Sleep Flashcards
What are brain rhythms?
Brain rhythms refer to distinct patterns of neuronal activity that are associated with specific behaviours, arousal level and sleep state.
The earth has a rhythmic environment that can vary with the seasons:
- Temperature
- Precipitation
- Daylight
In order to compete effectively and survive, an animals behaviour must oscillate with its environment
How do we measure brain rhythmicity?
The electroencephalogram
The electroencephalogram (EEG) is a measurement of electrical activity generated by the brain and recorded from the scalp.
EEG measures the combined activity of a large number (1000s) of similarly orientated neurons
Requires synchronous activity across groups of cells
EEG reflects summed post-synaptic activity of large cell ensembles
Involves non-invasive electrodes placed on standard positions on the head – connected to amplifiers and a recording device
How can EEG rhythms be categorised?
EEG rhythms can be categorised by their frequency range with each range named after a greek letter:
A high-frequency low-amplitude associated with alertness and waking
Beta is associated with a waking state
Alpha is associated with waking and resting state
A low-frequency high-amplitude associated with non-dreaming sleep
Theta is associated with non-dreaming sleep
Delta is associated with a deep sleep
What are the two different types of generation of synchronous brain rhythms?
Pacemaker (a):
- Synchronous rhythms can be led by a central clock or pacemaker (e.g. thalamus)
Collective behaviour (b):
- Synchronous rhythms can arise from the collective behaviour of cortical neurons themselves
How does the thalamic pacemaker work?
Thalamic pacemaker:
- 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 neurons can compel a much larger group of cortical neurons
What’s the alternative to the thalamic pacemaker for cortical neurons?
Collective behaviour of cortical neurons
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 cerebral cortex
What are the functions of the brain rhythms?
Cortical rhythms parallel many interesting human behaviours… but do they serve a functional purpose?
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 (e.g. epilepsy)
What is sleep defined as?
Sleep is a readily reversible state of reduced responsiveness to, and interaction with, the environment.
Sleep may be universal amongst all animals (e.g. fruit fly Drosophila sleeps)
Do we really 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 associated with a high frequency and low amplitude pattern
Non-REM sleep:
Body capable of involuntary movement, rarely accompanied by vivid, detailed dreams
“Idling brain in a moveable body”
Low frequency and high amplitude pattern
REM sleep:
Body immobilised, accompanied by vivid, detailed dreams
“An active, hallucinating brain in a paralysed body”
High frequency and low amplitude
What is the sleep cycle like?
EEG rhythms 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 happens during the different functional states of the brain?
Non-REM sleep: Temperature drop Larger heart rate drop than REM Larger breathing drop than REM Brain energy consumption drop REM sleep: Larger temperature drop than non REM Heart rate drop (irregular) Breathing drop (irregular) Brain energy consumption huge increase
Why do we sleep?
No single theory of the function of sleep is widely accepted, although most reasonable ideas fall into two categories – theories of restoration and adaptation.
Restoration- We sleep to rest and recover and to prepare to be awake again
Adaptation- We sleep to protect ourselves (e.g. hide from predators) and to conserve energy
What is the neural mechanism 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, norepinephrine 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 is the neural mechanism of sleep?
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 norepinephrine)
However, cholinergic neurons in pons shown to increase rate of firing to induce REM sleep – linked with dreaming
Rhythmic forms of firing in the thalamus shown to block the flow of sensory information up to the cortex
In addition to a decrease in brainstem activity, other sleep-promoting factors also involved in promoting sleep…
What are sleep-promoting factors?
Adenosine:
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)
Inhibitory effect on ACh, norepinephrine and 5-HT, which promote wakefulness
Adenosine receptor antagonists (e.g. caffeine) promote wakefulness
Nitric oxide (NO):
Nitric oxide (NO) is a potent vasodilator
Decreases smooth muscle tone (decreasing blood pressure)
NO also stimulates adenosine release
Inflammatory factors?:
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?
Melatonin:
Melatonin is a hormone secreted by the pineal gland at night
Shown to initiate and maintain sleep
Over-the-counter medication for symptoms of insomnia and jet-lag
