Sleep and Circadian Rhythms Flashcards
Why has the brain evolved a variety of systems?
The earth has a rhythmic environment – temperature, precipitation and daylight vary with the seasons.
In order to compete effectively and survive, an animals behaviour must oscillate with it’s environment.
The brain has evolved a variety of systems for rhythmic control – most striking example is our sleep/wake cycle.
How do we measure brain rhythmicity?
Using electroencephalogram (EEG)
What is EEG?
The electroencephalogram (EEG) is a measurement of electrical activity generated by the brain and recorded from the scalp.
What does the EEG involve?
Involves non-invasive electrodes placed on standard positions on the head – connected to amplifiers and a recording device.
What is the EEG used for today?
Today, the EEG is used primarily to help diagnose certain neurological disorders (e.g. seizures in epilepsy).
How does the EEG work?
EEG measures the combined activity of a large number (1000s) of similarly orientated neurons
What does the EEG require?
Requires synchronous activity across groups of cells.
What does the EEG reflect?
EEG reflects summed post-synaptic activity of large cell ensembles.
What does the amplitude of the EEG depend on?
The amplitude of an EEG signal depends upon how synchronous the activity of the neurons is.
When a group of cells is excited the tiny signals sum to generate a large surface signal.
Timing is everything – the same amount of excitation can occur, but at irregular intervals resulting in a small summed signal.
What do EEG rhythms correlate to?
EEG rhythms correlate with states of behaviours.
EEG rhythms are categorised by their frequency range.
What is a high frequency low amplitude associated with?
alertness and waking
What is low frequency high amplitude associated with?
non-dreaming sleep or coma
How are synchronous brain rhythms generated?
Synchronous rhythms can be led by a central clock or pacemaker (e.g. thalamus).
Synchronous rhythms can arise from the collective behaviour of cortical neurons themselves.
How does the thalamic pacemaker work?
The thalamus, with its vast input to the cerebral cortex, can act as a pacemaker.
Synaptic connections between excitatory and inhibitory thalamic neurons force each individual neuron to conform to the rhythm of the group.
Co-ordinated rhythms are then passed to the cortex by thalamocortical axons.
Thus, a relatively small group of centralised thalamic cells can compel a much larger group of cortical cells.
How does the collective behaviour of cortical neurons generate synchronous brain rhythms?
Some rhythms of the cerebral cortex do not depend on a thalamic pacemaker – rely instead on collective interactions of cortical neurons themselves.
Excitatory and inhibitory interconnections of neurons result in a co-ordinated, synchronous pattern of activity.
This can remain localised or spread to encompass larger regions of the cortex.
What are the functions of brain rhythms?
One plausible hypothesis is that most brain rhythms have no direct function – instead they are by-products.
Brain circuits are strongly interconnected with various forms of excitatory feedback – rhythms may be an unavoidable consequence of such circuitry.
However, even if brain rhythms don’t have a function, they provides us with a convenient window on the functional states of the brain.
What is sleep?
Sleep is a readily reversible state of reduced responsiveness to, and interaction with, the environment.
Sleep may be universal among higher invertebrates and perhaps amongst all animals (e.g. fruit fly Drosophila sleeps).
Do we need sleep?
Prolonged sleep deprivation can be devastating to proper functioning.
However, we can stave off sleep… but not forever
What are the functional states of the brain?
Wakefulness
Non-REM sleep: Body capable of involuntary movement, rarely accompanied by vivid, detailed dreams.
REM sleep: Body immobilised, accompanied by vivid, detailed dreams.
What is the difference between non-REM and REM sleep?
Non-REM sleep: Temp ↓ HR ↓ ↓ Breathing ↓ ↓ Brain energy consumption ↓
REM sleep: Temperature ↓ ↓ ↓ Heart rate ↓ (irregular) Breathing ↓ (irregular) Brain energy consumption ↑↑↑
Compare the EEG between the states of the brain
Awake: low-amplitude, high frequency
Non-REM: high amplitude, low frequency
REM: low-amplitude, high frequency
Compare sensation between the states of the brain
Awake:
vivid, externally generated
non-REM:
dull or absent
REM:
vivid, internally generated
Compare thought between the states of the brain:
Awake:
Logical, progressive
non-REM:
Logical, repetitive
REM:
vivid, illogical, bizzare
Compare movement between the states of the brain
Awake:
continuous, voluntary
non-REM:
occasional, involuntary
REM:
muscle paralysis: movemet commanded by the brain but not carried out
Compare REM between the different states of the brain
Awake:
often
non-REM:
rare
REM:
often
What is the sleep cycle?
EEG stages can be sub-divided to indicate depth of sleep (Stages 1-4).
Each night begins with a period of non-REM sleep.
Sleep stages are then cycled throughout the night, repeating approximately every 90 minutes
As night progresses, there is a shift from non-REM to REM sleep.
What can the theories on the purpose of sleep be split up to?
No single theory of the function of sleep is widely accepted, although most reasonable ideas fall into two categories – theories of restoration and adaptation.
What is the theory of restoration?
We sleep to rest and recover and to prepare to be awake again.
What is the theory of adaptation?
We sleep to protect ourselves (e.g. hide from predators) and to conserve energy.
What are the neural mechanisms of wakefulness?
During wakefulness, there is an increase in brainstem activity.
Several sets of neurons increase rate of firing in anticipation of wakening and enhance the wake state (e.g. ACh, 5-HT, NE and histamine).
Collectively, these neurons synapse directly brain regions including the thalamus and cerebral cortex.
Increase in excitatory activity supresses rhythmic forms of firing in the thalamus and cortex present during sleep.
What are the neural mechanisms of sleep?
During sleep, there is an decrease in brainstem activity.
Several sets of neurons decrease rate of firing during sleep (e.g. ACh, 5-HT and NE).
However, cholinergic neurons in pons shown to increase rate of firing to induce REM sleep.
Rhythmic forms of firing in the thalamus shown to block the flow of sensory information up to the cortex.
However, other sleep-promoting factors also involved in promoting sleep…
What are the 4 sleep promoting factors?
Adenosine
nitric oxide
inflammatory factors
melatonin
Describe adenosine as a sleep-promoting factor.
Adenosine is a building block for DNA, RNA and ATP.
Adenosine receptor activation decreases heart rate, respiratory rate and smooth muscle tone (decreasing blood pressure).
Adenosine receptor antagonists (e.g. caffeine) promote wakefulness.
Describe nitric oxide as a sleep-promoting factor
Nitric oxide (NO) is a potent vasodilator.
Decreases smooth muscle tone (decreasing blood pressure).
NO also stimulates adenosine release.
Describe inflammatory factors as a sleep-promoting factor
Sleepiness is a familiar consequence of infection (e.g. cold, flu).
Cytokines (e.g. interleukin-1) stimulates the immune system to fight infections.
Interleukin-1 levels shown to promote non-REM sleep –evidence for adaptation theory?
Describe melatonin as a sleep-promoting factor.
Melatonin is a hormone secreted by the pineal body at night.
Over-the-counter medication for symptoms of jet-lag.
Shown to initiate and maintain sleep – role in natural sleep-wake cycle remains unclear.
What are circadian rhythms?
Almost all land animals co-ordinate behaviour according to circadian rhythms – the daily cycles of daylight and darkness that result from the spin of the Earth.
Most physiological processes also rise and fall with daily rhythms (e.g. temperature, hormone levels).
What is so interesting about if cycles of daylight and darkness are removed from an animal’s environment?
If cycles of daylight and darkness are removed from an animals environment, circadian rhythms continue.
Primary clocks for circadian rhythms are biological (“brain clocks”).
What are zeitgebers?
Environmental time cues (e.g. light-dark, temperature, humidity) are collectively termed zeitgebers.
It is quite difficult to separate a human from all possible zeitgebers – even inside a laboratory (e.g. people coming/going provide time cues).
Isolation studies are therefore best conducted in deep caves.
If humans are separated from all possible zeitgebers, they are said to be in a “free-running” state – internal biological clock of approximately 24.5-25.5 hours.
What is the SCN?
The suprachiasmatic nucleus (SCN) is a small nucleus of the hypothalamus that receives retinal innervation and synchronises circadian rhythms with the daily light-dark cycle.
SCN inhibition does not abolish sleep – animals will continue to co-ordinate sleep with light-dark cycles if they are present.
Explain SCN mechanisms
Retinal cells synchronising the SCN are not rods or cones – they are specialised photoreceptor cells expressing the photopigment melanopsin.
Photoreceptors expressing melanopsin are slowly excited by light and can detect changes in luminosity.
Project directly to the SCN, inhibiting the production of melatonin by the pineal gland – melatonin involved in inducing the onset of sleep.
