Circadian Rhythms Flashcards

1
Q

How do the cycles of animals near equatorial regions and polar regions differ?

A
  • Equatorial: based on day/night cycles (circadian rhythm)

- Polar: Seasonal cycles (circannual rhythm)

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2
Q

What are the 4 types of biorhythms? Provide an example of each.

A
  1. Circannual: Yearly (e.g., migration of birds)
  2. Infradian: Less than a year (e.g., human menstrual cycle)
  3. Circadian: Daily (e.g., human sleep cycle)
  4. Ultradian: Less than a day (e.g., eating cycle)
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3
Q

What processes are regulated by circadian rhythms?

A
  • Sleep / wake
  • Body temperature
  • Blood flow
  • Urine production
  • Hormone regulation
  • Metabolic rate
  • Alertness
  • Growth hormones
  • Cortisol
  • Potassium
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4
Q

What are the 3 properties of circadian rhythms?

A
  1. 24 hour period: Responses repeat on a roughly 24 hour period
  2. Endogenous: Rhythmic responses continue in the absence of stimulation
  3. Entrainable: Circadian rhythms can be reset or adjusted by exposure to external stimuli
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5
Q

Is plant movement exogenous or endogenous?

A
  • Leaves open up during the day for photosynthesis and close at night
  • Endogenous behaviour: If a plant is placed in a constantly dim light and a pen is attached to the leaf to record when it moves, it will produce a rhythmic movement that occurs even in the absence of light
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6
Q

How are circadian rhythms studied in mammals?

A
  • Rat has access to a running wheel that is connected to a computer
  • Computer measures amount of revolutions
  • Produces attogram: Each line represents a day’s activity
  • Rats are active when lights are turned off but scurry away when lights go on (decreased activity)
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7
Q

What experiment with rats showed that circadian rhythms are entrainable?

A
  • When attogram recorded rat activity when there were 12 hours with lights on and 12 hours with lights off, there was a predictable pattern of activity
  • When rats were placed in constant darkness, there was still rhythmicity, but the animals’ behaviours ran a little slower. This is because the internal clock runs on about 24.5 to 25.5 hour cycles
  • Eventually there is a reversal of behaviour where they are active during periods where they weren’t active and vice versa
  • Circadian rhythms are endogenous but they are not perfect, so they must be entrained by other cues such as light
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8
Q

What are zeitgebers?

A
  • Means “time giver”
  • Endogenous rhythms are not accurate enough to be 100% reliable
  • Light entrains biological clocks to day/night cycle: Acts to “reset” the circadian clock to maintain its correspondence to the outside world
  • A circadian clock that is properly reset by a zeitgeber is said to be entrained
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9
Q

What is a free running rhythm?

A
  • A circadian rhythm in the absence of zeitgebers that is a little more / less than 24 hours
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10
Q

What did Aschoff and Weber’s bunker study show about circadian rhythms?

A
  • When isolated from the sun, people’s free-running rhythms tended to be slightly longer than 24 hours
  • Every night, tended to go to bed a bit later than the night before until sleep/wake cycle was completely reversed
  • In some cases, physiology and behaviour become out of sync: When master clock gets out of sync, hormone release becomes out of sync with behaviour
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11
Q

What are the 3 elements of a biological clock?

A
  1. Light sensor: Senses changes in lightness / darkness (eyes); entrains the clock and keeps its rhythm coordinated with the environment
  2. Clock: Continues to run and keeps a basic rhythm in the absence of any input
  3. Output pathway: Allows clock to control brain and body functions (e.g., signalling to the adrenal glands to release cortisol)
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12
Q

What is the suprachiasmatic nucleus?

A
  • Region above the hypothalamus located above the optic chiasm
  • Small cluster of neurons that serve as biological clock
  • Receives information from light that comes into the eyes
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13
Q

What happens when the SCN is lesioned?

A
  • Abolishes circadian rhythmicity of physical activity, sleep/wake cycles, feeding, drinking, etc.
  • Animals will sleep around the same amount but the distribution of sleep will be random
  • SCN from another organism can be transplanted and rhythms will resume within 2-4 weeks (suggests rhythms are endogenous rather than learned)
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14
Q

What is the retinohypothalamic tract?

A
  • Axons from ganglion cells in the retina that synapse onto dendrites on SCN neurons. This input is required for entrainment, and SCN neurons are sensitive to light in non-selective way.
  • Ganglion cells express a photopigment called melanopsin that is depolarized by light
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15
Q

Where does the SCN output to?

A
  • Nearby hypothalamic nuclei
  • Midbrain and other parts of diencephalon
  • GABAergic (inhibitory)
  • Also use vasopressin
  • Lesions to efferent SCN pathways disrupt circadian rhythms
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16
Q

Explain the pathway that leads to the production of melatonin.

A
  • SCN provides inhibitory tone to the PVN
  • PVN projects to preganglionic sympathetic neurons in spinal column
  • Preganglionic sympathetic neurons modulate neurons in pineal gland
  • Pineal gland synthesizes melatonin from tryptophan
  • Melatonin production goes up as environmental light goes down
  • When secreted into bloodstream, melatonin binds to melatonin receptors on SCN neurons and influences sleep wake cycles (indicates lack of light)
17
Q

How do SCN neurons keep time?

A
  • Raising SCN neurons in vitro demonstrates that they intrinsically keep time
  • Rhythmicity of glucose utilization, protein synthesis, vasopressin production, etc. remains intact in absence of zeitgebers
  • Rhythmicity also occurs after tetrodotoxin administration (doesn’t require APs between each other to maintain rhythmicity)
18
Q

What are clock genes of SCN neurons?

A
  • Used to endogenously keep time
  • Genes are transcribed and translated
  • After a delay, proteins send feedback by interacting with transcription factors, causing a decrease in gene expression (negative feedback loop)
  • Less protein production, less inhibition of gene expression and eventually the cycle renews in 24 hours
19
Q

What are the names of clock genes / proteins?

A
  1. Cryptochrome (CRY)
  2. circadian locomotor output cycles kaput (CLOCK)
  3. Brain and muscle ARNT-like (BMAL1)
  4. Period (PER1, PER2, PER3)
20
Q

Explain the negative feedback loop of clock genes.

A
  • BMAL1 and Clk translated into proteins and form heterodimer C-B
  • C-B re-enters nucleus and promotes transcription of other genes while inhibiting others
  • CRY-PER2 heterodimer goes back into nucleus and promotes transcription of BMAL1 and Clk and cycle continues
21
Q

What are the 3 key components of the feedback loop of clock genes?

A
  1. Concentration of BMAL and 3 PER proteins cycle in counterphase
  2. PER2 is positive regulator of BMAL1 loop
  3. CRY is negative regulator of the period and cryptochrome loops (shuts system down)