The role of vestibular system and the cerebellum in adapting to gravitoinertial, spatial orientation and postural challenges of REM sleep.

The underlying reasons for, and mechanisms of rapid eye movement (REM) sleep events remain a mystery. The mystery has arisen from interpreting REM sleep events as occurring in 'isolation' from the world at large, and phylogenetically ancient brain areas using 'primal' gravity-dependent coordinates, reflexes and stimuli parameters to relay and process information about self and environment. This paper views REM sleep as a phylogenetically older form of wakefulness, wherein the brain uses a gravitoinertial-centred reference frame and an internal self-object model to evaluate and integrate inputs from several sensory systems and to adapt to spatial-temporal disintegration and malignant cholinergic-induced vasodepressor/ventilatory threat. The integration of vestibular and non-vestibular sensory graviceptor signals enables estimation and control of centre of the body mass, position and spatial relationship of body parts, gaze, head and whole-body tilt, spatial orientation and autonomic functions relative to gravity. The vestibulocerebellum and vermis, via vestibular and fastigial nucleus, coordinate inputs and outputs from several sensory systems and modulate the amplitude and duration of 'fight-or-flight' vestibulo-orienting and autonomic 'burst' responses to overcome the ongoing challenges. Resolving multisystem conflicts during the unique stresses (gravitoinertial, hypoxic, thermal, immobilisation, etc.) of REM sleep enables learning, cross-modal plasticity, higher-order integration and multidimensional spatial updating of sensory-motor-cognitive components. This paper aims to generate discussion, reinterpretation and creative testing of this novel hypothesis, which, if experimentally confirmed, has major implications across medicine, bioscience and space physiology, from developmental, clinical, research and theoretical perspectives.