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DEFINITIONS:
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Sleep is defined as a natural, periodically recurring state of inactivity, characterised by the
loss of consciousness and reduced responsiveness to external stimuli
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EEG:
In 1929, a German psychiatrist called Hans Berger discovered that electrical activity of the brain
could be measured and thereby recorded and observed in the form of brain waves
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It is used to differentiate changes between periods of wakefulness and the various stages of sleep
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5 to 4 Hz

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Theta, 4 to 8 Hz

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Alpha, 8 to 12 Hz

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Sigma, 12 to 14 Hz

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Beta, 14 to 30 Hz

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Gamma, 30 to 50 Hz

EEG data are combined with those from concurrent recording of eye movements from the
electrooculogram (EOG), and muscle tone from the electromyogram (EMG) to define the states of
sleep and wakefulness
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Stage 1: (Shallow Sleep)
This is characterised by irregular frequency and smaller amplitude
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Stage 2:
The individual is less easily aroused at this stage, and the most notable characteristic of this stage is
the presence of sleep spindles which are bursts in the sigma band frequency
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The presence of K complexes on the EEG is also a hallmark feature of this stage of sleep, and these
suppress cortical arousal and promote sleep based memory consolidation
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Stage 3:
This stage is characterised by large amplitude with a low frequency within the delta band, with the
individual becoming less inclined to becoming aroused
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Stage 4:
The sleeper even harder to arouse, and this stage constitutes for 50% of normal sleep
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Periods of SWS increase after sleep deprivation, however the need for it rapidly declines through
age, with most individuals aged over 60 exhibiting no signs of slow wave sleep
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Most dreams occur at this stage, however muscles are rendered paralysed
therefore this stage is also known for inducing paradoxical sleep
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For example, sleep in the human new-born occupies twothirds of time, with REM sleep occupying half of their total sleep time which ranges from 17-18
hours
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BRAIN ANATOMY:
BRAIN STEM
Regions in the rostral reticular formation send projections to the forebrain through 2 main pathways
critical for regulation of sleep-wake cycles
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The dorsal ascending pathway projects to multiple thalamic
nuclei, which in turn have widespread projections to the cortex
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These
neurons fire rapidly during wakefulness, but slow down during slow-wave sleep and resume rapid
firing again during REM (active) sleep
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The ventral ascending pathways of the brain stem RAS project rostrally through the lateral
hypothalamus, terminating on magnocellular neurons in the substantia innominata, medial septum,
and the diagonal band
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This pathway
originates in the noradrenergic nucleus, the locus coeruleus, and in the serotoninergic dorsal and
median raphe nuclei
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Regions caudal to the pons also contribute to the regular cycling between sleep and
wakefulness
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al
who discovered that making a transection made just above the junction of the pons and midbrain
produced a state in which periodic occurrence of REM sleep was found in recordings made in the
isolated brainstem while, in contrast, recordings in the isolated forebrain showed no signs of REM
sleep
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THE ASCENDING AROUSAL SYSTEM INDUCES WAKEFULNESS
Contemporary models of the wake-sleep regulatory system are based on the seminal research
conducted as follows: In 1930, von Economo reported that a viral illness known as encephalitis
lethargica was caused by lesions of the posterior hypothalamus and rostral midbrain
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Moruzzi and Magoun confirmed that waking behaviour is indeed maintained by an “ascending
reticular activating system,” originating in the upper brainstem adjacent to the junction of the pons
and midbrain and continuing on to the diencephalon, where it separates into two branches
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They also found that electrical stimulation of the reticular core of the upper brainstem
produced immediate EEG desynchronization thereby promoting wakefulness
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The system contains two major branches, each comprising discrete cell populations and
neurotransmitters
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Two cholinergic structures in the brainstem and basal forebrain serve as the origin of these
projections to the principal thalamic nuclei – the PPT/LDT nuclei
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PPT/LDT neurons are most active during
wakefulness and rapid eye movement (REM) sleep and discharge more slowly during non-REM
(NREM) sleep, a period when cortical activity is reduced
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The second branch of the ascending arousal system projects into the lateral hypothalamus, basal
forebrain, and the cerebral cortex
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Several additional cerebrocortical afferents
have been identified: lateral hypothalamic peptidergic neurons, which contain melaninconcentrating hormone or orexin/hypocretin, and basal forebrain nuclei, which contain
acetylcholine or GABA
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Neurons in these monoaminergic systems have broad action potentials, discharging most rapidly
during wakefulness, slowing during NREM sleep, and showing little activity during REM sleep
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An
endogenous clock is located within the suprachiasmatic nucleus which regulates circadian rhythms
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These
proteins dimerise and promote the transcription of 2 genes known as Period and Cryptochrome
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These genes oscillate with a period of 24 hours, and it was
confirmed that they play a central role in the molecular mechanism of the Drosophila biological clock
and mutations in these genes can alter the period of the circadian rhythm
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The SCN promotes arousal during the day
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Most of SCN neurons project to the
dorsomedial hypothalamus, which in turn projects to the ventrolateral preoptic area; to a collection
of hypothalamic endocrine cells that secrete hypocretin, corticotrophinreleasing hormone,
thyrotropin-releasing hormone, and gonadotrophin-releasing hormone; and to autonomic neurons
that project to the brain stem and spinal cord autonomic (sympathetic and parasympathetic) nuclei
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Lateral Hypothalamus:
Located in the posterior hypothalamus, it is the exclusive source of arousal-promoting peptides
hypocretins I and II (otherwise known as orexin)
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Hypocretin-producing lateral hypothalamic neurons send numerous excitatory projections to wakepromoting adrenergic, histaminergic, dopaminergic, and cholinergic nuclei, and are involved in
regulating these centres
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The cholinergic neurons, on the other hand, excite
hypocretinergic neurons by a positive feedback loop
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Ventrolateral preoptic nucleus:
This nucleus is located in the preoptic area of the anterior hypothalamus and is a sleep generating
center, which opposes the arousing effect of the posterior hypothalamus
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Ventrolateral preoptic nucleus neurons are activated by sleep-inducing factors such as adenosine
and prostaglandin D2
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These neurons in the VLPO
contain inhibitory transmitters g-aminobutyric acid (GABA) and galanin and project to arousal
neurons in the hypothalamus and brain stem
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Because the activation of the VLPO is required for normal regulation of sleep, it is an essential
element of the sleep-wake central circuitry
...
determined that a group
of ventrolateral preoptic neurons is specifically activated during sleep
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Neurons of the extended VLPO connect with pontine sites implicated in REM sleep gating – the LDT,
dorsal raphe nucleus, and locus coeruleus, whereas the VLPO cluster provides output to
histaminergic neurons of the TMN
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However,
histaminergic neurons in the brain have been identified only recently [30,31]
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These neurons project throughout the CNS, with the heaviest projections going to the
cerebral cortex, the amygdala, and the substantia nigra
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Pineal gland:
The highly vascularized pineal gland is located on the posterodorsal aspect of the third ventricle
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Upon exposure to light, retinal ganglion cells release melanopsin into
the SCN, which in turn activates the sympathetic intermediolateral cell column in the thoracic spinal
cord, which has a negative feedback loop with the pineal gland, resulting in inhibition of melatonin
release
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They are mutually inhibitory and also self–reinforcing - i
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when each side is firing, it
reduces its own inhibitory feedback
During wakefulness, the monoaminergic nuclei inhibit the ventrolateral preoptic nucleus, thereby
relieving the inhibition of the monoaminergic cells, and that of the orexin neurons, and the
cholinergic pedunculopontine (PPT) and laterodorsal tegmental nuclei
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During sleep, the firing of the VLPO neurons inhibits the monoaminergic cell groups, thereby
relieving their own inhibition
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The direct mutual inhibition between the VLPO
and the monoaminergic cell groups forms a classic flip-flop switch, which produces sharp transitions
in state, but is relatively unstable
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FUNCTIONS OF SLEEP
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Sleep-dependent memory processing: These studies have been directed specifically toward
the role of sleep in memory encoding, memory consolidation, brain plasticity, and memory
reconsolidation, and have confirmed the new hypothesis that sleep contributes importantly
to processes of memory and brain plasticity
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Conservation of energy
Endocrine function
Thermoregulation



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