In mammals, the process of sleeping goes through stages of deepening sleep followed by shallower sleep. The cycle repeats during sleep in humans about every 90 to 120 minutes, or about 4 or 5 cycles per night on average during an eight-hour sleep period. At the shallowest phase of each cycle, mammals have a period of light sleep accompanied by relaxation and stillness of the body, except for the eyes. The eyes in this sleep phase have rapid, brief, nonrhythmic movements we call REM, for Rapid Eye Movements. The shallow sleep phase with REM is called REM sleep. Most dreaming in sleep in humans occurs during REM sleep.
During REM sleep, the body's muscles are kept relaxed due to action of the nucleus ceruleus in the pons of the brainstem, which inhibits the spinal cord's motor neurons. Otherwise we might act out (with body motions) our actions in our dreams.
The reason for REM remaining as eye movements while all other body motion is deactivated in sleep is not clear. One theory I had thought likely (until the publication below) is that REM is just due to incomplete motor inhibition. In that scenario, REM is just because the body does not specifically need to deactivate such eye movements, since eye movement with eyes closed does not jeopardize safety as other body movements in sleep might.
But that theory about REM sleep seems much less likely in view of the new information about the REM-specificity of the action of the cells of a small cluster of cells that make up the small nucleus papilio in the brainstem. If the rapid eye movements in sleep were just due to dream actions, there would seem little reason to expect there to be a separate set of motor neurons to maintain the movements of REM. The ordinary extraocular nuclei would be enough.
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Published: 19 November 2019
Neurons in the Nucleus papilio contribute to the control of eye movements during REM sleep
C. Gutierrez Herrera, F. Girard, A. Bilella, T. C. Gent, D. M. Roccaro-Waldmeyer, A. Adamantidis & M. R. Celio
Nature Communications volume 10, Article number: 5225 (2019)
Abstract
Rapid eye movements (REM) are characteristic of the eponymous phase of sleep, yet the underlying motor commands remain an enigma. Here, we identified a cluster of Calbindin-D28K-expressing neurons in the Nucleus papilio (NPCalb), located in the dorsal paragigantocellular nucleus, which are active during REM sleep and project to the three contralateral eye-muscle nuclei. The firing of opto-tagged NPCalb neurons is augmented prior to the onset of eye movements during REM sleep. Optogenetic activation of NPCalb neurons triggers eye movements selectively during REM sleep, while their genetic ablation or optogenetic silencing suppresses them. None of these perturbations led to a change in the duration of REM sleep episodes. Our study provides the first evidence for a brainstem premotor command contributing to the control of eye movements selectively during REM sleep in the mammalian brain.
