EEG sleep and circadian rhythms Flashcards
What is sleep?
An easily reversible state of inactivity characterized by lack of interaction with the external environment.
Why we need to sleep is an unsolved mystery but all mammals and birds show the same stages of sleep - meaning its something that developed yonks ago
(that part isn’t relevant but hey ho)
What causes sleep?
Sleep occurs due to an active inhibitory process in which inhibitory projections are sent to the cortex
It is believed to be coming from the Pons - from an area called the Reticular formation of the brainstem (hypothalamus also is involved but well get to that)
The reticular formation is closely associated with controlling consciousness and sends projections to the thalamus and higher cortical areas
What neurotransmitter is thought to play a vital role in sleep and why?
Serotonin
Drugs blocking serotonin release/transmission in Reticular formation caused an inability to sleep so we know it must play a vital role in sleep
Possibly because serotonin is a precursor of melatonin
When sleep-inducing peptides from a sleep-deprived animal are injected into the brain of a different species of animal, what will happen?
What does this indicate about sleeping?
Sleep inducing peptides will cause ‘natural sleep’ in other animals even of other species
This means the physiological response/effect of that peptide is very similar/identical across many species
This indicates that sleeping is an ancient process that evolved very early on
What is the involvement of the hypothalamus in sleep?
The Suprachiasmatic nuclei (SNC) in the hypothalamus is (thought to be) involved in the induction of sleep
SCN activity demonstrates ~24hr circadian rhythm and controls release of melatonin from the pineal gland.
What is the Pineal gland?
Endocrine gland which is part of the epithalamus
How does the SCN act on the pineal gland?
Inhibitory neurons in SCN are stimulated by light and act to inhibit pineal gland.
Darkness therefore corresponds with decreased activity in the SCN and increased melatonin release and feelings of sleepiness in humans.
What is meant by the term ‘Circadian rhythm’?
A circadian rhythm is a natural, internal process that regulates the sleep-wake cycle and repeats roughly every 24 hours
The term can be applied to any 24hr repeating physiological cycle but in this sense, it is to do with sleep
How does melatonin control our sleep?
Circadian rhythm of melatonin release likely linked to inhibition of orexin
Orexin (aka hypocretin) is an excitatory neurotransmitter released from the hypothalamus - required for wakefulness
What happens to Orexin neurons when we are awake and asleep?
Orexin neurons are active during the waking state and stop firing during sleep
What happens if there is defective orexin signalling?
Narcolepsy
The individual will suddenly fall asleep, sometimes even when talking
How are the levels of consciousness assessed?
Behaviour:
- Look at their behaviour, general alertness, speech patterns, speech content, reading, writing and calculating skills. Spell words backwards or count backwards
Electroencephalogram (EEG):
- EEG uses electrodes placed on the scalp to record activity of underlying neurons
How do the readings on an EEG correspond to consciousness?
Recordings reflect the electrical activity of the underlying neurones
Waves can be analysed by…
Amplitude:
- in µV
- tends to decrease with neuronal excitation
Frequency:
- in Waves per second
- tends to increase with neuronal excitation
What are the 4 main types of wave patterns seen with EEG
Alpha:
- High Frequency - High amplitude
- Relaxed, awake
Beta:
- Very high frequency - low amplitude
- Asynchronous
- Alert, awake
Theta
- Low frequency - varying amplitude
- Children, stress/frustration, sleep
Delta
- Very low frequency - high amplitude
- Deep sleep
How does visual stimulus (ie having eyes open) affect the level of brain activity shown on an EEG?
Visual stimulus will increase neuronal activity
This means the EEG wave will have decreased amplitude
Why is it that Higher alertness (cortical activity) shows a decreased amplitude but high frequency?
Alertness causes the wave pattern to become asynchronous - as the brain is doing a lot more things than if relaxed (alpha)
Opposite polarities of signals travelling in different neurones decrease/cancel eachother out often so the amplitude is smaller
Frequency is high because all the neurones have ADHD and are rapid firing out tons of signals
The sleep cycle moves through 5 successful stages
Describe the features of Stage 1 of the sleep cycle
Low frequency, High amplitude Theta waves
‘S-sleep’
Light sleeping so you’re easily roused
Slow eye movements
What are the features of stage 2 sleep?
Frequency slows lower than Stage 1
However, random rapid bursts of High-frequency (12-14 Hz) waves called sleep spindles occur
Eye movements stop from stage 1
What are the features of Stage 3 sleep
Delta waves:
- Very low frequency (2 Hz) - maintaining High amplitude from stages 1 & 2
Interspersed with short episodes of faster waves - however, spindle activity decreases from stage 2
What happens to the EEG waves in stage 4?
Exclusively delta waves from stage 3
No spindle activity or any of that jazz
When is deep sleep?
Stages 3 and 4 are deep sleep
This means its very difficult to rouse them
It is also when sleepwalking/talking occurs
What is stage 5 of the sleep cycle?
REM sleep
Rapid eye movements
Dreams
25% of our sleep is REM
High-frequency state - similar to when we are awake
Describe the vascular, respiratory and metabolic changes that occur in the body during deep, slow-wave sleep
In deep, slow wave sleep there is:
- Decreased vascular tone and thus lower BP
- Decreased respiratory rate
- Decreased basal metabolic rate and thus Body temperature drops slightly
