Neural Basis of Behavior: Sleep Department of Rehabilitation Medicine

Neural Basis of Behavior: Sleep

Department of Rehabilitation Medicine

Rhythms of Waking and Sleep

•Animals generate endogenous 24 hour cycles of wakefulness

and sleep.

•Some animals generate endogenous circannual rhythms, internal

mechanisms that operate on an annual or yearly cycle.

–Prepares animals e.g. birds for seasonal changes


All animals produce endogenous circadian rhythms, internal

mechanisms that operate on an approximately 24 hour cycle.

–Regulates the sleep/ wake cycle.

–Also regulates the frequency of eating and drinking, body

temperature, secretion of hormones, volume of urination, and

sensitivity to drugs.


Circadian rhythms:

•Remains consistent despite lack of environmental cues

indicating the time of day

•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.


•The duration of the rhythm depends on the amount of light

•Experiments designed to determine the length of the circadian

rhythm place subjects in environments with no cues to time of

day.

•Results depend upon the amount of light to which subjects are

artificially exposed.

–Rhythms run faster in bright light conditions and subjects have

trouble sleeping.

–In constant darkness, people have difficulty waking.


•Biological clock – the brain generates its own rhythm

•Human circadian clock generates a rhythm slightly longer than

24 hours when it has no external cue to set it.

•Most people can adjust to 23- or 25- hour day but not to a 22- or

28- hour day.

•Bright light late in the day can lengthen the circadian rhythm.


•Mechanisms of the circadian rhythms include the following:

–The Suprachiasmatic nucleus.

–Genes that produce certain proteins.

–Melatonin levels.


•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.


•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.


•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 phase-advance the internal

clock and can be used as a sleep aid.


•The purpose of the circadian rhythm is to keep our internal

workings in phase with the outside world.

•Light is critical for periodically resetting our circadian rhythms.

•A zeitgeber is a term used to describe any stimulus that resets

the circadian rhythms.

•Exercise, noise, meals, and temperature are others zeitgebers.


•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.


•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.

Stages of Sleep And Brain Mechanisms

•The electroencephalograph (EEG) allowed researchers to

discover that there are various stages of sleep.

•Over the course of about 90 minutes:

– a sleeper goes through sleep stages 1, 2, 3, and 4

–then returns through the stages 3 and 2 to a stage called REM.


•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.



•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-complexes - a sharp high-amplitude negative wave followed

by a smaller, slower positive wave.



•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.



•Rapid eye movement sleep (REM) are periods characterized by

rapid eye movements.

•EEG waves are irregular, low-voltage and fast.

•Postural muscles of the body are more relaxed than other stages.

•Strongly associated with dreaming, but people also report

dreaming in other stages of sleep.

•Also known as “paradoxical sleep” because it is deep sleep in

some ways, but light sleep in other ways.



•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.


•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.


•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.


•The hypothalamus contains neurons that release “histamine” to

produce widespread excitatory effects throughout the brain.

–Anti-histamines produce sleepiness.

•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 forebrain and

brain stem to increase wakefulness and arousal.


•During REM sleep:

–Activity increases in the pons triggers the onset of REM sleep,

evident by the presence of pons-geniculate-occipital (PGO)

waves

–Activity decreases in the primary visual cortex, the motor

cortex, and the prefrontal cortex.


•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.

•Input from the pons activates the amygdala giving the dream an

emotional content.

•Because much of the prefrontal cortex is inactive, memory of

dreams is weak.

–Also explains sudden scene changes that during occur in

dreams.


•Decreased arousal required for sleep is accomplished via the

following ways:

1.Decrease of body temperature

2.Decrease of external stimulation

3.Increase of adenosine in the brain to inhibit the basal

forebrain cells responsible for arousal. (Caffeine blocks

adenosine receptors)

4.Accumulation of prostaglandins that inhibit the hypothalamic

cells responsible for increased arousal. (The immune system

increase its concentration in response to infection)


•Abnormalities of sleep

–Insomnia: a sleep disorder associated with inability to fall

asleep or stay asleep.

–Related to abnormality of biological rhythms.

–Onset insomnia

–Maintenance insomnia

–Termination insomnia


•Sleep apnea: inability to breathe while sleeping for a prolonged

period of time.

•Narcolepsy: frequent periods of sleepiness.

–Symptoms: sudden sleepiness; cataplexy; sleep paralysis;

hypnagogic hallucination

•Night terrors: experiences of intense anxiety usually occurs in

NREM sleep.

•Sleep talking: occurs in both REM and NREM sleep.

•Sleepwalking: occurs mostly in stage 3 or 4 sleep. (Does not

occur in REM sleep)

Why Sleep? Why REM? Why Dreams?

•Sleep is a specialized state that serves a variety of important

functions including:

–conservation of energy

–repair and restoration

–learning and memory consolidation

•Animals sleep habits and are influenced by particular aspects of

their life.

•Some animals also increase their sleep time during food

shortages.

–sleep is analogous to the hibernation of animals.


•People vary in their need for sleep.

–Most sleep about 8 hours.

•Sleep enables restorative processes in the brain to occur.

–Proteins are rebuilt.

–Energy supplies are replenished.

•Sleep also plays an important role in enhancing learning and

strengthening memory.


•Research is inconclusive regarding the exact functions of REM.

•During REM:

–The brain may discard useless connections

–Learned motor skills may be consolidated.


•Biological perspectives of dreaming

•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.

•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.

Thank you.

Documents

Neural Basis of Behavior: Sleep Department of Rehabilitation Medicine