Basic principles of chronobiology

Question 1

What is a possible advantage of cyclical biological processes in life?

Answer 1

an adaptation to living in a cyclical world, to be able to predict rather than react

Question 2

Define phase, amplitude and period, and the 3 different classifications of a rhythm based on its length

Answer 2

Phase = in sync
Amplitude = departure from baseline
Period = length from two points of same amplitude

Ultradian = T < 20h
Circadian = 20h < T < 28h

Question 3

What is an actogram?

Answer 3

A spectrum over the course of x days to show eg activity

Question 4

What is the three main properties of circadian rhythms?

Answer 4

The are endogenous, temperature compensated, and responsive to environmental cues

Question 5

What does it mean if a rhythm observed is not exactly 24 hours?

Answer 5

It is not environmentally determined (and therefore endogenous*)
*unless there is an environmental factor that cycles every 22-23 hours

Question 6

How did bees prove the concept of a biological clock?

Answer 6

They do a figure 8 dance in the direction of the food, relative to the sun
But the sun changes position during the day, yet the bees compensate (must be an internal compensation mechanism)

Question 7

What is the difference between a clock and a rhythm?

Answer 7

A clock is the endogenous “gears” that are continuously ongoing
A rhythm is the measurable output (the hands on the clock”

You can have pacemaker cells (the clock) that regulate body temperature (rhythm)

Question 8

How are clock outputs and the environment related?

Answer 8

The environment is the “adjuster knob” of the clock and can fine tune the rhythm

The rhythms can themselves change the environment too, and the clock itself

Question 9

What is the Imposed time?

Answer 9

The imposed duration of a day cycle (not light)

Question 10

What is Zeitgeber time?

Answer 10

something in the environment that clues a person in for the current time (eg 9:00, if there’s a sun it’s AM if it’s dark it’s PM)

Question 11

Why isn’t there Zeitgeber time during constant conditions? What is used instead?

Answer 11

Nothing to cue into time

Use Circadian Time instead

Question 12

What is subjective day/night?

How can we deduce this from an animal?

Answer 12

The time during an LL or DD experiment that would normally be day/night if there was zeitgeber

Depending on if the animal is diurnal or nocturnal, spikes of activity will determine the subjective day/night

Question 13

On an actogram, what does it mean for the onset of activity for a nocturnal animal to shift to the left? What is happening to CT?

Answer 13

CT12 is shifting away from ZT12
The subjective night is starting earlier and earlier
The FRT (free running time) is not exactly 24 hours, and SHORTER since it is moving to the left

Question 14

What is the free running (or endogenous) period?

Answer 14

The time between two onsets of activity, anything that is not exactly 24 hours will cause a shift
Represented by Tau

Question 15

What happens if Tau is less than 24 hours?
Exactly 24 hours?
Greater than 24 hours?

Answer 15

Less - leftwards shift on actogram, starting earlier
exactly - no change
more - rightwards shift on actogram, starting later

Question 16

Why is it advantageous to have clocks be temperature compensated?

Answer 16

It wouldn’t be viable to have our clocks change with the environment that much

eg a change in 10C usually causes a reaction to go 2-3x faster, the weather varies much more than that

Question 17

Why is it useful for circadian rhythms to respond to environmental cues?

Answer 17

eg Jetlag, human clock isn’t actually 24 hours but the sun “resets” it

Question 18

What is a phase delay? a phase advance? how do they occur and when would a change in phase not be observed? Use the model of a nocturnal animal

Answer 18

A phase delay is a delay in the onset of activity, caused by a light pulse in the early subjective night
A phase advance is the opposite, caused by a light pulse in the late subjective night

If the light was pulsed exactly in the middle of the subjective night there would be no effect

Question 19

What is the difference between a Photoperiod and a T-Cycle?

Answer 19

A photoperiod is always 24 hours, a T cycle is not

Question 20

Which is easier to reset, a phase advance or a phase delay?

Answer 20

A phase advance

Question 21

How can you test if something is endogenous?

Answer 21

Test it under constant conditions