In a nutshell: The floppiness we experience when we sleep, called muscle atonia, is triggered by nerve cell projections running from a small region of the brain called the PnO to the spinal cord.

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Ever noticed how floppy a person goes when they sleep? This almost complete lack of muscle tone, called muscle atonia, happens in REM sleep, which is characterized by rapid eye movement and dreaming, and occurs 4 or 5 times during a night’s sleep.

A small cluster of nerve cells between the midbrain and hindbrain called the oral pontine reticular nucleus, or PnO, is key to cycling between REM and non-REM sleep. Changes in the activity of chemical messengers in the PnO, or damage to it, increase or decrease the amount of time spent in REM sleep

Nerve cell projections from the PnO to sites along the entire length of the spinal cord have been found in rats and some other species, showing the pathway by which this tiny brain region could also affect muscle tone in the whole body.

The study described here, by a team led by CIBF chief investigator George Paxinos, confirm those findings, showing that the same is also true for mice.

“It suggests an anatomical basis for the loss of muscle tone while we sleep,” says Andy (Huazheng) Liang, a post-doc in Paxinos’ lab and lead author on the study.

Next steps:
These neuroscientists will continue mapping PnO nerve connections, shifting their focus to incoming connections now that the outgoing ones have been identified. They will also investigate how the whole PnO-spinal cord system influences different sleep stages and muscle atonia.

Liang, H., Watson, C., & Paxinos, G. (2015). Projections from the oral pontine reticular nucleus to the spinal cord of the mouse. Neuroscience letters, 584, 113-118.

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