Download Stages of Sleep And Brain Mechanisms

Chapter 9
Wakefulness and Sleep
Rhythms of Waking and Sleep
• Some animals generate endogenous
circannual rhythms, internal mechanisms that
operate on an annual or yearly cycle.
– Example: Birds migratory patterns, animals
storing food for the winter.
Rhythms of Waking and Sleep
• All animals produce endogenous circadian
rhythms, internal mechanisms that operate on
an approximately 24 hour cycle.
• Animals generate endogenous 24 hour cycles
of wakefulness and sleep.
– Also regulates the frequency of eating and
drinking, body temperature, secretion of
hormones, urination, and sensitivity to
drugs.
Rhythms of Waking and Sleep
• Can differ between people and lead to
different patterns of wakefulness and
alertness.
• Change as a function of age.
– Example: sleep patterns from childhood to
late adulthood.
Rhythms of Waking and Sleep
• The purpose of the circadian rhythm is to
keep our internal workings in phase with the
outside world.
• Human circadian clock generates a rhythm
slightly longer than 24 hours when it has no
external cue to set it.
• Resetting our circadian rhythms is sometimes
necessary.
Rhythms of Waking and Sleep
• Free-running rhythm is a rhythm that occurs
when no stimuli resets it.
• A zeitgeber is a term used to describe any
stimulus that resets the circadian rhythms.
• Light is the primary one.
• Exercise, noise, meals, and temperature are
others zeitgebers.
Rhythms of Waking and Sleep
• Jet lag refers to the disruption of the circadian
rhythms due to crossing time zones.
– Stems from a mismatch of the internal
circadian clock and external time.
• Characterized by sleepiness during the day,
sleeplessness at night, and impaired
concentration.
• Traveling west “phase-delays” our circadian
rhythms.
• Traveling east “phase-advances” our
circadian rhythms.
Rhythms of Waking and Sleep
• Circadian rhythms remain consistent despite
lack of environmental cues indicating the time
of day
• Most people can adjust to 23- or 25- hour day
but not to a 22- or 28- hour day.
• People who engage in shift work often fail to
adjust completely.
Rhythms of Waking and Sleep
• Mechanisms of the circadian rhythms include
the following:
– The Suprachiasmatic nucleus
– Genes that produce certain proteins
– Melatonin levels
Rhythms of Waking and Sleep
• The suprachiasmatic nucleus (SCN) is part of
the hypothalamus and the main control center
of the circadian rhythms of sleep and
temperature.
– Located above the optic chiasm.
– Damage to the SCN results in less
consistent body rhythms that are no longer
synchronized to environmental patterns of
light and dark.
Rhythms of Waking and Sleep
• The SCN generates circadian rhythms in a
genetically controlled, unlearned manner.
• Single cell extracted from the SCN and raised
in tissue culture continues to produce action
potential in a rhythmic pattern.
• Various cells communicate with each other to
sharpen the circadian rhythm.
Rhythms of Waking and Sleep
• Light resets the SCN via a small branch of the
optic nerve known as the retinohypothalamic
path.
– Travels directly from the retina to the SCN.
• The retinohypothalamic path comes from a
special population of ganglion cells that have
their own photopigment called melanopsin.
– The cells respond directly to light and do
not require any input from the rods or
cones.
Rhythms of Waking and Sleep
•
Two types of genes are responsible for
generating the circadian rhythm.
1. Period - produce proteins called Per.
2. Timeless - produce proteins called Tim.
• Per and Tim proteins increase the activity of
certain kinds of neurons in the SCN that
regulate sleep and waking.
• Mutations in the Per gene result in odd
circadian rhythms.
Rhythms of Waking and Sleep
• The SCN regulates waking and sleeping by
controlling activity levels in other areas of the
brain.
• The SCN regulates the pineal gland, an
endocrine gland located posterior to the
thalamus.
• The pineal gland secretes melatonin, a
hormone that increases sleepiness.
Rhythms of Waking and Sleep
• Melatonin secretion usually begins 2 to 3
hours before bedtime.
• Melatonin feeds back to reset the biological
clock through its effects on receptors in the
SCN.
• Melatonin taken in the afternoon can phaseadvance the internal clock and can be used
as a sleep aid.
Stages of Sleep And Brain
Mechanisms
• Sleep is a state that the brain actively
produces.
• Characterized by a moderate decrease in
brain activity and decreased response to
stimuli.
• Sleep differs from the following states:
– Coma
– Vegetative state
– Minimally conscious state
– Brain death
Stages of Sleep And Brain
Mechanisms
• Coma – extended period of unconsciousness
caused by head trauma, stroke, or disease
characterized by low brain activity that
remains fairly steady
– Person shows little response to stimuli
• Vegetative state – person alternates between
periods of sleep and moderate arousal but no
awareness of surrounding
– Some autonomic arousal to painful
stimulus
– No purposeful activity/ response to speech
Stages of Sleep And Brain
Mechanisms
• Minimally conscious state - one stage higher
than a vegetative state marked by occasional
brief periods of purposeful action and limited
speech comprehension
• Brain death - no sign of brain activity and no
response to any stimulus
Stages of Sleep And Brain
Mechanisms
• The electroencephalograph (EEG) allowed
researchers to discover that there are various
stages of sleep.
• Allows researchers to compare brain activity
at different times during sleep.
• A polysomnograph is a combination of EEG
and eye-movement records
Stages of Sleep And Brain
Mechanisms
• Alpha waves are present when one begins a
state of relaxation.
• Stage 1 sleep is when sleep has just begun.
– the EEG is dominated by irregular, jagged,
low voltage waves.
– brain activity begins to decline.
Stages of Sleep And Brain
Mechanisms
• Stage 2 sleep is characterized by the
presence of:
– Sleep spindles - 12- to 14-Hz waves during
a burst that lasts at least half a second.
– K-complex - a sharp high-amplitude
negative wave followed by a smaller,
slower positive wave.
Stages of Sleep And Brain
Mechanisms
• Stage 3 and stage 4 together constitute slow
wave sleep (SWS) and is characterized by:
– EEG recording of slow, large amplitude
wave.
– Slowing of heart rate, breathing rate, and
brain activity.
– Highly synchronized neuronal activity.
Stages of Sleep And Brain
Mechanisms
• Rapid eye movement sleep (REM) are
periods characterized by rapid eye
movements during sleep.
• Also know as paradoxical sleep is deep sleep
in some ways, but light sleep in other ways.
• EEG waves are irregular, low-voltage and
fast.
• Postural muscles of the body are more
relaxed than other stages.
Stages of Sleep And Brain
Mechanisms
• Stages other than REM are referred to as
non-REM sleep (NREM).
• When one falls asleep, they progress through
stages 1, 2, 3, and 4 in sequential order.
• After about an hour, the person begins to
cycle back through the stages from stage 4 to
stages 3 and 2 and than REM.
• The sequence repeats with each cycle lasting
approximately 90 minutes.
Stages of Sleep And Brain
Mechanisms
• Stage 3 and 4 sleep predominate early in the
night.
– The length of stages 3 and 4 decrease as
the night progresses.
• REM sleep is predominant later in the night.
– Length of the REM stages increases as the
night progresses.
• REM is strongly associated with dreaming,
but people also report dreaming in other
stages of sleep.
Stages of Sleep And Brain
Mechanisms
• Various brain mechanisms are associated
with wakefulness and arousal.
• The reticular formation is a part of the
midbrain that extends from the medulla to the
forebrain and is responsible for arousal.
Stages of Sleep And Brain
Mechanisms
• The pontomesencephalon is a part of the
midbrain that contributes to cortical arousal.
– Axons extend to the thalamus and basal
forebrain which release acetylcholine and
glutamate
– produce excitatory effects to widespread
areas of the cortex.
• Stimulation of the pontomesencephalon
awakens sleeping individuals and increases
alertness in those already awake.
Stages of Sleep And Brain
Mechanisms
• The locus coeruleus is small structure in the
pons whose axons release norepinephrine to
arouse various areas of the cortex and
increase wakefulness.
– Usually dormant while asleep.
Structure
Neurotransmitter(s) it
releases
Effects on Behavior
Pontomesencephalon
Acetylcholine, glutamate
Increases cortical arousal
Locus coeruleus
Norepinephrine
Increases information
storage during
wakefulness; suppresses
REM sleep
Excitatory cells
Acetylcholine
Excites thalamus and
cortex; increases
learning, attention;
shifts sleep from NREM
to REM
Inhibitory cells
GABA
Inhibits thalamus and
cortex
Hypothalamus (parts)
Histamine
Increases arousal
(parts)
Orexin
Maintains wakefulness
Basal forebrain
Dorsal raphe and pons Serotonin
Interrupts REM sleep
Stages of Sleep And Brain
Mechanisms
• The basal forebrain is an area anterior and
dorsal to the hypothalamus containing cells
that extend throughout the thalamus and
cerebral cortex.
• Cells of the basal forebrain release the
inhibitory neurotransmitter GABA.
• Inhibition provided by GABA is essential for
sleep.
• Other axons from the basal forebrain release
acetylcholine which is excitatory and
increases arousal.
Stages of Sleep And Brain
Mechanisms
• The hypothalamus contains neurons that
release “histamine” to produce widespread
excitatory effects throughout the brain.
– Anti-histamines produce sleepiness.
Stages of Sleep And Brain
Mechanisms
• Orexin is a peptide neurotransmitter released
in a pathway from the lateral nucleus of the
hypothalamus highly responsible for the
ability to stay awake.
– Stimulates acetylcholine-releasing cells in
the basal forebrain to stimulate neurons
responsible for wakefulness and arousal.
– The basal forebrain is an area just anterior
and dorsal to the hypothalamus
Stages of Sleep And Brain
Mechanisms
•
Functions of the inhibitory neurotransmitter
GABA are also important:
1. Decreasing the temperature and
metabolic rate
2. Decreasing stimulation of neurons.
Stages of Sleep And Brain
Mechanisms
• During REM sleep:
– Activity increases in the pons (triggers on
set of REM sleep) and limbic system
(emotional systems), parietal cortex and
temporal cortex.
– Activity in the pons triggers onset of REM
sleep
– Activity decreases in the primary visual
cortex, the motor cortex, and the
dorsolateral prefrontal cortex.
Stages of Sleep And Brain
Mechanisms
• REM sleep is also associated with a
distinctive pattern of high-amplitude electrical
potentials known as PGO waves.
• Waves of neural activity are detected first in
the pons and then in the lateral geniculate of
the hypothalamus, and then the occipital
cortex.
• REM deprivation results in high density of
PGO waves when allowed to sleep normally.
Stages of Sleep And Brain
Mechanisms
• Cells in the pons send messages to the
spinal cord which inhibit motor neurons that
control the body’s large muscles.
– Prevents motor movement during REM
sleep.
• REM is also regulated by serotonin and
acetylcholine.
– Drugs that stimulate Ach receptors quickly
move people to REM.
– Serotonin interrupts REM.
Stages of Sleep And Brain
Mechanisms
• Insomnia is a sleep disorder associated with
inadequate sleep.
– Caused by a number of factors including
noise, stress, pain medication.
– Can also be the result of disorders such as
epilepsy, Parkinson’s disease, depression,
anxiety or other psychiatric conditions.
– Dependence on sleeping pills and shifts in
the circadian rhythms can also result in
insomnia.
Stages of Sleep And Brain
Mechanisms
• Sleep apnea is a sleep disorder characterized
by the inability to breathe while sleeping for a
prolonged period of time.
• Consequences include sleepiness during the
day, impaired attention, depression, and
sometimes heart problems.
• Cognitive impairment may result from loss of
neurons due to insufficient oxygen levels.
• Causes include, genetics, hormones, old age,
and deterioration of the brain mechanisms
that control breathing and obesity.
Stages of Sleep And Brain
Mechanisms
• Narcolepsy is a sleep disorder characterized
by frequent periods of sleepiness.
• Four main symptoms include:
– Gradual or sudden attack of sleepiness.
– Occasional cataplexy - muscle weakness
triggered by strong emotions.
– Sleep paralysis- inability to move while
asleep or waking up.
– Hypnagogic hallucinations- dreamlike
experiences the person has difficulty
distinguishing from reality.
Stages of Sleep And Brain
Mechanisms
(Insomnia cont’d)
• Seems to run in families although no gene
has been identified.
• Caused by lack of hypothalamic cells that
produce and release orexin.
• Primary treatment is with stimulant drugs
which increase wakefulness by enhancing
dopamine and norepinephrine activity.
Stages of Sleep And Brain
Mechanisms
• Periodic limb movement disorder is the
repeated involuntary movement of the legs
and arms while sleeping.
– Legs kick once every 20 to 30 seconds for
periods of minutes to hours.
– Usually occurs during NREM sleep.
Stages of Sleep And Brain
Mechanisms
• REM behavior disorder is associated with
vigorous movement during REM sleep.
– Usually associated with acting out dreams.
– Occurs mostly in the elderly and in older
men with brain diseases such as
Parkinson’s.
– Associated with damage to the pons
(inhibit the spinal neurons that control large
muscle movements).
Stages of Sleep And Brain
Mechanisms
• Night terrors are experiences of intense
anxiety from which a person awakens
screaming in terror.
– Usually occurs in NREM sleep.
• “Sleep talking” occurs during both REM and
NREM sleep.
• “Sleepwalking” runs in families, mostly occurs
in young children, and occurs mostly in stage
3 or 4 sleep.
Why Sleep? Why REM? Why Dreams?
• Functions of sleep include:
– Energy conservation.
– Restoration of the brain and body.
– Memory consolidation.
Why Sleep? Why REM? Why Dreams?
• The original function of sleep was to probably
conserve energy.
• Conservation of energy is accomplished via:
– Decrease in body temperature of about 1-2
Celsius degrees in mammals.
– Decrease in muscle activity.
Why Sleep? Why REM? Why Dreams?
• Animals also increase their sleep time during
food shortages.
– sleep is analogous to the hibernation of
animals.
• Animals sleep habits and are influenced by
particular aspects of their life including:
– how many hours they spend each day
devoted to looking for food.
– Safety from predators while they sleep
• Examples: Sleep patterns of dolphins,
migratory birds, and swifts.
Why Sleep? Why REM? Why Dreams?
• Sleep enables restorative processes:
– Proteins rebuilt in the brain
– Energy supplies replenished
• Moderate sleep deprivation results in
impaired concentration, irritability,
hallucinations, tremors, unpleasant mood,
and decreased immune system functioning.
• Caffeine increases arousal by blocking the
receptors for adenosine (accumulate during
wakefulness and increase drowsiness)
Why Sleep? Why REM? Why Dreams?
• Sleep also plays an important role in
enhancing learning and strengthening
memory.
– Performance on a newly learned task is
often better the next day if adequate sleep
is achieved during the night.
• Increased brain activity occurs in the area of
the brain activated by a newly learned task
while one is asleep.
– Activity also correlates with improvement in
activity seen the following day.
Why Sleep? Why REM? Why Dreams?
• Humans spend one-third of their life asleep.
• One-fifth of sleep time is spent in REM.
• Species vary in amount of sleep time spent in
REM.
– Percentage of REM sleep is positively
correlated with the total amount of sleep in
most animals.
• Among humans, those who get the most
sleep have the highest percentage of REM.
Why Sleep? Why REM? Why Dreams?
• Research is inconclusive regarding the exact
functions of REM.
• During REM:
– The brain may discard useless connections
– Learned motor skills may be consolidated.
• Maurice (1998) suggests the function of REM
is simply to shake the eyeballs back and forth
to provide sufficient oxygen to the corneas.
Why Sleep? Why REM? Why Dreams?
•
Biological research on dreaming is
complicated by the fact that subjects can not
often accurately remember what was
dreamt.
• Two biological theories of dreaming include:
1. The activation-synthesis hypothesis.
2. The clinico-anatomical hypothesis.
Why Sleep? Why REM? Why Dreams?
• The activation-synthesis hypothesis suggests
dreams begin with spontaneous activity in the
pons which activates many parts of the
cortex.
– The cortex synthesizes a story from the
pattern of activation.
– Normal sensory information cannot
compete with the self-generated
stimulation and hallucinations result.
Why Sleep? Why REM? Why Dreams?
• Input from the pons activates the amygdala
giving the dream an emotional content.
• Because much of the prefrontal cortex is
inactive during PGO waves, memory of
dreams is weak.
– Also explains sudden scene changes that
occur in dreams.
Why Sleep? Why REM? Why Dreams?
• The clinico-anatomical hypothesis places less
emphasis on the pons, PGO waves, or even
REM sleep.
– Suggests that dreams are similar to
thinking, just under unusual circumstances.
• Similar to the activation synthesis hypothesis
in that dreams begin with arousing stimuli that
are generated within the brain.
– Stimulation is combined with recent
memories and any information the brain is
receiving from the senses.
Why Sleep? Why REM? Why Dreams?
• Since the brain is getting little information
from the sense organs, images are generated
without constraints or interference.
• Arousal can not lead to action as the primary
motor cortex and the motor neurons of the
spinal cord are suppressed.
• Activity in the prefrontal cortex is suppressed
which impairs working memory during
dreaming.
Why Sleep? Why REM? Why Dreams?
• Activity is high in the inferior part of the
parietal cortex, an area important for visualspatial perception.
– Patients with damage report problems with
binding body sensations with vision and
have no dreams.
– Activity is also high in areas outside of V1,
accounting for the visual imagery of
dreams.
Why Sleep? Why REM? Why Dreams?
• Activity is high in the hypothalamus and
amygdala which accounts for the emotional
and motivational content of dreams.
• Either internal or external stimulation
activates parts of the parietal, occipital, and
temporal cortex.
• Lack of sensory input from V1 and no
criticism from the prefrontal cortex creates the
hallucinatory perceptions.