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Sleep is a complex process that involves various physiological changes, including a decrease in muscle tone during the REM (rapid eye movement) stage. This loss of muscle tone, also known as atonia, has been a subject of interest in sleep research. It is characterised by reduced tonic electromyographic (EMG) activity, particularly in the mentalis and orbicularis oris muscles. The sudden drop in muscle tone during the transition from non-REM (NREM) to REM sleep has been observed in both human and animal studies, with potential implications for understanding sleep disorders and behaviours. The phenomenon of atonia has also been linked to the hypothesis that poor sleep may contribute to irritability and increased aggression. Additionally, the measurement of tonic muscle activity during REM sleep has clinical significance, especially in the context of diagnosing and predicting neurodegenerative diseases such as REM sleep behaviour disorder and Parkinson's disease.
| Characteristics | Values |
|---|---|
| Type of muscle tone loss | Sudden drop of muscle tone |
| Muscle affected | Mentalis, orbicularis oris, suprahyoid, masseter, temporalis |
| Occurrence | During transition from NREM to REM sleep |
| Time taken for loss of muscle tone | No longer than 2 seconds |
| Nature of muscle tone loss | Atonia, intermittently interrupted by twitching |
| Relation to REM sleep | One of the three electrophysiological signs for identifying REM sleep |
| Relation to NREM sleep | Occurs at sleep onset |
| Relation to sleep deprivation | May be a causal factor for irritability, anger, and increased aggression |
| Relation to dreams | Historically, atonia was thought to prevent humans and animals from acting out their dreams |
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What You'll Learn
- REM sleep is characterised by low muscle tone, rapid eye movements, low EEG amplitude and twitching
- Low muscle tone during sleep can be identified through amplitude analysis of mentalis and orbicularis oris muscles
- Sleep deprivation may cause increased aggression and irritability in humans and animals
- REM sleep behaviour disorder (RBD) is characterised by violent movements during sleep, and is linked to the development of Parkinson's disease
- Cataplexy causes loss of muscle tone without loss of conscious awareness
REM sleep is characterised by low muscle tone, rapid eye movements, low EEG amplitude and twitching
Rapid eye movement (REM) sleep is characterised by several distinct features, including low muscle tone, rapid eye movements, low EEG amplitude, and twitching.
REM sleep is associated with a sudden drop in muscle tone, which occurs due to a decrease in motor neuron activity. This loss of muscle tone, also known as atonia, results in temporary paralysis or inhibition of movement. The mentalis muscle, for instance, exhibits the lowest values of muscle tone during REM sleep, with brief phasic activations. This loss of muscle tone is one of the key criteria for identifying REM sleep, along with low-voltage, mixed-frequency EEG activity.
The occurrence of rapid eye movements is another defining feature of REM sleep. These eye movements are caused by bursts of electrical activity, known as ponto-geniculo-occipital (PGO) waves, which originate in the brain stem. PGO waves exhibit their highest amplitude when they reach the visual cortex, resulting in the rapid eye movements characteristic of REM sleep.
REM sleep is also characterised by low EEG amplitude. During this sleep stage, cerebral neurons fire with similar intensity to wakefulness, resulting in low-amplitude, mixed-frequency brain waves. This neural activity resembles the patterns observed during wakefulness, contributing to the paradoxical nature of REM sleep.
Additionally, twitching may occur during REM sleep, interspersed with periods of muscle atonia. These twitches are associated with brain activity in regions related to sensorimotor function, such as the hippocampus, cerebellum, and red nucleus. While the specific function of twitching in adults is not yet understood, it is speculated that sensory feedback from these twitches may contribute to the development and maintenance of the sensorimotor system.
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Low muscle tone during sleep can be identified through amplitude analysis of mentalis and orbicularis oris muscles
Sleep is characterised by different patterns of brain activity, and the changes in muscle tone during sleep have been studied extensively. One of the key indicators of the onset of REM sleep is a sudden drop in muscle tone, which is one of the three electrophysiological signs used to identify REM sleep. This loss of muscle tone is also referred to as atonia and is intermittently interrupted by twitching.
The mentalis and orbicularis oris muscles play a role in identifying the onset of REM sleep. The mentalis muscle, in particular, shows the lowest values of tonic electromyographic (EMG) activity during REM sleep, with very low variability during the same REM sleep episode and across REM episodes.
Amplitude analysis of the mentalis and orbicularis oris muscles can be used to study the interference pattern of these facial muscles. This analysis involves the use of a concentric needle electrode to study the motor unit action potential (MUAP) and interference patterns. By comparing the data from healthy subjects with those from patients with pathological conditions, researchers can differentiate between neurogenic and myogenic lesions.
Therefore, low muscle tone during sleep can be identified through amplitude analysis of the mentalis and orbicularis oris muscles. This analysis provides insights into the EMG activity of these muscles, which is crucial for understanding the onset of REM sleep and the associated loss of muscle tone.
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Sleep deprivation may cause increased aggression and irritability in humans and animals
Sleep is an essential part of our lives. Typically, people need seven to eight hours of sleep each night to maintain peak mental and physical health. Consecutive days of sleep deprivation can lead to "sleep debt", which is a cumulative effect of insufficient sleep. Research has indicated a connection between sleep deprivation, sleep debt, and mental health.
Several studies have found that sleep deprivation may cause increased aggression and irritability in humans and animals. For example, in a study conducted by Webb in 1962, rats started to exhibit aggressive behaviour after 16 days of total sleep deprivation. Similar studies on mice and rats have confirmed these findings.
The relation between sleep problems and aggression may be due to the negative effect of sleep loss on prefrontal cortical functioning, which likely contributes to a loss of control over emotions, including the regulation of aggressive impulses. Other potential contributing mechanisms connecting sleep problems to aggression and violence may be found within the central serotonergic and the hypothalamic-pituitary-adrenal-axis. Individual variation within these neurobiological systems may explain why certain individuals experience amplified aggressive responses induced by sleep loss.
In humans, irritability increases with insufficient sleep, but aggression itself is either unaffected or reduced. This may be due to the impact of sustained activity or stress rather than sleep deprivation per se, or REM sleep loss vs total sleep loss.
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REM sleep behaviour disorder (RBD) is characterised by violent movements during sleep, and is linked to the development of Parkinson's disease
REM sleep behaviour disorder (RBD) is characterised by violent movements during sleep. It is a sleep disorder in which individuals physically act out their dreams while in the rapid eye movement (REM) stage of sleep, often violently. The condition can cause injury to the individual or their bed partner. RBD is associated with a loss of normal muscle atonia during REM sleep, which can result in violent dream-enacting behaviours.
RBD is strongly associated with certain neurodegenerative disorders. About 97% of people with isolated (idiopathic) RBD will develop Parkinson's disease, Lewy body dementia, or multiple system atrophy within 14 years of their diagnosis. The link between RBD and Parkinson's disease is particularly notable, with RBD considered a highly specific marker of future synucleinopathy. Studies indicate that over 80% of people who develop isolated RBD will go on to develop an alpha-synuclein-related neurodegenerative disorder, with Parkinson's disease being the most common outcome.
The onset of RBD often precedes the motor symptoms of Parkinson's disease and other neurodegenerative disorders. This has led researchers to believe that RBD may be an important indicator of prodromal Parkinson's disease. The presence of RBD can significantly impact the quality of life for individuals with early Parkinson's disease, as it is associated with worse overall sleep quality and an increased risk of injury.
The underlying cause of RBD is thought to be related to an issue in the brainstem, specifically in the pons region, which controls muscle paralysis during REM sleep. Lesions or damaged tissue in the pons are also associated with Parkinson's disease and other neurodegenerative disorders. This shared pathology provides further evidence of the link between RBD and the development of Parkinson's disease.
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Cataplexy causes loss of muscle tone without loss of conscious awareness
Cataplexy is a brain disorder that causes a sudden and brief loss of muscle tone and control. It is one of the main symptoms of narcolepsy, a chronic sleep and neurological disorder. Cataplexy episodes are triggered by strong emotions such as anger, stress, anxiety, fear, joy, or laughter. During an episode, an individual may experience a sagging jaw, mild head drop, slurred speech, or total collapse. However, it is important to note that people usually remain conscious and awake during a cataplexy attack, which distinguishes it from fainting or seizures.
The loss of muscle tone in cataplexy resembles the natural loss of muscle control that occurs during REM sleep. However, cataplexy episodes occur when an individual is awake. This phenomenon has been attributed to the disruption of hypocretin neurons and their pathways. Hypocretin, also known as orexin, is a sleep-wake regulating brain hormone. Individuals with narcolepsy type 1 have abnormally low levels of hypocretin, which can cause REM sleep to occur at inappropriate times.
The role of histaminergic neurons is crucial in preserving consciousness during cataplexy. In narcolepsy, there may be an increase in these neurons, possibly to compensate for the loss of hypocretin. Research suggests that enhancing histamine release in the hypothalamus through certain therapies may increase alertness and wakefulness in individuals with cataplexy.
Cataplexy can be challenging to live with as it is difficult to control emotional responses, and cataplectic attacks can occur randomly. Individuals with cataplexy must be constantly vigilant to avoid injury during an attack. While there is no cure for cataplexy, medications can help manage the symptoms. These medications target either excessive daytime sleepiness or the improvement of cataplexy symptoms directly.
In summary, cataplexy causes a loss of muscle tone without a corresponding loss of conscious awareness. It is a symptom of narcolepsy, triggered by strong emotions, and results from a disruption in brain chemicals and pathways. While cataplexy can be managed through medication, individuals affected by it must remain vigilant to prevent injuries during attacks.
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Frequently asked questions
REM stands for rapid eye movement sleep. It is characterised by rapid eye movements, a mixed frequency, low-amplitude EEG, and a low submental muscle tone with phasic twitches.
The loss of muscle tone during REM sleep has been observed in several studies. This loss of muscle tone is one of the three electrophysiological signs for identifying REM sleep.
The flower pot method is a technique used in sleep deprivation studies. Animals are placed on top of a flower pot surrounded by water. When the animals enter REM sleep and experience a loss of muscle tone, they fall into the water.
REM sleep behaviour disorder is a parasomnia characterised by violent or potentially violent movements during REM sleep. It is associated with a partial or total loss of normal muscle atonia during REM sleep. RBD predominantly affects elderly individuals and can indicate future neurodegenerative diseases such as Parkinson's disease.
