Lecture 3 Flashcards
What are the functional characteristics of all biological clocks (ultradian, circadian, circannual clocks, etc.)?
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How have past investigators experimentally confirmed that the daily clock is indeed an
endogenous timer?
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What is the experimental evidence that the SCN is an important biological clock?
What is the evidence that other extra-SCN clocks might also exist in mammals?
–in other vertebrates?
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Actiwatch data in humans
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Actiwatch data in humans Adult Locomoter Activity
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Undergraduate Student
Locomotor Activity Data
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Effect of SCN Lesions
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Circadian Variations in Body Functions
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Exogenous versus Endogenous
Control of Biological Clocks
Some behavioral rhythms have been recognized since ancient times, but they have generally been attributed to exogenous (outside the organism) factors.
Recent evidence indicates that endogenous (inside the organism) timing mechanisms mediate many of the observed rhythms in physiology and behavior.
Time Sense
- Endogenous and exogenous clocks in Bees
- Endogenous clock constant
• Exogenous clock depends on celestial cues
Renner (1960)
• Long Island, NY hive.
Foraged 12:54 - 2:24 EST
• Fly hive to Davis, CA 3hr and 15min time difference Foraged 9:45 - 12:15 Jet lagged bees Exogenous clock gradually adjusts
Zeitgedachtnis:
August Forel
In 1910, August Forel reached the conclusion that bees were able to remember things after he had a bothersome encounter with them.
He liked having breakfast outside on a patio with his family each morning, but he finally had to retreat into the house because bees kept trying to eat his food.
From inside the house, he noted that the bees continued to visit the patio each morning at the same time even though there was no longer any food for them.
How could bees remember the exact time to come each day looking for sweet jam?
Zeitgedachtnis
von Frisch and Beling
In 1929 Karl von Frisch and I. Beling developed Forel’s observations into laboratory experiments.
They fed bees sugar water at precisely the same time each day.
As Forel had observed, when they removed the food, the bees still came to feed at exactly the same time each day.
The only variable in this experiment was the presence or absence of food; the bees’
behavior remained the same.
von Frisch and Beling then repeated the experiment in a salt mine where there was no
change in light or temperature.
The variable was still food, and the environmental conditions were held constant.
The bees continued to feed at the same time each day.
What conclusions could they make if the bees continued to feed at the same time in unchanging environmental conditions?
How did the bees know what time it was?
Sun Compass
In other early experiments, Gustave Kramer trained
starlings to fly in a particular direction for its source
of food using the sun as a compass.
When placed in a laboratory where an electric light
replaced the real sun as a direction giver, hour after hour the bird added 15° of arc counter clockwise to the angle it made relative to the artificial sun.
He concluded that the bird knew that the angular velocity of the sun is on average 15° per hour and that it had access to some reliable clock to compensate for its constantly shifting ‘compass’.
Sun Arc Hypothesis
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Temperature compensation of
biological clocks:
Intro
More often than not, chemical reaction rates depend on temperature, with a doubling of rate with every 10°C rise in temperature.
In places like Ohio, where temperature changes are quite common and sudden, a temperature dependent clock would be very unreliable.
Scientists have now suggested that the biological clock is temperature compensated, and so varies very little at different constant temperatures.
Temperature compensation of
biological clocks:
Bünning
In 1931, Bünning created a hypothesis of temperature-dependence of biological clocks.
He conducted experiments on bean plants and found them to show a change in period length of leaf movement with changes in temperature.
Bünning found that for a 10° rise in temperature, there was a 20% change in the period length of leaf movement.
Temperature compensation of
biological clocks
Brown & Webb
In contrast to this temperature-dependent hypothesis, Brown & Webb (1948) forwarded the temperature-independent hypothesis which stated that the function of the biological clock does not vary with temperature.
Their experimental subject was Fiddler crabs.
Fiddler crabs, during the daytime are dark in color, but
turn pale at night exhibiting a daily light-dark cycle.
Brown & Webb found that in these crabs, the period of the daily cycles of change in color was completely independent of temperature over a range of 20° C.
