25. Sleep and arousal Flashcards
What is meant by ‘consciousness’ or ‘arousal’?
This is the ability of an individual to react appropriately to stimuli in the outside world
How is ‘consciousness’ or ‘arousal’ clinically assessed?
Assessed using the Glasgow Coma Score
Classified as either severe, moderate or mild unconsciousness
What is important to remember about being unconscious vs. being conscious
Consciousness is not an all or nothing phenomena but it is a graded phenomena - need to ask how conscious is a person rather than are they conscious
How is the electroencephalograph (EEG) used to measure consciousness?
Used to record very very small voltages from the brain through the scalp (hence it is subject to interference such as noise, muscle movements, eye movements)
Measures voltages of the cerebral cortex
Cannot be used to detect what individual neurones are doing but can show when the different regions of the brain are active
How does an EEG trace appear when someone is asleep/awake and why
When aroused, excited and alert the EEG shows a low amplitude and a high frequency - there is a desynchronisation of neuronal firings
When drowsy and closer to sleep, the EEG shows a high amplitude and a low frequency - there is a synchronisation of neuronal firings
Which part of the brain was originally thought to control consciousness and why is this no longer the case?
The cerebral cortex - thought this because when someone is in a coma, the EEG shows no electrical activity
BUT when at certain stages when asleep, the EEG of the cortex is almost identical to when we are awake
What is normal sleep and why does sleep occur?
Normal sleep is known as slow-wave sleep or non-REM sleep
This is a heightened anabolic state during which there is increased growth and rejuvenation of the immune, nervous and muscular systems and the secretion of growth hormone
SO involved with growth (why you sleep when you are younger) and wound repair (why you sleep when you are injured)
There are also decreased cortisol levels whilst we sleep
What adverse effects can occur due to a lack of sleep?
Will result in general behavioural problems such as a lack of concentration
Alters the blood pressure and can hence result in the development of cardiovascular disease, chronic kidney disease, diabetes
There is a reduced blood pressure during sleep due to reduced sympathetic output and a lack of sleep can result in a state of hypertension
Can result in obesity - a lack of sleep results in decreased secretion of leptin (secreted when asleep) and a lack of sleep results in an increased hunger (lack of satiety)
What are the different stages of sleep?
How do these appear on an EEG?
There are four stages of normal/slow-wave sleep and then there is a stage of REM sleep
From stage 1 to stage 4, we become less and less conscious and so the EEG trace changes from a low amplitude and a high frequency to a high amplitude and a low frequency
Nb. The amplitude correlates with the ease of being able to awaken the person - this indicates synchronisation of cortical activity
Then have REM sleep (after stage IV) - rapid eye movement sleep
The eyes move back and forth and it is even harder to awaken the person
NB. during REM sleep is when you dream
How is REM sleep shown on an EEG trace?
Interestingly, the EEG of someone in REM sleep is very similar to that of an awake person
In sleep vs. wake - when is the cortical activity synchronised and when is it not?
In sleep - the cortical activity is synchronised
When awake - the cortical activity is desynchronised
How often do the stages of the sleep cycle repeat?
Stages 1-4 repeats in 90 minute cycles throughout the night
Which region of the brain is responsible for the control of sleep and wakefulness?
The reticular formation of the pons is the key control area for sleep and wakefulness
This is modulated by signals from the hypothalamus
What are the different stimuli for sleep?
Chemicals in the blood
Diurnal rhythms
How do chemicals in the blood act as a stimuli for sleep?
Around the preoptic nuclei, the BBB is leaky and large molecules can penetrate and act on local neurones
E.g. Ghrelin hunger hormone and low levels of blood glucose have an inhibitory action on sleep
E.g. Cholecystokinin associated with satiety increases sleepiness and induces sleep
E.g. High adenosine levels induce sleep (Caffeine is an antagonist at A1 adenosine receptors so keeps you awake)
E.g. Histamine firing from histaminergic neurones