Sleep Flashcards
Define brain rhythms
Brain rhythms refer to distinct patterns of neuronal activity that are associated with specific behaviours, arousal level and sleep state.
What are 3 types of rhythmic environment?
- Temperature
- Precipitation
- Daylight
How do we measure brain rhythmicity?
The electroencephalogram (EEG) is a measurement of electrical activity generated by the brain and recorded from the scalp.
What does the amplitude of an EEG depend on?
The amplitude of an EEG signal depends upon how synchronous the activity of a group of cells is
What happens when a group of cells are excited and synchronous
When a group of cells are excited and synchronous, the tiny signals sum to generate a large surface signal
How can EEGs be categorised by there range?
- A high-frequency low-amplitude associated with alertness and waking
- A low-frequency high-amplitude associated with non-dreaming sleep
Give to examples of the generation of synchronous brain waves
Pacemaker
• Synchronous rhythms can be led by a central clock or pacemaker (e.g. thalamus)
Collective behaviour
• Synchronous rhythms can arise from the collective behaviour of cortical neurons themselves
Describe the 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
• 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 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 (e.g. epilepsy)
Define sleep
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)
What can lack of sleep cause?
- 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?
On image
Describe the sleep cycle
• EEG rhythms can be sub-divided to indicate depth of sleep (Stages 1-4)
• Each night begins with a period of non-REM sleep
• As night progresses, there is a shift from non-REM to REM sleep
these switch every 90mins appox
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
Describe 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, 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
Describe 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 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
- However, other sleep-promoting factors also involved in promoting sleep…
What are the 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
What is the term for environmental time cues?
What is the term if you remove all environmental cues?
Environmental time cues (e.g. light-dark, temperature, humidity) are collectively termed zeitgebers.
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.
What do the SCN clock genes do?
What does it recieve from the retina?
- SCN clock genes produce proteins that send feedback to the SCN and inhibit further production of those proteins – occurs over a period of 24 hours
- Light information from the retina serves to rest the SCN neuron clocks each day
- SCN has control over circadian clocks throughout the body (e.g. liver)
