Understanding how the brain paralyses you while you sleep

Our body muscles become temporarily paralysed during sleep

sleep rep Representational image

Rapid eye movement sleep (REMS) is a unique phase of sleep in mammals and birds, characterised by periodic recurrence of rapid eye movements, accompanied by low muscle tone throughout the body. REMS first occurs about 90 minutes after falling asleep.

REM sleep correlates when we dream. Our eyes move rapidly move back and forth behind closed eyelids, but our bodies remain still. This near-paralysis of muscles while dreaming is called REM-atonia, and is lacking in people with REM sleep behaviour disorder.

Instead of being still during REM sleep, muscles move around, often going as far as to stand up and jump, yell, or punch. Researchers set out to find the neurons in the brain that normally prevent this type of behaviour during REM sleep.

Researchers at the University of Tsukuba led by Professor Takeshi Sakurai have found neurons in the brain that link sleep disorders and could provide a target for treatments.

In reality though, sleep-related illnesses like narcolepsy, cataplexy, and rapid eye movement (REM) are linked to overwhelming daytime drowsiness. Narcolepsy is a chronic sleep disorder that causes overwhelming daytime drowsiness. Cataplexy is a sudden episode of muscle weakness that occurs while fully aware and conscious.

Working with mice, the team identified a specific group of neurons as likely candidates. These cells were located in an area of the brain called the ventral medial medulla and received input from another area called the sublaterodorsal tegmental nucleus, or SLD.

“The anatomy of the neurons we found matched what we know," explains Sakurai. "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,” he said. When the researchers blocked the input to these neurons, the mice began moving during their sleep, just like someone with REM sleep behaviour disorder.

Narcolepsy, as demonstrated by Homer Simpson, is characterised by suddenly falling asleep at any time during the day, even in mid-sentence (he was diagnosed with narcolepsy). Cataplexy is a related illness in which people suddenly lose muscle tone and collapse.

Although they are awake, their muscles act as if they are in REM sleep. Sakurai and his team suspected that the special neurons they found were related to these two disorders. They tested their hypothesis using a mouse model of narcolepsy in which cataplexic attacks could be triggered by chocolate. "We found that silencing the SLD-to-ventral medial medulla reduced the number of cataplexic bouts," says Sakurai.

Overall, the experiments showed these special circuits control muscle atonia in both REM sleep and cataplexy.

"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 behaviour disorder," says Sakurai.