Rem Sleep: A Paradox Of Brain Activity And Inactivity

Rapid Eye Movement (REM) sleep is called paradoxical sleep because it involves seemingly contradictory states of an active mind and a sleeping body. During REM sleep, the brain acts as if it is awake, with cerebral neurons firing with the same overall intensity as in wakefulness. However, the body remains completely still, with many muscles becoming temporarily paralysed. This state of paralysis is known as REM atonia and is thought to prevent sleepers from acting out their dreams.

Brain is active, body is still

The term "paradoxical sleep" was coined by French researcher Michel Jouvet in the late 1950s to describe the seemingly contradictory states of an active mind and a sleeping body during REM sleep. During this stage, the brain exhibits high levels of metabolic activity and brain waves similar to those seen during wakefulness, while the body remains mostly inactive, with muscles relaxed and temporarily paralysed.

During REM sleep, the brain's cerebral neurons fire with similar intensity to that of wakefulness, resulting in complex and vivid dreams. Brain energy use during this stage equals or exceeds that of wakefulness, with oxygen and glucose metabolism rates surpassing those of non-REM sleep by 11-40%. The brainstem, particularly the pontine tegmentum and locus coeruleus, is believed to be the origin of the electrical and chemical activity regulating REM sleep. This phase is characterised by an abundance of the neurotransmitter acetylcholine and a near absence of monoamine neurotransmitters such as histamine, serotonin, and norepinephrine.

While the brain is highly active, the body is temporarily paralysed during REM sleep. This state, known as REM atonia, is achieved through the inhibition of motor neurons, which undergo hyperpolarisation, raising the threshold for stimulation. Most muscles are affected, including those responsible for movement, with the notable exceptions of the diaphragm and eye muscles. This paralysis is essential for preventing sleepers from acting out their dreams and keeping them safely still.

The paradox of REM sleep lies in the contrasting states of the brain and body. While the brain exhibits wakefulness-like activity, the body remains inactive, with muscles relaxed and temporarily paralysed. This contrast has led to the term "paradoxical sleep," highlighting the seemingly contradictory nature of this sleep stage.

Dreaming vs. no visual or auditory stimulation

During the REM sleep phase, the brain is highly active, and brain activity speeds up, resembling the activity during waking hours. This leads to vivid dreams. However, the body remains completely still, with temporary muscle paralysis, except for the muscles that help with breathing and eye movement. This state of muscle atonia or paralysis is called REM atonia.

The term "paradoxical sleep" was coined by French researcher Dr. Michel Jouvet in the 1950s due to the contradictory states of an active mind and a sleeping body. The brain behaves as if it is awake, with cerebral neurons firing with the same intensity as during wakefulness, but the body is paralysed.

The absence of visual and auditory stimulation or sensory deprivation during REM sleep can cause hallucinations. The brain acts as if it is processing external stimuli, but the body is immobilised. This is similar to the tonic immobility reflex, a defensive mechanism where animals feign death to escape predators.

REM sleep is also associated with memory consolidation and creativity. The thalamus, which processes sensory information during waking hours, becomes active during REM sleep, contributing to the rich sensory experiences of dreams.

While the purpose of REM sleep is still debated, it is associated with various important functions, including learning, memory, and creativity. The high levels of brain activity and energy consumption during REM sleep remain a mystery, especially since the suppression of REM sleep through drugs or brain injuries does not seem to have any striking effects on behaviour or memory.

Brainstem's role in REM sleep

The brainstem plays a crucial role in REM sleep, the fourth and final stage of the sleep cycle. It is during this stage that the brain is highly active, resembling the brain activity of someone who is awake. The brainstem, comprised of the pons, medulla, and midbrain, controls the transitions between being awake and falling asleep.

During REM sleep, the brainstem sends signals to relax the muscles essential for body posture and limb movements, preventing sleepers from acting out their dreams. This temporary paralysis, called atonia, affects most muscles, but those involved in breathing remain active, and the eyes dart rapidly from side to side.

The brainstem was first identified as playing a role in REM sleep by French researcher Dr. Michel Jouvet in the 1950s. Jouvet discovered that the brainstem sends signals to initiate muscle atonia during REM sleep. He observed that cats with lesions on the brainstem lost muscle atonia and appeared to act out their dreams.

The brainstem is also involved in regulating REM sleep. The pons, a structure within the brainstem, is both necessary and sufficient for REM sleep. The glutamatergic neurons in the sublaterodorsal tegmental nucleus of the pons are active during REM sleep and are involved in muscle tone suppression. The cessation of activity in the inhibitory GABA neurons in this region can trigger REM sleep.

The brainstem's role in REM sleep is further highlighted by its involvement in REM sleep behaviour disorder (RBD). RBD is a sleep disorder where individuals do not experience muscle paralysis during REM sleep and may act out their dreams. This disorder is often caused by a breakdown in the area of the brainstem responsible for regulating REM sleep.

REM sleep deprivation effects

REM sleep is a stage of sleep that is characterised by increased brain activity and muscle paralysis. During this stage, the brain is active, and vivid dreams may occur. However, the body remains still as many muscles become temporarily paralysed.

REM sleep deprivation can have several effects on the body and mind. Firstly, it can lead to fatigue, irritability, and changes in mood, memory, and cognition. Research has also shown that it may contribute to cardiovascular issues, type 2 diabetes, cancer, stroke, and neurodegenerative diseases such as Alzheimer's.

REM sleep deprivation has also been linked to an increase in nociceptive behaviour and pain sensitivity. This suggests that a lack of REM sleep may cause hyperalgesia and reduce the effectiveness of opioid-mediated pain relief.

In addition, REM sleep deprivation can affect the body's ability to regulate stress. Studies have shown that 96 hours of REM sleep deprivation in rats resulted in increased levels of plasma corticosterone, a hormone that is released in response to stress.

Finally, REM sleep deprivation can impact learning and memory consolidation. Tasks that require complex associations or cognitive processing are particularly sensitive to REM sleep deprivation, and a lack of REM sleep has been linked to lower binding of noradrenaline to adrenergic receptors, which may impair memory.

Overall, while the specific purpose of REM sleep remains unknown, REM sleep deprivation can have significant and wide-ranging effects on the body and mind.

REM sleep's link to memory

REM sleep is associated with memory consolidation, a process that stabilises recently acquired information into long-term storage.

During REM sleep, the brain processes new learnings and motor skills from the day, deciding which to commit to memory, which to maintain, and which to delete. This is also the stage of sleep where emotional memories are processed, which can help us cope with difficult experiences.

The link between REM sleep and memory consolidation was first suggested in the 1950s when scientists studying sleeping infants noticed distinct periods of rapid eye movement. They hypothesised that the brain was active during these periods, similar to when awake, and that this activity was linked to dreaming and memory consolidation.

While the exact nature of the link between REM sleep and memory remains unclear, recent studies have provided more evidence. Using optogenetic techniques in a mouse model, one study found that neural activity during REM sleep is critical for normal memory consolidation. Another study of healthy college students found that those who napped between tests performed better, and the more time they spent in REM sleep during their nap, the higher their accuracy.

REM sleep is also associated with brain development, as newborns spend most of their sleep time in this stage.

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