Researchers discover how the brain paralyzes you while you sleep

TSUKUBA, Japan — For a television character like Homer Simpson, having narcolepsy can lead to all sorts of ridiculous situations that make the audience laugh. In real life however, suddenly falling asleep while driving, working, or even in mid-sentence is no laughing matter. Now, researchers in Japan have discovered that a group neurons in the brain actually paralyzes the body while we sleep. Their study finds problems with these neurons can lead to conditions including narcolepsy, cataplexy, and rapid eye movement (REM) sleep behavior disorder.

A team from the University of Tsukuba says REM sleep goes hand-in-hand with dreaming. When people are in this deep sleep, their eyes keep moving but their body stays still. Researchers call this partial paralysis of the muscles REM-atonia.

For patients dealing with REM sleep behavior disorder however, their bodies don’t experience the paralyzing effects of sleep. In fact, people with the disorder keep moving; often yelling, punching, and even jumping or standing up in their sleep.

Using lab mice, Professor Takeshi Sakurai and his team examined what regions of the brain typically prevent various species from moving during REM sleep. Their study revealed that the brain’s ventral medial medulla plays a major role in paralyzing people when they sleep. This brain area receives signals from the sublaterodorsal tegmental nucleus (SLD), which shuts down movement functions.

“The anatomy of the neurons we found matched what we know,” Sakurai explains in a university release. “They were connected to neurons that control voluntary movements, but not those that control muscles in the eyes or internal organs. Importantly, they were inhibitory, meaning that they can prevent muscle movement when active.”

Finding the link to sleep disorders

When study authors blocked signal inputs to the ventral medial medulla in mice, they started moving while asleep — just like a patient with REM sleep behavior disorder.

In the opposite extreme, narcolepsy causes patients to fall asleep without warning at any point of the day. Cataplexy is a similar condition which causes people to suddenly collapse after they lose muscle control. Although patients with cataplexy are not asleep, their paralyzed bodies act like they’re in normal REM sleep.

Sakurai’s team tested a theory that these disorders have a connection to the same brain neurons which impact REM sleep. Researchers studied the brains of mice which suffer cataplexic attacks after eating chocolate. Their experiments again revealed a link between brain circuits controlling REM-atonia and cataplexy.

“We found that silencing the SLD-to-ventral medial medulla reduced the number of cataplexic bouts,” Sakurai reports. “The glycinergic neurons we have identified in the ventral medial medulla could be a good target for drug therapies for people with narcolepsy, cataplexy, or REM sleep behavior disorder.”

The study author adds future tests will need to examine how emotions, which can also trigger cataplexy, affect these neurons.

The study appears in the Journal of Neuroscience.

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