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)
2
Q
What are the 4 types of biorhythms? Provide an example of each.
A
- Circannual: Yearly (e.g., migration of birds)
- Infradian: Less than a year (e.g., human menstrual cycle)
- Circadian: Daily (e.g., human sleep cycle)
- Ultradian: Less than a day (e.g., eating cycle)
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
4
Q
What are the 3 properties of circadian rhythms?
A
- 24 hour period: Responses repeat on a roughly 24 hour period
- Endogenous: Rhythmic responses continue in the absence of stimulation
- Entrainable: Circadian rhythms can be reset or adjusted by exposure to external stimuli
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
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)
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
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
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
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
11
Q
What are the 3 elements of a biological clock?
A
- Light sensor: Senses changes in lightness / darkness (eyes); entrains the clock and keeps its rhythm coordinated with the environment
- Clock: Continues to run and keeps a basic rhythm in the absence of any input
- Output pathway: Allows clock to control brain and body functions (e.g., signalling to the adrenal glands to release cortisol)
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
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)
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
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
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
- Cryptochrome (CRY)
- circadian locomotor output cycles kaput (CLOCK)
- Brain and muscle ARNT-like (BMAL1)
- 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
- Concentration of BMAL and 3 PER proteins cycle in counterphase
- PER2 is positive regulator of BMAL1 loop
- CRY is negative regulator of the period and cryptochrome loops (shuts system down)
