Sleep & Circadian Rhythms Flashcards
What are Brain Rhythms?
Refer to distinct patterns of neuronal activity that are associated with specific behaviours, arousal level and sleep rate
List some environmental rhythms
The earth has a rhythmic environment that can vary with the seasons
- Temperature
- Precipitation
- Daylight
What must an animal do in order to survive and compete efficiently?
- In order to compete effectively and survive, an animals behaviour must oscillate with its environment
- The brain has evolved a variety of systems for thymine control such as sleep/wake cycle
What does an electroencephalogram (EEG) do?
It’s a measurement of electrical activity generated by the brain and recorded from the scalp
What does an electroencephalogram involve?
- Involves non invasive electrodes placed on standard positions on the head
- Connected to amplifiers and a recording device
Briefly describe how an electroencephalogram works (PART 1)
- 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
Briefly describe how an electroencephalogram works (PART 2)
- The amplitude of an EEG signal depends upon how synchronous the activity of a group of cells is
- When a group of cells are excited and synchronous, the tiny signals sum to generate a large surface signal
- However, timing is everything, the same amount excitation can occur at irregular intervals and result in a small summed signal
How can the EEG rhythms be categorised?
By their frequency range:
- A high frequency low amplitude associated with alertness and waking
- A low frequency high amplitude aaaociated with non dreaming sleep
What is the electroencephalogram used for today?
Primarily used to help diagnose certain neurological disorders (Seizures in epilepsy)
What are the 2 types of synchronous base rhythms?
- 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 pacemakers
- The thalamus can act as a pacemaker with its vast input to the cerebral cortex
- Synaptic connections between excitatory and inhibitory thalamic neurons force each individual neurons to conform to the rhythm of the group
- Co-ordinated rhythms are then passed to the cortex by thalamuocrotical axons
- Therefore a relatively small group of centralised thalamic neurons can compel a much larger group of cortical neurons
Describe the collective behaviour of cortical neurons
- Some rhythms of the cerebral cortex do not depend on a thalamic pacemaker
- Instes they rely on collective interactions of cortical neurons themselves
- Excitatory and inhibitory interconnections of neurons result in a coordinated 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 hypothesis is that most brain rhythms have no direct function but are by products instead
- Brain rhythms are strongly interconnected with various forms of excitatory feedback
- Rhythms must be an unavoidable consequence of such circuitry
- However even if brain rhythms don’t have a function, they provide us with a convenient window on the functional states of the brain
Define sleep
A readily reversible state of reduced responsiveness and interaction with the environment
Do we really need sleep?
- Prolonged sleep deprivation can be devastating to proper functioning
- However we can starve off sleep but not for long
- Longest record without sleep was 264.4 hours by Randy Gardner
- Sleep is be universal amongst all animals
What are the 3 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
List some features of Non REM sleep
- Decreased temperature
- Decreased heart rate
- Decreased breathing
- Brain energy consumption
List some features of REM sleep
- Decreased temperature
- Decreased irregular heart rate
- Decreased irregular breathing
- Increased brain energy consumption
Describe the sleep cycle
- EEG rhythms can be 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
Why do we sleep?
No single theory of the function of sleep is widely accepted although most reasonable ideas fall into two categories
- Restoration: We sleep to rest and recover to prepare to be awake again
- Adaptation: We sleep to protect ourselves and to conserve energy
What are the Neural mechanisms of wakefulness?
- During wakefulness, there is an increase in brain stem activity
- Several sets of neurons increase the rate of firing in anticipation of wakening and enhance the wake state
- Collectively, these neurons synapse direct brain regions including the thalamus and cerebral cortex
- Increase in excitatory activity suppresses rhythmic forms of firing in the thalamus and cortex present during sleep
What are the Neural mechanisms of sleep? (PART 1)
- During sleep, there is a decrease in brain stem activity
- Several sets of neurons decrease rate of firing during sleep
- However cholinerguc neurons in pons shown to increase rate of firing to induce REM sleep: linked with dreaming
What are the Neural mechanisms of sleep? (PART 2)
- 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
List the 2 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)
- Inhibitory effect on ACh, norepinephrine and 5-HT which promote wakefulness
- Adenosine receptor antagonists promote wakefulness
Describe Nitric Oxide as a sleep promoting factor
- Nitric Oxide is a potent vasodilator
- Decreases smooth muscle tone (Decreasing blood pressure)
- Nitric Oxide also stimulates adenosine release
Describe inflammatory factors as sleep promoting factors
- Sleepiness is a familiar consequence of infection
- Cytokines stimulate 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 gland at night
- Shown to initiate and maintain sleep
- Over the counter medication for symptoms of insomnia and jet lag
What is a circadian rhythm?
Refers to any rhythm with a period of approximately 24 hours
Describe circadian rhythms
- Almost all land animals coordinate behaviour according to circadian rhythms: Daily cycles of daylight and darkness
- Most physiological processes also rise and fall with daily rhythms
What happens to the circadian rhythm, if daylight and darkness are removed from an animals environment ?
- Circadian rhythms continue
- Primary clocks for circadian rhythms are biological (Brain clocks)
What is meant by the term brain clocks?
Environmental time cues (light-dark, temperature, humidity) are collectively termed Zeitgebers
What is the problem with Brain clocks?
- It is quite difficult to separate a human from all possible Zeitgebers, even in a lab
- Isolation studies are therefore best conducted in deep caves
- If humans are separated from all Zeitgebers, they’re said to be in a free running state, internal biological clocks of approximately 24.5 hours-25.5 hours
What is the Suprachiasmatic nucleus (SCN)?
A small nucleus of the hypothalamus that receives retinal inner action and synchronises circadian rhythms with the daily light dark cycle
How do neurons in the SCN regulate circadian rhythmicity?
- SCN clock genes produce proteins that send feedback to the SCN
- This inhibits further protein production 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
