Do Now

What does it mean when an organism is exhibiting a free-running period?

What is the benefit of entrainment?

What zeitgebers might an organism with a circadian rhythm use?

What is the name of the graph or figure used to depict an organisms activity record?

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Actograms

What they are & how to read them

How we know biological clocks exist

The earliest and most convincing evidence that internal biological clocks exist was obtained by placing organisms in isolation, and then removing environmental cues, to the greatest degree possible.

When environmental cues are eliminated, the clock is no longer reset by a zeitgeber, and its own pace is revealed.

We record this using actograms!

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Why use actograms

• We've talked about the fact that an internal (biological) rhythm doesn't exactly match the external (environmental) rhythm, and needs to be entrained (re-set) by a zeitgeber (environmental cue)
• An endogenous (internal) rhythm has a free running period.
• The free running period is the length of a period in the absence of environmental cues.
• An exogenous (external) rhythm will not continue without environmental cues

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Actograms can tell us:

• Whether a rhythm is endogenous/

exogenous

• The length of the free running period
• The duration of an active phase
• What environmental cues act as the zeitgeber
• and more...

Actograms

In Actograms, animal activity (e.g. use of running wheel) is represented as bars.

These bars are placed within a 24 hour timeframe so we can see rhythms.

Sometimes the graph is duplicated (as with the example 4x) so we can see a pattern easily.

Actogram Showing the Transition to a Free-running System

The red number 22 on the vertical axis represents the day at which environmental cues were experimentally withdrawn.

In the laboratory-generated actogram, these crab's locomotor activity can be seen to follow tidal rather than circadian rhythms.

The circles indicate high tide, which occurs every 12.4 hours, while the bar at the top of the page indicates sunrise and sunset.

Actogram

The actogram represents a rhythm of activity initially entrained to a LD 12:12 light cycle (12 hours light 12 hours dark).

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Upon transfer to constant DD - constant dark conditions (represented by the black bar in the center of the actogram), circadian rhythm resume with their endogenous period.

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What does this tell us about

• Whether the rhythm is endogenous/ exogenous(what would we see if it were the other?)
• The length of the free running period (how would we calc. it?)
• The duration of an active phase
• Does this show entrainment and if so what environmental cues act as the zeitgeber
• Is there a phase shift (movement of onset activity earlier/later each period)

1200hrs 1800hrs 2400hrs 600hrs 1200hrs

Note: You will need to be able to identify entrainment, free running period and phase shift in actograms

Now For Some Practice

Reading Actograms

From the previous slide:

1. What kind of biological rhythm does the cockroach show
2. Is the cockroach nocturnal, diurnal or crepuscular
3. Is the cockroaches zeitgeber
1. Onset of darkness (sunset/dusk)
2. Temperature
3. Onset of light (sunrise/dawn)
4. Calculate the active phase and period
5. Describe the phase shift
6. Calculate the free running period
7. Endogenous or exogenous?

1. What kind of biological rhythm does the cockroach show

Circadian (only 1 active period in 24h)

1. Is the cockroach nocturnal, diurnal or crepuscular

Nocturnal (active during dark)

1. Is the cockroaches zeitgeber
1. Onset of darkness (sunset/dusk)
2. Calculate the active phase and period

Active phase = 10 hours period = 24 hours

1. Describe the phase shift

Phase delay - activity starts later each cycle/period

1. Calculate the free running period

26-17 = 7 hours delay

Over 14 days

7/14 = 0.5 hours delay

Free running period

= 24h + 0.5h = 24.5 hours

1. Endogenous or exogenous?

Endogenous (still shows a rhythm)

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