EEG, Sleep and Circadian Rhythms Flashcards
Define sleep
State of unconsciousness from which an individual can be aroused by normal stimuli, light, touch, sound etc. and is cyclical
How does sleep occur?
Due to active inhibitory processes that originate in the Reticular Formation of the brain stem (pons)
What is the reticular formation?
Closely associated with controlling the state of consciousness. It sends projections to the thalamus and higher cortical areas.
Why is it thought that serotonin is critical to sleep induction?
As many neurones with the reticular formation are serotonergic.
Drugs that block serotonin formation inhibit sleep.
Serotonin is a precursor of melatonin –> its release corresponds with sleep cycle (greater when falling asleep and less in the morning)
What is the suprachiasmatic nuclei (SCN)?
Nerve cells of the hypothalamus which are important in the induction of sleep and the 24hr circadian rhythm
What does damage to the SCN cause?
Damage to the SCN will disrupt the sleep-wake cycle
How does the SCN induce sleep?
Electrical stimulation can promotor sleep –> activity of SCN stimulates release of melatonin from the pineal gland (feeling of sleepiness in humans)
What is orexin?
Excitatory neurotransmitter released from the hypothalamus (only active during waking state and stop firing during sleep)
What does defective orexin signalling cause?
Narcolepsy - individual will suddenly fall asleep, sometimes even when walking
Describe induction of sleep through to wakefulness
Melatonin of the SCN and orexin induce sleep. When these active cells become fatigued and excitatory signals fade, inhibitory peptide signals from the reticular formation lead to progression into sleep state
Opposite happens during waking
What are two ways to assess consciousness levels?
- Look at their behaviour, general alertness, speech patterns, speech content, reading, writing and calculating skills.
- Record pattern of brain activity using electroencephalogram (EEG)
What is an electroencephalogram (EEG)?
EEG uses electrodes placed on the scalp to record activity of underlying neurones
Recording shows wave patterns that reflect electrical activity of the brain
How are wave recordings analysed on EEGs?
By amplitude and frequency
How does amplitude and wavelength change with decreasing frequency?
Wavelength increases and frequency decreases
What are the four main types of wave patterns found on EEGs?
- Alpha
- Beta
- Theta
- Delta
What waves are present during the relaxed, awake state?
High frequency, high amplitude waves – alpha waves state
What waves are present during the alert, awake state?
Higher frequency, low amplitude asynchronous waves – beta waves
There is low amplitude because of the brain’s high activity, it fires impulses without sequence and so the opposing polarities of the signals cancel each other out and do not get recorded on EEG.
What are theta waves?
- Low frequency with varying amplitude
* Common in children and during emotional stress and frustration in adults
What are delta waves?
- Low frequency, high amplitude
* Occur in deep sleep
What are the five stages of the sleep cycle?
Stage 1:
• Slow wave, non-REM, s-sleep
• Slow eye movements, light sleep and easily aroused.
• High amplitude, low frequency theta waves
Stage 2:
Bursts of rapid waves called sleep spindles (clusters of higher frequency rhythmic waves)
Stage 3:
• High amplitude, very slow delta waves interspersed with short episodes of faster waves
• Spindle activity declines
Stage 4:
Exclusively delta waves
Difficult to rouse from stage 3 and 4 sleep – deep sleep – and sleep walking/talking occurs during these stages
Stage 5:
• Rapid Eye Movements (REM) aka paradoxical sleep
• Dreams occur during REM sleep
What are physiological characteristic of deep, slow wave sleep?
- Deep sleep that occurs in the first hours of sleep
- Most restful type of sleep
- Association with decreased vascular tone (and therefore BP), respiratory and basal metabolic rate (hence drop in BT)
- Dreams may occur but are rarely remembered
Describe paradoxical sleep
Following initial S-wave sleep of stages 3 and 4, there is a gradual awakening towards Stage 1, but not like the initial Stage 1, instead REM sleep during which EEG waves are desynchronized, high frequency, low amplitude, very like the awake state
What are the physiological characteristics of REM sleep?
Lasts 5-30mins every 90mins or so during normal nights sleep
• Dreams mostly occur
• Eye muscles show bursts of rapid activity
• Profound inhibition of all other skeletal muscles (other than eyes) due to inhibitory projection from pons to spinal cord – prevents acting out of dreams
• Dependent on cholinergic pathway within the reticular formation and their projection to the thalamus, hypothalamus and cortex
• HR/RR become irregular and brain metabolism increases.
• Very difficult to arouse from REM sleep, although might awaken spontaneously
What are four outcomes of sleep deprivation?
- Impairment of cognitive function
- Impairment of physical performance
- Sluggishness
- Irritability
What six functions does sleep support?
- Neuronal plasticity
- Learning and memory
- Cognition
- Clearance of waste product from CNS
- Conservation of whole body energy (however, cerebral O2 consumption increases in REM sleep)
- Immune function
How does sleep change over a lifetime?
Total sleep decreases rapidly through childhood and adolescence. Percentage of REM sleep also declines:
- 80% in 10week premature infant
- 50% at full term
- 25% in adulthood
- May be absent in 80+ yrs old
Total time asleep is greatest during development when brain is maturating and synaptic formation is occurring rapidly.
What is insomnia?
Chronic inability to obtain the necessary amount of quality sleep to maintain adequate daytime behaviour
What are two types of insomnia?
- Chronic, primary insomnia where there is usually no identifiable psychological or physical cause
- Temporary, secondary insomnia in response to pain, bereavement or other crisis
What are two drugs which treat insomnia?
- Barbituates (side effects on REM sleep)
* Benzodiazepines (addictive)
In what stage of sleep do nightmares occur?
REM sleep
In what stage of sleep do night terrors occur?
Deep, delta sleep and are common in children 3-8yrs, usually early in the night
What is somnambulism?
Sleep walking
In what stage of sleep does somnambulism occur?
Exclusively in non-REM sleep, mainly stage 4 and more common in children, probably due to decline in stage 4 in adults
Describe somnambulism
They walk with their eyes open, can see and will avoid objects and will often obey instruction but have not recall of the episode when woken
Describe narcolepsy
Patients enter directly into REM sleep with little warning. Symptoms could be interpreted as intrusion of REM sleep characteristics onto the waking state
Linked to dysfunctional orexin release from the hypothalamus
What is the primary clock for circadian rhythm?
Suprachiasmatic nucleus (SCN) of the hypothalamus
These neurons have an inherent 24hr cycle which is entrained by external cues such as light/dark cycles (cant be only one because blind people have circadian rhythm)
