Japanese Researchers Found Out How the Brain Paralyzes Muscles While in REM Sleep

The brain sends signals to the muscles during Rapid Eye Movement or REM sleep to cause REM sleep muscle paralysis or muscle atonia or REM-atonia. According to Stanford Health Care, this is considered a normal function of REM sleep.

But how does it do that, and why?

Professor Takeshi Sakurai of the University of Tsukuba in Japan and his colleagues have found a group of neurons in the brain that link suppressed unwanted movement during REM sleep that could link narcolepsy, cataplexy, and rapid eye movement (REM) sleep behavior disorder for future treatments.

Finding the Neurons Responsible for REM-atonia

REM sleep is directly correlated when a person starts to dream while sleeping. The eyes move back and forth, and the body remains still. People who cannot maintain a near-paralysis of muscles during this stage of sleeping have a REM sleep behavior disorder because instead of being still, they start moving to the extent of walking, standing up, jumping, or yelling.

Working with mice, Sakurai and his team have found a group of neurons in the mouse's brain that is likely the cause of near-paralysis during REM sleep. The neurons are located in a region of the brain called the ventral medial medulla that receives input from the sublaterodorsal tegmental nucleus or SLD, EurekAlert! reported.

Sakurai said that the structure of the neurons matched what they previously knew. The neurons were connected to neurons that are responsible for voluntary movements. He added that the neurons were inhibitory neurons that prevent muscle movement when active.

They tried blocking these neurons' input and observed that the mice started moving when they were asleep. It was as if they have the REM sleep behavior disorder, the researchers said.

The team suspected that the group neurons were also connected to narcolepsy and cataplexy. To know for sure, they tested their hypothesis using a mouse model of narcolepsy in which, when given chocolates, it can trigger its cataplexy. Sakurai said that they found that inhibiting the SLD-to-ventral medial medulla reduced the cataplexic attacks.

In general, these groups of neurons can control muscle atonia in both cataplexy and REM sleep. Therefore, it can be a good target for developing treatments for cataplexy, narcolepsy, and REM sleep behavior disorder.

"Future studies will have to examine how emotions, which are known to trigger cataplexy, can affect these neurons," Sakurai said.

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What Would Happen If the Body Did Not Paralyze During Sleep?

According to Stanford Health Care, when the body is not under REM sleep, muscle paralysis, or muscle atonia, it could mean that they have the REM sleep behavior disorder (RBD). It is characterized by unusual behaviors even during REM sleep that causes disruption and injury as a manifestation of action-filled or violent dreams.

Behaviors in RBD could include twitching, flailing, kicking, utterances, sitting up, and leaving the bed. The person could wake up after experiencing that and provide a coherent dream story.

RBD could also sometimes be caused by certain drugs or drug withdrawal, but it is more common with age and, in some cases, neurological disorders. Understanding what causes muscle atonia during REM sleep can help people with RBD resolve their condition.