EEG, Sleep and Circadian Rhythms Flashcards
Asleep
State of unconsciousness from which individual can be aroused by normal stimuli, light, touch, sound etc. Cyclical
Coma
State of unconsciousness from which an individual cannot be aroused and does not respond to stimuli. Coma is not a deep sleep it is very complex
What does sleep occur due to?
Active inhibitory processes that originate in the pons
What creates a brain that nerve sleeps?
Destruction of the brainstem at the level of the mid-pons.
Where does the inhibitory impulses originate form in the brainstem?
Reticular foramen
What is the reticular foramen closely associated with?
Controlling the state of consciousness
What does the reticular foramen do?
Sends projections to the thalamus and higher cortical areas
Give an example of a precursor of melatonin.
Serotonin
What are many neurons within the reticular foramen?
Serotonergic
What do drugs that block serotonin do?
Inhibit sleep
What does the suprachiasmatic nuclei demonstrate?
~24hr circadian rhythm
Where is the suprachismatic nuclei found?
Hypothalamus
What can electrical stimulation of the SCN promote?
Sleep
What happens if the SCN is damaged?
Disruption of the sleep-wake cycle
What does activity in the SCN stimulate?
The release of melatonin from the pineal gland
What does release of melatonin promote?
Feelings of sleepiness
What excitatory neurotransmitter does the hypothalamus release?
Orexin (aka hypocretin)
What is orexin required for?
Wakefulness
Describe the action of orexin neurones.
- Active during the waking state
- Stop firing during sleep
What does defective orexin firing cause?
Narcolepsy
How is wakefulness established in the sleep-wake cycle?
- Excitatory neurones in the ascending reticular activating system (ARAS) part of the reticular formation are released from inhibition from sleep centres in the reticular formation.
- Stimulates excitatory pathways both CNS and PNS
- Positive feedback from CNS and PNS sustains wakefulness in individual for many hours
How is sleep established in the sleep-wake cycle?
- Eventually like all cells these active cells become fatigued and excitatory signals fade.
- At this point inhibitory, peptide signals from the sleep centres in the reticular formation likely take over and rapidly dominate the weakening excitatory signals leading to rapid progression into sleep state.
After being asleep, how does the wakefulness part of the cycle commence?
The inhibitory cells fatigue and the excitatory cells are reinvigorated
Assessing level of consciousness in an awake person involves…
- Look at their behaviour, general alertness, speech patterns, speech content, reading, writing and calculating skills. Spell words backwards or count backwards
- Record patterns of brain activity using ElectoEncepheloGram (EEG). EEG uses electrodes placed on the scalp to record activity of underlying neurones
What do EEGs show?
Wave patterns that reflect the electrical activity of the brain.
How can the waves of an EEG be analysed?
- Amplitude: the size of the wave (ranges from 0-200uV)
- Frequency: number of waves per second (ranges from 1-50+)
How does the general frequency of EEG waves vary with neuronal excitation?
Least frequent -Anaesthesia -Sleep -Awake-relaxed -Awake focussed -Tonic-clonic epilepsy Most frequent
What are the 4 main types of wave pattern seen on EEGs?
- Alpha
- Beta
- Theta
- Delta
When do alpha waves occur?
Relaxed awake state
How are alpha waves characterised?
High frequency and high amplitude
When do beta waves occur?
Alert awake state
How are beta waves characterised?
- Even higher frequency than alpha
- Low amplitude asynchronous waves
How are beta waves formed?
- Low amplitude comes about not because of low activity but precisely the opposite
- However this increase in activity is asynchronous as brain is doing so many things at once and opposing polarities of the signals cancel each other out and do not get recorded on EEG
How are theta waves characterised?
- Low frequency
- Vary in amplitude
When do theta waves occur?
- They are common in children and during times of emotional stress and frustration in adults.
- They also occur during sleep in both adults and children
How are delta waves characterised?
- Very low frequency
- High amplitude
When do delta waves occur?
During deep sleep
How many stages of sleep are there ?
5 stages which occur in a cyclical fashion
Stage 1 of sleep
- Slow wave, non REM, S-sleep
- Slow eye movements
- Lights sleep. Easily aroused
- High amplitude, low frequency theta waves
Stage 2 of sleep
- Eye movements stop
- Frequency slows further but EEG shows bursts of rapid waves called sleep spindles (clusters of rhythmic waves, ~12-14Hz)
Stage 3 of sleep
- High amplitude, very slow (2Hz) delta waves interspersed with short episodes of faster waves
- Spindle activity declines
Stage 4 of sleep
- Exclusively delta waves
- Very difficult to rouse from stage 3-4
- Known as deep sleep
- Sleep walking/talking occurs during these stages
REM sleep
- Rapid eye movements occur
- Paradoxical sleep
- Dreams occur during REM sleep
- 25% of sleep is REM
What is REM sleep characterised by?
Fast waves, eerily similar to those of the awake state
How are stages 1-4 of sleep characterised?
Slow wave sleep
What are the physiological characteristics of deep, slow wave sleep?
- Deep sleep that occurs in the first hours of sleep
- Most restful type of sleep
- Associated with decreased vascular tone (and therefore BP), respiratory and basal metabolic rate (hence drop in body temperature)
- Dreams may occur but are rarely remembered
What happens following initial S-wave sleep of stage 3 and 4?
There is a gradual awakening towards Stage 1 but not like the initial stage, instead REM sleep during which EEG waves are desynchronised, high frequency, low amplitude, very like the awake state hence the term paradoxical sleep
Describe the cycle of REM sleep.
- Lasts 5-30 minutes every 90 minutes or so during a normal nights sleep
- Becomes more frequent as the night progresses and rest and recovery are established
When do dreams mostly occur?
REM sleep
Why do people not act their dreams out?
- Eye muscles show bursts of rapid activity
- profound inhibition of all other skeletal muscles due to inhibitory projections from pons to spinal cord prevents acting out of dreams
What does REM sleep behavioural disorder result in?
Lack of inhibition of the skeletal muscles during REM sleep leading to people acting out their dreams, sometimes with disastrous consequences
What is REM sleep dependent on?
-Cholinergic pathways within the reticular formation and their projection to the thalamus, hypothalamus and cortex. -Anticholinesterases increase time spent in REM sleep
How can you tell when someone is in REM sleep?
- HR/RR become irregular and brain metabolism increases
- EEG pattern mimics beta waves associated with highly alert, awake state
How can we vividly recall dreams?
Its is very difficult to arouse an individual from REM sleep although we often spontaneously awaken inn the morning from this stage of sleep
What happens when someone is deprived of REM sleep??
There is always a catch up when able to sleep again indicating that REM sleep must have an important physiological function
Sleep deprive subjects demonstrate, without exception…
- Impairment of cognitive function
- Impairment of physical performance
- Sluggishness
- Irritability
- Psychosis has also been observed in some subjects
What does sleep support?
- Neuronal plasticity
- Learning and memory
- Cognition
- Clearance of waste products from CNS
- Conservation of whole body energy (although cerebral O2 consumption may actually increase, especially during REM sleep)
- Immune function
How does sleep change over a lifetime?
- Total sleep time decreases rapidly through childhood and adolescence
- Percentage of REM sleep also declines: 80% in 10 week premature infant, 50% at full term declining to a stable 25% in adulthood. May be absent by 80+ years
When is total time asleep greatest?
During development when brain maturation and synaptic formation is occurring rapidly
How many people are affected by sleep disorders?
Very common ~25%
Insomnia
A chronic inability to obtain the necessary amount or quality of sleep to maintain adequate daytime behaviour. Very subjective, very common. Affects ~33% of adults
What types of insomnia do you need to distinguish between?
- Chronic, primary insomnia where there is usually no identifiable psychological or physical cause
- Temporary, secondary insomnia in response to pain, bereavement or other crisis. Usually short lived
What do drugs used to treat insomnia show?
- Barbiturates depress REM and delta sleep. Chronic use increases time taken to fall asleep, decreases time spent in REM and stage 3 and 4 sleep and increases awakening per night
- Benzodiazepines have less effect on REM sleep but are addictive and cause many problems on withdrawal.
Nightmares
Have a strong visual component and are seen during REM sleep, typically occurring quite far on through the night. Waking will stop the nightmare and the individual will have a clear recollection of the dream
Night Terrors
- Occur in deep, delta sleep and are common in children 3-8 years, typically occurring early in the night.
- Children thrash and scream and may sit or stand up with their eyes open but are not properly awake ad often fail to recognise their parents.
- the child does not remember the episode on waking the following morning
Somnambulism (sleep walking)
- Occurs exclusively in non-REM sleep, mainly in Stage 4 sleep and is more common in children ad young adults, probably due to the decline in stage 4 sleep with age.
- Walk with their eyes open and can see and will avoid objects, can carry out reasonably complex tasks such as prepare food and will often obey instruction but have no recall of the episode when woken.
Narcolepsy
- Patients enter directly into REM sleep with little warning. Symptoms could be interpreted as intrusion of REM sleep characteristics onto waking state
- Very dangerous because of accident risk
- Linked to dysfunctional orexin release from the hypothalamus
Circadian rhythm
Biological systems show oscillations with an ~24 hour period
Where is the master clock of the circadian rhythm located?
In the SCN of the hypothalamus lying just above the optic chiasma
How does the master clock of the SCN work?
- These neurons have an inherent ~24hour cycle which is entrained by external cues such as light/dark cycle. Probably because some of the nerve fibres in the optic nerve pass to the SCN.
- However this cannot be the only entrainment as blind people have circadian rhythms
What is the most likely cause of loss of circadian rhythm?
- Destruction of SCN
- Loss of melatonin signalling and disruption of orexin signalling in the hypothalamus
What are the long term health effects of disruption of sleep due to frequent changing of time zones or shift work?
- Attention deficits
- 3x increase in CVS and GI disease after 10 years of shift work
How do people with loss of vision dream?
- People who are born blind have auditory dreams
- People who go blind eventually lose the ability to have visual dreams
How do dolphins and birds sleep?
With half their brain at a time
