Compare and contrast the two sleep disorders REM sleep behavioural disorder and sleep walking.

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Compare and contrast the two sleep disorders REM sleep behavioural disorder and sleep walking.

Sleep is a state of natural rest in most vertebrates, characterized by a decreased reaction to external stimuli and a decrease in voluntary body movements. Sleep is necessary for survival. Sleep deprivation or disruption leads to impaired cognitive functioning and compromised physical, emotional and mental well-being. For decades it was thought that brain activity during sleep was reduced or absent. It was only in the 1950s and 60s that this assumption was abandoned when electroencephalogram (EEG) recordings revealed regular cyclic alterations of brain activity during sleep: the rapid-eye movement- (REM) and non-REM sleep phases. Since then, advances in neuroimaging methods enabled a much more precise investigation of sleep and its related disorders. This essay will start with describing normal sleep physiology since this is crucial for the understanding of sleep pathologies. A short overview is given about sleep disorders in general, before discussing two common parasomnias, REM sleep behavioural disorder (RBD) and sleep walking. RBD will be introduced first and while subsequently introducing sleepwalking it is contrasted to RBD.

Sleep is an active state in which the brain is still roughly 80% activated and capable of robust and elaborate information processing. Sleep involves different stages, which are divided into the earlier mentioned, general types REM and non-REM sleep. Non-REM sleep is characterized by  nervous system activity. This results in a slowing of heart rate, blood pressure and breathing. The brain also seems at rest; the firing rates of neurons are at their lowest rate. Non-REM sleep accounts for 75– 80% of sleeping time and consists of four stages. Stage one is thought as the gateway between wake and sleep. During this period people start losing  and conscious awareness of the external environment. Sudden twitches and jerks can be experienced and the EEG is characterized by disappearance of alpha waves (normally present in relaxed but awake adults) and first appearance of theta waves (which have a much lower frequency but higher amplitude). This stage normally last only a few minutes. In stage two, conscious awareness of the environment is completely lost and muscle tone (as can be measured with electromyogram –EMG-) lowers further. In the EEG, sleep spindles and K-complexes appear, which are bursts of activity produced by the brain to keep the sleeper tranquil. They are further thought to inhibit unnecessary information processing which could lead to waking up. This stage may last 5-15 minutes. Stage three serves as a transition into stage four, which is the deepest stage of sleep in the sense that it is extremely difficult to be woken up. It accounts only for 10-15% of total sleep time. In the EEG, delta waves predominate. In contrast to REM sleep, dreaming in non-REM sleep is rare and muscles are not paralyzed. The body is capable of movement but only rarely does. This is the stage when sleepwalking, night terrors and sleep talking occur (Bear, Connors & Paradiso, 1996).

Sleep proceeds in cycles of REM and non-REM phases, one cycle lasting approximately 90 minutes. REM sleep is characterized by rapid eye movements, loss of skeletal  and  nervous system activity. The summed activity of the brains neurons is similar to that of an awake person and the EEG reflects that with high frequency, low amplitude waves. Adults spend approximately 80-120 min of their sleeping time in REM sleep (Rosenzweig, Breedlove & Leiman, 2002).

The brain structures that regulate the sleep-wakefulness cycle are found in the brainstem, thalamus, hypothalamus and the basal forebrain. They belong to a network of neurons called the “ascending reticular activating system” which was identified in the 1950s as the brainstem control system for the arousal level of the two hemispheres (Pace-Schott & Hobson, 2002). This network consists of several subsystems, which can be best distinguished by their respective neurotransmitters acetylcholine, dopamine, noradrenalin, serotonin, histamine and hypocretin. These systems either innervate the cortex directly or via the thalamus and hence regulate the sleep- wake cycle. The timing of sleep onset and its duration is thought to be controlled by two processes, a circadian and a homeostatic process. The circadian process determines periods of high and low sleep susceptibility and high and low REM sleep propensity. The  of the hypothalamus receives projections from the retina and sends messages to the  which affect the secretion of . That is, the presence of light contains signals to cease secretion of melatonin which in turn signals the body to wake up. The homeostatic process is regulated by the time spent awake. That is, the more time has passed since the last sleep the more somnogens such as adenosin accumulate in the brain which in turn cause tiredness (Zeman & Reading, 2005).

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The two distinct sleep phases, REM and non-REM sleep have different neuroanatomic substrates. The cells that activate REM sleep are cholinergic neurons located in the pedunculopontine tegmental nucleus and the laterodorsal tegmental nucleus in the pontomesencephalic region. These cells fire maximally during REM sleep. The cells that deactivate REM sleep are aminergic neurons located in the locus coeruleus and raphe nuclei. These cells are inactive during REM sleep. The reciprocal interaction in the brain stem between REM-activating and REM-deactivating neurons is responsible for REM generation and maintenance. The release of certain  (,  and ), is completely shut down ...

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