Sleep & Circadian rhythms

Question 1

Brain rhythms

Answer 1

Brain rhythms refer to distinct patterns of neuronal activity that are associated with specific behaviours, arousal level and sleep state.

Question 2

The earth has a rhythmic environment

Answer 2

• Temperature
• Precipitation
• Daylight

Question 3

EEG

Answer 3

The electroencephalogram (EEG) is a measurement of electrical activity generated by the brain and recorded from the scalp.
•	Today, the EEG is used primarily to help diagnose certain neurological disorders (e.g. seizures in epilepsy)

Question 4

How EEG works?

Answer 4

• 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
• 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 of excitation can occur at irregular intervals and result in a small summed signal

Question 5

Collective behaviour brain rhythms

Answer 5

Synchronous rhythms can arise from the collective behaviour of cortical neurons themselves

Question 6

Pacemaker brain rhythms

Answer 6

Synchronous rhythms can be led by a central clock or pacemaker (e.g. thalamus)

Question 7

Thalamic

Answer 7

• 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

Question 8

Collective behavior of cortical neurons

Answer 8

• 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

Question 9

The function of brain rhythms

Answer 9

Theories but One plausible hypothesis is that most brain rhythms have no direct function – instead they are by-products

Question 10

Sleep

Answer 10

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)

Question 11

Do we really need sleep?

Answer 11

• Prolonged sleep deprivation can be devastating to proper functioning
• However, we can stave off sleep… but not forever…

Question 12

Functional states of the brain

Answer 12

Wakefulness
Non-REM sleep
REM

Question 13

Non-REM sleep

Answer 13

Body capable of involuntary movement, rarely accompanied by vivid, detailed dreams
“Idling brain in a moveable body”

Question 14

REM sleep

Answer 14

Body immobilised, accompanied by vivid, detailed dreams

“An active, hallucinating brain in a paralysed body”

Question 15

The sleep cycle

Answer 15

EEG rhythms can be sub-divided to indicate the 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 the night progresses, there is a shift from non-REM to REM sleep

Question 16

Restoration

Answer 16

We sleep to rest and recover and to prepare to be awake again

Question 17

Adaption

Answer 17

We sleep to protect ourselves (e.g. hide from predators) and to conserve energy

Question 18

Neural mechanisms of wakefulness

Answer 18

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

Question 19

Neural mechanisms of sleep

Answer 19

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

Question 20

Adenosine

Answer 20

• 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

Question 21

Nitric oxide (NO)

Answer 21

• Nitric oxide (NO) is a potent vasodilator
• Decreases smooth muscle tone (decreasing blood pressure)
• NO also stimulates adenosine release

Question 22

Inflammatory factors

Answer 22

• 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

Question 23

Melatonin

Answer 23

• 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

Question 24

Circadian rhythms

Answer 24

A circadian rhythm refers to any rhythm with a period of approximately 24 hours.

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

Question 25

Brain clocks

Answer 25

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

Question 26

The suprachiasmatic nucleus (SCN)

Answer 26

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.

Question 27

SCN and sleep

Answer 27

SCN inhibition does not abolish sleep – animals will continue to co-ordinate sleep with light-dark cycles if they are present.

Question 28

How do nucleus in the SCN regulate circadian rhythmicity?

Answer 28

• 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)