Central mechanisms of paradoxical sleep.

Paradoxical sleep (PS) is composed of several characteristic "tonic" and "phasic" events. In this paper, it was shown that each of these "PS-subsystems" might be generated by distinct brainstem neuron groups and involved in the cholinergic, cholinoceptive, and monoaminoceptive mechanisms. In addition, it was suggested that the central mechanisms of PS consist of: cholinergic PS-on neurons that discharge tonically and specifically during the periods of PS; and monoaminergic (serotonergic, noradrenergic, and presumably also adrenergic) PS-off neurons that cease firing selectively during this period of sleep. The PS-on neurons play an executive role, while the PS-off ones play a permissive role in the generation of PS. The PS-on neurons are located both in the mediodorsal pontine tegmentum, especially the peri-LC alpha and medial part of the LC alpha, and in the ventromedial medulla, particularly the Mc. The PS-off neurons are located rather diffusely in the lower brainstem. There exist both excitatory interactions between the PS-on neurons, between the PS-off neurons and reciprocal inhibitory interactions between the PS-on and PS-off neurons. Periodically, these interactions lead to, by still unknown mechanisms, the excitation of all PS-on neurons and inhibition of all PS-off neurons, and, consequently, the appearance of PS.