Verified on 08/09/2022 by PasseportSanté
REM sleep occurs after the slow wave sleep phase; it ends, thus, each cycle of sleep. It is characterized by significant electroencephalogram activity, similar to that of the waking state, associated, at the same time, with muscle atony which causes the sleeper to remain motionless during this paradoxical sleep. It is during this phase that most dreams occur, which can be remembered upon awakening. Its mechanisms have been discovered since the 1950s, in particular thanks to the work of scientist Michel Jouvet. According to different theories, multiple functions are allocated to this phase of sleep also called REM (Rapid Eye Movement). Specific brain regions and neurons are involved in its mechanism of action.
What is REM sleep?
Sleep is a drop in the state of consciousness, separating two periods of wakefulness. It alternates phases of slow wave sleep and paradoxical sleep. In fact, paradoxical sleep, also called REM sleep (for Rapid Eye Movement sleep in English, because it is characterized by rapid movements of the eye) is notably marked by rapid fluctuations and of small amplitude of the electroencephalogram. This EEG translates the electrical activity of the brain. REM sleep is therefore associated with an intense period of dreams, rapid eye movements, as well as muscle atony, ie a decrease in muscle tone.
Its name is due to the fact that during this phase of sleep, the activity of the electroencephalogram resembles that observed during the waking state, but without activity or muscle tone. The REM phase of sleep was originally discovered in the 1950s in humans and later confirmed through the study of cats. It is in particular the scientist Michel Jouvet who described the main characteristics of paradoxical sleep.
Sleep: a succession of cycles
Sleep consists of a succession of cycles. Thus, three to six cycles, each lasting 60 to 120 minutes, follow each other during the night. These cycles include alternating slow-wave sleep and REM sleep. The first cycles during the night are mainly made up of deep slow-wave sleep, while those marking the end of the night are more conducive to REM sleep. On an electroencephalography tracing, the electrical waves traveling through the brain look different depending on the stage of sleep.
So called because it is characterized by slow waves, it is itself made up of several stages, first a transition phase of a few minutes which separates waking and sleep, then a phase of light sleep which ends installed. This will then be followed by a phase of progressively deeper sleep lasting several tens of minutes: the waves visible on the EEG will then be of high amplitude and low frequency.
During this phase of sleep, the brain inhibits the motor system: thus, the sleeper remains completely still. Muscle tone is totally abolished during this phase, except for some movements of the extremities. On the other hand, cerebral activity is close to that of the waking phase. As for blood pressure and respiratory rate, they experience frequent fluctuations. It should also be noted that REM sleep is conducive to dreams: thus, it includes the most intense dreams, and those that can be remembered when awake.
In addition, paradoxical sleep is longer during the first years of life, its duration being reduced in adulthood.
Michel Jouvet: a scientific approach for the discovery of paradoxical sleep
Following in the tradition of Claude Bernard, Michel Jouvet, a great pioneer in sleep research, truly insisted on the experimental rigor of approaches in medicine and for scientific questions. He began his research career in neurology and neurosurgery, examining loss of consciousness in patients with coma, and he established for the first time the subcortical electrophysiological criterion for brain death.
Michel Jouvet then worked in California on experimental approaches on animals to determine the regions that were necessary for consciousness as well as the mechanisms by which consciousness was naturally altered during sleep, before characterizing, in his laboratory in Lyon, which parts of the brain, in the cat, were necessary for sleep. This is how he discovered a third state of the brain that stood out. Indeed, he was characterized by a rapid cortical activity, similar to that of the waking state, yet accompanied by a behavior similar to that of sleep, and more notably still by an atony of the postural muscles: this led him to qualify this phase, in 1959, as “paradoxical sleep”.
REM sleep is the body’s greatest mystery since, unlike slow-wave sleep, the brain is not at rest, but unlike wakefulness, the body cannot move. Michel Jouvet has, in fact, developed multiple approaches in his laboratory in order to examine the phylogenetic, ontogenetic, as well as phenomenological aspects, but more particularly the mechanistic aspects of sleep and wakefulness: he has thus studied the structures, and more later, the chemical elements that would be responsible for the different phases of sleep. In particular, he was able to demonstrate that the serotonin neurons located in the raphe nuclei were essential for slow-wave sleep.
Later, on the basis of pharmacological studies, he first demonstrated that neurons in the area of the locus coeruleus, a subcortical nucleus of the brain located in the brainstem, must be involved in sleep. paradoxical. Finally, he also shed light on the importance of certain acetylcholine neurons for REM sleep. In addition, acetylcholine neurons are thought to be equally important for both slow-wave and REM sleep, and in particular may stimulate cortex activation while promoting muscle atony during this phase of REM sleep. Finally, other neuro-modulating substances could be essential for the different phases of sleep, and in particular orexin as well as MCH (melanin concentrating hormone) in the hypothalamus.
What is the role of REM sleep?
REM sleep appeared late in evolution, as did homeothermy (the fact that body temperature does not depend on ambient temperature). In fact, paradoxical sleep plays other roles than slow-wave sleep, or at least can complete them. Several theories have been put forward as to the role of REM sleep.
- For so-called psychodynamic theories, the Freudian theory predominates, namely that paradoxical sleep, or dreaming, would be the expression of a “liberation of instinctive drives”, normally blocked by the preconscious. Furthermore, he would be the guardian of sleep;
- for other theories, paradoxical sleep plays a fundamental role either in memorization or in forgetting;
- in addition, by means of a process of “endogenous stimulation” of the brain, this paradoxical sleep would therefore play a role in the development of the brain, during the first years of life;
- finally, paradoxical sleep could make it possible to maintain the genetic bases of personality, like an iterative programming process.
Finally, as rarely in the history of physiology, there are many questions on the question of the functions of sleep, and in particular paradoxical sleep; and if the how, that is the acquired data relating to the mechanisms of sleep and dreams, is important, on the other hand the why remains hypothetical and ignorance remains ultimately almost total as to the question of the functions of paradoxical sleep.
What mechanisms explain REM sleep?
The regions of the brain which contain the neurons responsible for REM sleep are located in the brainstem, and more precisely at the level of the pontine reticular formation (a nervous structure of the brainstem at the interface of the autonomic, motor and cerebral systems, and with which half of the neurons of the central nervous system come into contact): the two anterior thirds of this bridge thus seem to contain the structures triggering REM sleep. The caudate nucleus, which is one of the three major components of the basal ganglia, is also involved.
Neurons generating REM sleep are said to be glutamatergic, since they use glutamate as a neurotransmitter. Furthermore, it has also been suggested that levels of glycine but also those of GABA (gamma-aminobutyric acid, a very common chemical messenger in cortical neurons) could contribute to the hyperpolarization of motoneurons during REM sleep.
However, hyperpolarization corresponds to a change in the membrane potential from a negative value to a value which is even more so: this hyperpolarization will therefore move the membrane potential away from the value of the stimulation threshold; in summary, no action potential is triggered at the level of the axon, so there is no triggering of motor neuron activity and therefore no movement is set in motion. On the other hand, cholinergic neurons, or, as mentioned above, acetylcholine neurons, contribute to the activation of the brain during this phase of sleep.
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