MODULE 3: ENTRAINMENT BY LIGHT

Question 1

what 3 words can describe the mechanisms responsible for circadian rhythms?

Answer 1

oscillator
pacemaker
clock

Question 2

why is it important to know the subtle distinctions between the words oscillator, pacemaker, and clock?

Answer 2

multicellular organisms have circadian time systems with many, different, and sometimes hierarchical roles

Question 3

define an oscillator

Answer 3

any device that produces a rhythm

Question 4

why can oscillators be categorized along the range of slave to master?

Answer 4

they may be incapable of keeping a rhythm alone and or depend on signals from a pacemaker

Question 5

what are secondary/local oscillators?

Answer 5

the position of the oscillator in a complex multi-oscillator system

Question 6

where might someone find a seocndary/local oscillator?

Answer 6

a specific body tissue/organ or brain region

Question 7

what is the role of a secondary oscillator?

Answer 7

inducing circadian rhythms on a particular region

Question 8

if someone were to remove a secondary oscillator, what would happen?

Answer 8

the rhythmicity of the organ/tissue or region would be affected

Question 9

what is a pacemaker?

Answer 9

the master oscillator that controls the other timing of other oscillators and direct processes

Question 10

if a pacemaker is to be removed, what would happen?

Answer 10

the whole oragnism’s rhythms would be affected

Question 11

true or false: removing a pacemaker would only result in local oscillators to stop

why?

Answer 11

false: they may continue but would desynchronize

Question 12

what do pacemakers use to coordinate rhythms?

Answer 12

cycles of nature

Question 13

what happens when an oscillator is synchronized to the external world?

Answer 13

a specific phase of the oscillator corresponds to a particular time of day

Question 14

define a clock

Answer 14

a device that allows us to know the time of day even without knowing the sun’s position

Question 15

what is an interval timer?

Answer 15

device that can time one cycle, stops and needs to be reset

Question 16

what are 2 examples of interval timers?

Answer 16

hour glass and stopwatch

Question 17

what are the 3 defining characteristics of circadian clocks?

Answer 17

1. they are self-sustaining and persist with a 24 hour periodicity in constant conditions
2. they are temperature compensated
3. they are entrainable by environmental stimuli

Question 18

circadian clocks ___ behaviour and physiology with predictable daily events in the real world

Answer 18

coordinate

Question 19

when a circadian clock triggers a behaviour or physiological process with predictable events in the real world, they are analogous to what?

Answer 19

alarm clocks

Question 20

when circadian clocks are being consulted continuously to allow memory/recognition each phase of their internal clock, what are they analogous to?

Answer 20

wrist watches

Question 21

what are circadian clocks measuring when they permit species to organize their behavior/physiological processes into annual rhythms?

Answer 21

day length or photoperiodism

Question 22

using circadian clocks to measure photoperiodism requires what?

Answer 22

a stable relationship with the solar day

Question 23

what would happen if a clock is not synchronized with the light/dark cycle?

Answer 23

a particular phase would not be predictive of the local environment time

Question 24

what 2 problems does entrainment need to solve?

Answer 24

1. circadian clocks cycle with a periodicity that only approximates 24hrs
2. must ensure that organisms do the right thing at the right time of day

Question 25

describe how entrainment would address the approximation of 24hrs problem

Answer 25

must adjust the clock on a regular basis so it does not drift out of synchrony with the outside world

Question 26

describe how entrainment would address the problem of matching an organism's actions to a time of day

Answer 26

synchronizing to the outside world allowing for sleep and waking states to occur at the appropriate time

Question 27

in diurnal animals, what is their dominant sense that helps with entrainment?

Answer 27

vision

Question 28

what is the objective of entrainment?

Answer 28

control both period and phase of the circadian clock to align it with the environment

Question 29

define zeitgeber

Answer 29

an external stimulus that can entrain a circadian clock

Question 30

what is masking?

Answer 30

the direct effect of an environmental stimulus on a behavioural state

Question 31

in diurnal animals ___ promotes alertness and ___ promotes sleep

Answer 31

light; dark

Question 32

why do chronobiologists say the direct effect of light is considered masking?

Answer 32

light can mask the true phase of the circadian clock

Question 33

what is negative masking?

Answer 33

when the environmental stimulus suppresses a behaviour that would normally be present during that time of day

Question 34

what is positive masking?

Answer 34

when an environmental stimulus stimulates behaviour that would normally be absent during that time of day

Question 35

what is the consequence of direct effect of light/dark on an animal's behaviour who has no functional circadian clock?

Answer 35

a light-dark cycle could induce a daily rest-activity rhythm

Question 36

what might be a reason that an animal may appear to be entrained by light/dark despite lacking any internal clock susceptible to entrainment?

Answer 36

a brain lesion or gene mutation

Question 37

how can someone determine if an observed synchrony of a rest-activity cycle with light/dark is due to entrainment, masking, or both?

Answer 37

looking at how fast the rhythm synchronizes with the light/dark cycle

Question 38

if the synchronization to the LD cycle is instantaneous, what is it mostly likely due to?

Answer 38

masking

Question 39

how might someone investigate if a synchronization is due to masking or entrainment?

Answer 39

replace the light dark cycle with a constant condition and compare the timing of the activity onset between the first day of constant conditions and the last day of LD cycle

Question 40

what is the hypothesis that supports the mechanism of entrainment?

Answer 40

if 24hr LD cycles can entrain circadian rhythms that differ from 24hrs, the light and dark must shift or reset the that circadian clock to correct its phase each day or adjust the periodicity to make it equal to 24hrs

Question 41

what happens when light is introduced to a nocturnal animal during early subjective night?

Answer 41

cycle would be delayed and activity would begin later the next day in constant dark

Question 42

what happens when light is introduced to a nocturnal animal during late subjective night?

Answer 42

cycle would advance and begin earlier the next day in constant dark

Question 43

describe the difference between a phase response curve and dose-response curve

Answer 43

the x-axis plots the circadian phase at which the stimulus was administrated

Question 44

what is a point phase?

Answer 44

when the light is being presented at a particular point in the organism's circadian cycle

Question 45

what is a phase shift?

Answer 45

when a phase of a circadian cycle is shifted after being introduced to a light pulse

Question 46

true or false: a light pulse shifts only one phase in a cycle

Answer 46

false: shifts all of the phases in a cycle together

Question 47

circadian hours are ___/24th of a full circadian cycle

Answer 47

1

Question 48

if an organism's tau is 25 hours, what is one circadian hour for that organism?

Answer 48

25/24 = 1.04

Question 49

when studying nocturnal animals the beginning of subjective night is ____ and subjective day is ___

Answer 49

CT12 CT0

Question 50

a light pulse in early half of subjective night causes a ___?

Answer 50

phase delay shift

Question 51

a light pulse in the later half of the subjective night causes a___?

Answer 51

phase advance shift

Question 52

true or false: general features of PRC shape are universal to circadian clocks in all creatures

Answer 52

true

Question 53

describe how the PRC to light pulses suggests that entrainment to a daily light/dark cycle can be achieved by what?

Answer 53

adjusting the circadian clock phase each day in response to light at dawn or dusk

Question 54

what is a phase in a circadian cycle?

Answer 54

position of the oscillator within its cycle

Question 55

what is a parameter of a circadian cycle?

Answer 55

period or amplitude that can be set to a particular value which then defines a rhythm

Question 56

can a phase be a parameter?

Answer 56

no, it cannot be set to a permanent value

Question 57

what is the non-parametric model of entrainment?

Answer 57

hypothesis that entrainment can be achieved by small daily phase adjustments

Question 58

why is it called a non-parametric model?

Answer 58

the thing being adjusted is not a parameter of the clock

Question 59

what is the discrete entrainment mofdel?

Answer 59

light has a discrete/time-limited action on the clock as opposed to the continuous action of light

Question 60

what are the 3 properties of entrainment to LD cycles that can be predicted when one knows tau of a circadian clock and the shape of the phase response curve to light pulses

Answer 60

1. limits to entrainment 2. phase of entrainment 3. gradual re-entrainment

Question 61

what are limits to entrainment?

Answer 61

circadian rhythms will only entrain to to LD cycles that have a period relatively close to 24 hours

Question 62

if a clock can only be shifted by 1/2 hours at most in a cycle then what happens when an LD cycle requiring a daily phase shift larger than that?

Answer 62

it won't entrain

Question 63

what is the phase of entrainment?

Answer 63

the period of the clock should determine the timing of circadian rhythms when they are stably entrained to the LD

Question 64

what is gradual re--entrainment?

Answer 64

when a LD cycle is shifted, the circadian clock should re-train gradually over several days instead of instantly

Question 65

what is relative coordination/entrainment?

Answer 65

light affecting the clock by alternately delaying or advancing it but not enough to offset the difference between T and tau

Question 66

what are T-cycle experiments?

Answer 66

when the period of Zeitgeber is being manipulated

Question 67

what does the phase of entrainment depend on?

Answer 67

the difference between the period of the circadian clock and of the LD cycle

Question 68

what is the aftereffect of entrainment?

Answer 68

an indicator that the circadian clock has the capacity to adapt to the outside world even though they are genetically determined

Question 69

does a shift in the LD cycle result in a gradual or immediate re-entrainment?

Answer 69

gradual

Question 70

what indicates that the circadian clock can shift by only a certain amount in one cycle?

Answer 70

the light pulse in a phase-response curve

Question 71

what are the 4 main factors that determine the length of time it takes to fully shift to a new time zone?

Answer 71

1. magnitude if the shift 2. direction of travel 3. timing of light exposure 4. parameters of light stimulus

Question 72

in regards to the magnitude of a LD cycle shift: if it is a large shift that is required, will it take longer or shorter to re-entrain?

Answer 72

longer

Question 73

which way would someone fly to be able to re-entrain rapidly?

Answer 73

west

Question 74

which way would someone fly to be able to re-entrain more slowly?

Answer 74

east

Question 75

what would accelerate re-entrainment?

Answer 75

the right timing of light exposure

Question 76

what are the complementary factors of light exposure timing?

Answer 76

travel direction the shift being delayed or advanced

Question 77

what are the 2 parameters of light stimulus?

Answer 77

amplitude and wavelength

Question 78

the brighter the light, the ___ the shift

Answer 78

greater

Question 79

which colour light is the most effective kind of stimulus?

Answer 79

blue light

Question 80

at what 2 times to nocturnal animals need light exposure to exhibit stable synchrony?

Answer 80

dawn and dusk

Question 81

what can induce phase shifts without altering the wavelength/amplitude but will alter the period?

Answer 81

light pulses

Question 82

what is the parametric entrainment model?

Answer 82

hypothesizes that light entrains circadian rhythms by altering both the phase and the period of the clock

Question 83

is the period of a clock a parameter?

Answer 83

yes

Question 84

what are the 3 experiments that observed parametric effects of light?

Answer 84

1. tau in constant light/dark 2. tau changing after light pulses 3. aftereffects of entrainment on tau

Question 85

what was observed in tau in constant conditions for nocturnal and diurnal animals?

Answer 85

nocturnal: constant light slows/increases cycle length diurnal: light exposure can lengthen or shorten the cycle to help it match the LD period

Question 86

what was observed of tau after light pulse application? (2)

Answer 86

1. light pulses cause a change in phase shift and tau | 2. light pulses that delay a clock lengthen a cycle and light pulses that advance a clock shorten a cycle

Question 87

what is an aftereffect of entrainment on tau?

Answer 87

LD cycle can alter the rate at which the clock oscillates (closer to frequency of the entraining LD cycle)

Question 88

when is entrainment most stable?

Answer 88

when light affects both phase and period

Question 89

what is entrainment in nature most likely due to?

Answer 89

both non-parametric and parametric effects

Question 90

why is studying entrainment difficult?

Answer 90

one must track both the activity rhythms and the actual light exposure to animal habitats

Question 91

what is biotelemetry?

Answer 91

the use of miniature activity and light sensors that can be attached to a collar or jacket and monitored remotely