MODULE 3: ENTRAINMENT BY LIGHT Flashcards
what 3 words can describe the mechanisms responsible for circadian rhythms?
oscillator
pacemaker
clock
why is it important to know the subtle distinctions between the words oscillator, pacemaker, and clock?
multicellular organisms have circadian time systems with many, different, and sometimes hierarchical roles
define an oscillator
any device that produces a rhythm
why can oscillators be categorized along the range of slave to master?
they may be incapable of keeping a rhythm alone and or depend on signals from a pacemaker
what are secondary/local oscillators?
the position of the oscillator in a complex multi-oscillator system
where might someone find a seocndary/local oscillator?
a specific body tissue/organ or brain region
what is the role of a secondary oscillator?
inducing circadian rhythms on a particular region
if someone were to remove a secondary oscillator, what would happen?
the rhythmicity of the organ/tissue or region would be affected
what is a pacemaker?
the master oscillator that controls the other timing of other oscillators and direct processes
if a pacemaker is to be removed, what would happen?
the whole oragnism’s rhythms would be affected
true or false: removing a pacemaker would only result in local oscillators to stop
why?
false: they may continue but would desynchronize
what do pacemakers use to coordinate rhythms?
cycles of nature
what happens when an oscillator is synchronized to the external world?
a specific phase of the oscillator corresponds to a particular time of day
define a clock
a device that allows us to know the time of day even without knowing the sun’s position
what is an interval timer?
device that can time one cycle, stops and needs to be reset
what are 2 examples of interval timers?
hour glass and stopwatch
what are the 3 defining characteristics of circadian clocks?
- they are self-sustaining and persist with a 24 hour periodicity in constant conditions
- they are temperature compensated
- they are entrainable by environmental stimuli
circadian clocks ___ behaviour and physiology with predictable daily events in the real world
coordinate
when a circadian clock triggers a behaviour or physiological process with predictable events in the real world, they are analogous to what?
alarm clocks
when circadian clocks are being consulted continuously to allow memory/recognition each phase of their internal clock, what are they analogous to?
wrist watches
what are circadian clocks measuring when they permit species to organize their behavior/physiological processes into annual rhythms?
day length or photoperiodism
using circadian clocks to measure photoperiodism requires what?
a stable relationship with the solar day
what would happen if a clock is not synchronized with the light/dark cycle?
a particular phase would not be predictive of the local environment time
what 2 problems does entrainment need to solve?
- circadian clocks cycle with a periodicity that only approximates 24hrs
- must ensure that organisms do the right thing at the right time of day
describe how entrainment would address the approximation of 24hrs problem
must adjust the clock on a regular basis so it does not drift out of synchrony with the outside world
describe how entrainment would address the problem of matching an organism’s actions to a time of day
synchronizing to the outside world allowing for sleep and waking states to occur at the appropriate time
in diurnal animals, what is their dominant sense that helps with entrainment?
vision
what is the objective of entrainment?
control both period and phase of the circadian clock to align it with the environment
define zeitgeber
an external stimulus that can entrain a circadian clock
what is masking?
the direct effect of an environmental stimulus on a behavioural state
in diurnal animals ___ promotes alertness and ___ promotes sleep
light; dark
why do chronobiologists say the direct effect of light is considered masking?
light can mask the true phase of the circadian clock
what is negative masking?
when the environmental stimulus suppresses a behaviour that would normally be present during that time of day
what is positive masking?
when an environmental stimulus stimulates behaviour that would normally be absent during that time of day
what is the consequence of direct effect of light/dark on an animal’s behaviour who has no functional circadian clock?
a light-dark cycle could induce a daily rest-activity rhythm
what might be a reason that an animal may appear to be entrained by light/dark despite lacking any internal clock susceptible to entrainment?
a brain lesion or gene mutation
how can someone determine if an observed synchrony of a rest-activity cycle with light/dark is due to entrainment, masking, or both?
looking at how fast the rhythm synchronizes with the light/dark cycle
if the synchronization to the LD cycle is instantaneous, what is it mostly likely due to?
masking
how might someone investigate if a synchronization is due to masking or entrainment?
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
what is the hypothesis that supports the mechanism of entrainment?
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
what happens when light is introduced to a nocturnal animal during early subjective night?
cycle would be delayed and activity would begin later the next day in constant dark
what happens when light is introduced to a nocturnal animal during late subjective night?
cycle would advance and begin earlier the next day in constant dark
describe the difference between a phase response curve and dose-response curve
the x-axis plots the circadian phase at which the stimulus was administrated
what is a point phase?
when the light is being presented at a particular point in the organism’s circadian cycle
what is a phase shift?
when a phase of a circadian cycle is shifted after being introduced to a light pulse
true or false: a light pulse shifts only one phase in a cycle
false: shifts all of the phases in a cycle together
circadian hours are ___/24th of a full circadian cycle
1
if an organism’s tau is 25 hours, what is one circadian hour for that organism?
25/24 = 1.04
when studying nocturnal animals the beginning of subjective night is ____ and subjective day is ___
CT12
CT0
a light pulse in early half of subjective night causes a ___?
phase delay shift
a light pulse in the later half of the subjective night causes a___?
phase advance shift
true or false: general features of PRC shape are universal to circadian clocks in all creatures
true
describe how the PRC to light pulses suggests that entrainment to a daily light/dark cycle can be achieved by what?
adjusting the circadian clock phase each day in response to light at dawn or dusk
what is a phase in a circadian cycle?
position of the oscillator within its cycle
what is a parameter of a circadian cycle?
period or amplitude that can be set to a particular value which then defines a rhythm
can a phase be a parameter?
no, it cannot be set to a permanent value
what is the non-parametric model of entrainment?
hypothesis that entrainment can be achieved by small daily phase adjustments
why is it called a non-parametric model?
the thing being adjusted is not a parameter of the clock
what is the discrete entrainment mofdel?
light has a discrete/time-limited action on the clock as opposed to the continuous action of light
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
- limits to entrainment
- phase of entrainment
- gradual re-entrainment
what are limits to entrainment?
circadian rhythms will only entrain to to LD cycles that have a period relatively close to 24 hours
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?
it won’t entrain
what is the phase of entrainment?
the period of the clock should determine the timing of circadian rhythms when they are stably entrained to the LD
what is gradual re–entrainment?
when a LD cycle is shifted, the circadian clock should re-train gradually over several days instead of instantly
what is relative coordination/entrainment?
light affecting the clock by alternately delaying or advancing it but not enough to offset the difference between T and tau
what are T-cycle experiments?
when the period of Zeitgeber is being manipulated
what does the phase of entrainment depend on?
the difference between the period of the circadian clock and of the LD cycle
what is the aftereffect of entrainment?
an indicator that the circadian clock has the capacity to adapt to the outside world even though they are genetically determined
does a shift in the LD cycle result in a gradual or immediate re-entrainment?
gradual
what indicates that the circadian clock can shift by only a certain amount in one cycle?
the light pulse in a phase-response curve
what are the 4 main factors that determine the length of time it takes to fully shift to a new time zone?
- magnitude if the shift
- direction of travel
- timing of light exposure
- parameters of light stimulus
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?
longer
which way would someone fly to be able to re-entrain rapidly?
west
which way would someone fly to be able to re-entrain more slowly?
east
what would accelerate re-entrainment?
the right timing of light exposure
what are the complementary factors of light exposure timing?
travel direction
the shift being delayed or advanced
what are the 2 parameters of light stimulus?
amplitude and wavelength
the brighter the light, the ___ the shift
greater
which colour light is the most effective kind of stimulus?
blue light
at what 2 times to nocturnal animals need light exposure to exhibit stable synchrony?
dawn and dusk
what can induce phase shifts without altering the wavelength/amplitude but will alter the period?
light pulses
what is the parametric entrainment model?
hypothesizes that light entrains circadian rhythms by altering both the phase and the period of the clock
is the period of a clock a parameter?
yes
what are the 3 experiments that observed parametric effects of light?
- tau in constant light/dark
- tau changing after light pulses
- aftereffects of entrainment on tau
what was observed in tau in constant conditions for nocturnal and diurnal animals?
nocturnal: constant light slows/increases cycle length
diurnal: light exposure can lengthen or shorten the cycle to help it match the LD period
what was observed of tau after light pulse application? (2)
- 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
what is an aftereffect of entrainment on tau?
LD cycle can alter the rate at which the clock oscillates (closer to frequency of the entraining LD cycle)
when is entrainment most stable?
when light affects both phase and period
what is entrainment in nature most likely due to?
both non-parametric and parametric effects
why is studying entrainment difficult?
one must track both the activity rhythms and the actual light exposure to animal habitats
what is biotelemetry?
the use of miniature activity and light sensors that can be attached to a collar or jacket and monitored remotely
