This is a summary of notes obtained in the lecture theatre and further online research. It includes explanation on how human biological clocks work and the mechanisms underlying physiological processes such as sleep and eating behaviours. It explains the general properties of circadian clocks of hu...
Biological rhythms which take place over 24 hours and are an adaptation of the 24 hours of our environment. The
rotation of the earth on its axis results in daily changes in light and temperature environmental conditions. Many
organisms have grown to anticipate these predictable changes by adjusting their behaviour, physiology and
metabolism ahead of time. So they have developed timing mechanisms which allow them to adapt in anticipation of
daily changes and environmental conditions.
SLEEP-WAKE CYCLE
The rhythm that we are more aware of is our own sleep-wake cycle. We tend to be more awake during the day and
more sleepy at night time. Some tend to wake up at the same time every day even at the absence of an alarm.The
time that you wake up is not the same for everyone. Some people tend to wake up early and some later, because
there is a lot of diversity in nature and daily rhythms of nature. This is part of Chron-types, a type of endogenous
clock which differs in every individual.
JET LAG
One of the manifestations of daily rhythms is jet lag. It happens when you travel quickly across many time zones.
Hat happens is that your own sleep wake cycle is out of synchrony with the local environment. As a result you feel
tired, dissorientated and unwell and this will persist for several days until your own rhythm synchronises with the
local environment.
RHYTHMS IN PLANTS
Daily rhythms in leaf position were first reported by Androsthenes around 400 BC. An example is shown in the plant
pictures. In panel A it is a picture at night and on panel B we can see the plant during the day when the leaves
are horizontal and maximises photosynthesis. This
graph shows us the quantification of the rhythm and
shows the changes of vertical position of the plant
over time.each bar corresponds to one day. The
lees are wide open in the morning but then they
closed during night time.
Many organisms exhibit daily rhythmic changes in their
behaviour, physiology and metabolism to match their
environment. Organisms like that are micro bacteria as
shown in the first picture, and animals such as hamsters,
flies, fish and even Blindomore rats.
,1729 - Jean Jacques d’Ortous de Martin
We are going to see that these rhythms have an endogenous
nature. This was first demonstrated in 1729. He found that
mimosa plants which has very clear ad visible rhythms continues
to exhibit leaf movement in constant darkness. That was the first
clue that rhythms are not driven by changes in night time
conditions but are controlled by an endogenous biological clock.
1950-1970 - Jurgen Ashoff
He used to place students in isolation in bunkers that were left from WW2. These were in a constant life
conditions, constant temperature ad light etc. So whoever was in there had no idea of the time of the day
and the outside environment. In the absence of any time blue from the environment, he showed that humans
demonstrated to assist daily rhythms in activity, body temperature and level of hormones in the blood.
2008 - Michel Siffre
He used to go down in caves for weeks and he basically showed that the activity records while he was in the
caves. We are looking at the graph, the activity records, were the dark bars are the time that he was asleep
and the white bars was when he was awake and active.
We can see that over time the onset of sleep drifted later and later. So the first observation is that there is a
rhythm and a pattern which persisted for the first few days. The second observation is that there was a daily
shift every day that pushed the onset of sleep a bit later every day. This match that his daily cycle was not 24
hours, but it was a bit longer. This was a very interesting observation because it showed that the daily rhythm
was not exactly 24 hours but it was 25.3 hours and there is no way that it was caused by the rotation of the
earth because this is 24 hours.
, - Plant and humans have daily rhythms that have an endogenous rhythm and that the duration of the cycle is
to exactly 24 hours and therefore it is not caused by the earth. These rhythms are called “circadian rhythms”.
Inspite of the fact that these rhythms persist at specific conditions and that it doesn’t last exactly 24 hours,
many people doubted that they are endogenous until the late 1980s. Some people suggested that the rotation
of the earth could be causing daily variation in the magnetic field that were sensed by organisms and changed
the rhythmicity. However, in the late1980s it was shown that they have an endogenous nature as these
rhythms have a genetic basis. And depending on the genetic nature of the organism the cycles were longer or
shorter than 24 hours.
KEY TERMINOLOGY
- Period: is the length of time that is required to complete a cycle
- Phase: describes a particular point of the cycle (such as peak phase)
- Amplitude: the difference between the peak and trough of a cycle
All circadian rhythms exhibit the same properties:
1. Entrainment: synchronisation of the circadian rhythm to a periodic signal from the environment.
Eg. Activity rhythms in animals. They have been widely studied using running weals because they can provide
Eg. Activity rhythms in animals. They have been
widely studied using running weals because they
can provide an easy way to monitor the cycle of an
animal. The picture on the right shows us the
patters of the activity pattern of a hamster on the
running wheel. The black bars indicate the interval
when the animal is active. On day 1 we can see
that it is inactive during the day and starts running
at 7 pm and runs until about 1-2 am in the
morning. The pattern repeats itself into
consecutive days. This is called an “actogram"
This part of the figure 7 days into the
demonstrates how the experiment, the light-
activity rhythms are dark cycle was shifted
synchronised in the 24 by 6 hours.We can see
hour light-dark cycle. that the animal had
We are looking at an actively adjusted its
actogramof an inactive pattern of activity and
rat. We can see that the onset of activity
during the first few days synchronised with
the animal is active lights on time.
during the day and the
onset of activity
corresponds to when
the lights are on.
Then the light-dark cycle was shifted back to its original time and once again the animal adjusted its cycle with
the light dark cycle. This shows tht th activity rhythm can become synchronised to the local light-dark cycle.
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