09. Sleep and circadian rhythms - Part 1 (Sleep) Flashcards
What are circadian rhythms?
Rhythms (regular patterns of activity) associated with a 24-hour cycle
What are endogenous cycles?
Rhythms/patterns generated from within
Examples: Circadian rhythms, migration, breeding season
What does the human circadian rhythm control?
- sleep
- wakefulness
- body temperature
- hormone secretion
- urination
What evidence is there for plant biorhythms?
- flowers open during the day and close at night
- evidence: d’Ortous (1729) found that the mimosa plant continued its rhythmic behaviour in the absence of light, dark or temperature cues
What evidence is there for human biorhythms?
- Aschoff (1965) found that when humans were placed in an underground bunker (no external cues) they still showed sleep-activity rhythms, even if they were >24hrs
- Indicates presence of endogenous biological clock
Cues that help set our biological clock are called…
Zeitgebers
German for ‘time givers’
What are some important Zeitgebers for humans?
Light (most important)
Meals
Activity
Temperature
When a zeitgeber resets a biorhythm, that rhythm is said to be…
entrained
How does jet lag affect our biorhythms?
- Disrupts biorhythm due to crossing a timezone
- ‘Phase-delays’ - we wake up later
- ‘Phaseadvances’ - we wake up earlier (we find this harder)
What is a chronotype?
A variation in biorhythms
E.g. early birds vs night owls
How do chronotypes change throughout life?
- Everyone starts off as an early bird
- Teenagers become night owls
- Old people become early birds again
- Morning people are happier than evening people
- Some schools in the US have changed their curriculum to a later start in order to accommodate teenagers (A. Carskadon)
Who looked in the brain for the biological clock?
Richter, 1927
What location of the brain contains the biological clock according to Richter (1927)?
- The hypothalamus
- He lesioned the brains of wild rats - damage to the hypothalamus caused them to lose their rhythmic behaviour
He hypothesised that damage to the biological clock causes disorders
Where is the hypothalamus is the biological clock?
The Suprachiasmatic Nucleus
discovered by Moore & Eichler and Stephan & Zucker in 1972
lesions disrupted circadian rhythms
What have we discovered about how the Suprachiasmatic Nucleus (SCN) functions?
- it is more active during a light period than a dark
- a single cell removed from the SCN will continue to function this way
- transplantation of an SCN into a donor organism makes the recipient follow the donor’s rhythm
How does light reach the SCN?
- The SCN receives light info through the retinohypothalamic tract
- The tract is made of special ganglion cells - “photosensitive retinal ganglion cells” or PRGCs (Berson et al 2002)
- They have a photopigment (melanopsin) and respond directly to light, especially blue light (Provencio et al 2000)
Does the SCN rely on rods and cones?
No
- Experiments show no effect on circadian rhythms when rods and cones inactivated (so blind people are still ‘entrained’)
- Part of the tract terminates in the midbrain and controls how open the pupil is
What makes the SCN ‘clock’ tick?
Two genes are important:
- the ‘per’ gene, which produces the ‘PER’ protein (period) (Hall & Rosbash, 1984)
- the ‘tim’ gene, which produces the ‘TIM’ protein (timeless) (Young, 1994)
Discovered by studying the SCN in drosophila
How does the SCN control rhythmic behaviours?
It drives ‘slave oscillators’ that control rhythmic behaviour, such as:
- digestion
- body temp
- activity
(Hastings, Maywood & Brancaccio, 2018)
What other brain areas does the SCN interact with?
The pituitary gland
- Causes melatonin release
- Dark phase (rest)
The pineal gland
- Causes glucocorticoid release
- Light phase (wakefulness)
What other effects does the SCN have on breeding?
(Animals)
- the SCN (pineal gland) controls breeding
- in winter, increased melatonin levels shrink the testes
- in spring, decreased melatonin levels enlarge gonads
How do circadian rhythms affect cognition and disease?
- time of day can affect performance on cognitive tasks
- time of day can affect treatment of disease (toxicity of drugs varies from 20-80%)
