Biology Reference
In-Depth Information
with malfunctioning circadian mechanisms, will be at a disadvantage.
What happens to the mouse that ventures out of his home in the
daytime? The hawk eats him for lunch!
In this chapter, we examine the mechanisms controlling the circadian
rhythms in living organisms and some of the mathematical tools used to
characterize and study rhythmic phenomena.
I. BIOLOGICAL CLOCKS
A. Introduction
Circadian rhythms were first described at the level of whole
organisms. In 1729, the French astronomer Jean Jacques d'Ortous de
Mairan (1678-1771) observed that the leaves of certain plants were
perpendicular to the stem during the day, but parallel to the stem at
night. This cycle continued even when the plant was placed in a dark
closet. The French natural scientist Henri-Louis Duhamel du Monceau
(1700-1782) was intrigued by de Mairan's observations and repeated the
experiment in 1758 in a wine cellar, to confirm that the movements
had not been caused by light leaking into de Mairan's closet. In 1832, the
Swiss botanist Augustin Pyramus de Candolle (1778-1841) reported that
the rhythms of the leaves of the sensitive plant, Mimosa pudica, persisted
even when the plants were subjected to continuous artificial light.
He also noted that, over time, the biological clocks of these plants ran
faster than normal, with a period of 22 to 22.5 hours. Further
experimentation allowed de Candolle to alter the clocks of the plants
by changing the cycle of lighting, demonstrating that the plants could
obtain cues from the exogenous light/dark cycle.
The existence of such circadian rhythms in animals has also been
well-documented. For example, in 1914, J. S. Szymanski reported that
goldfish swimming occurs with a daily rhythm. During the 1950s,
Janet Harker used cockroaches, with their precisely timed nocturnal
running activity, to physically locate the biological clock within these
organisms. In the late 1950s, Patricia DeCoursey observed that flying
squirrels' emergence from their dens exhibited closely controlled
timing.
In the 1960s, Jurgen Aschoff of the Max Planck Institute conducted his
so-called bunker studies, demonstrating the existence of human
circadian rhythms. Participants in his studies were isolated from all
external cues in an underground bunker, thus allowing their
endogenous rhythms to be studied in a controlled manner. The
volunteers remained in the underground apartment for one month,
during which time their temperature, urine excretion, activity patterns,
and performance on psychological tests were measured. The volunteers
demonstrated undeniable circadian rhythms, with an average period of
Search WWH ::




Custom Search