors suggest .
The mechanism has not yet been observed in human neurons, but the findings could help scientists better understand why conditions like post traumatic stress disorder and sleep disturbance are so closely linked .
The study was published in Science .