Figure 1.
Model of Some of the Major Systems Involved in Regulating Muscle Activity in REM Sleep Drawn on a Sagittal Section of the Brainstem
Cholinergic (ACh) neurons in the pons, which are under the inhibitory control of noradrenergic (NE) and serotonergic (5-HT) neurons, trigger REM sleep. They activate descending glutamatergic neurons, which in turn activate glycinergic and GABAergic neurons. Other glycinergic interneurons in the spinal cord are also activated by unknown descending inputs. The release of glycine and GABA inhibits motoneurons. The descending glutamatergic pathway also activates GABAergic interneurons, which inhibit noradrenergic and serotonergic neurons. The reduction in norepinephrine and serotonin release during REM sleep disfacilitates motoneurons. Descending glutamatergic neurons that connect directly to motoneurons produce phasic excitation during REM sleep. The net result of the action of this network is an absence of muscle tone in the “antigravity” muscles in REM sleep, interrupted by twitches (see text for references).