Infradian rhythms Flashcards
AO1: Infradian rhythms
Infradian rhythms are biological rhythms that last longer than 24 hours. One example is the female menstrual cycle, which operates on a 28-35 day cycle and the rhythm is regulated by glands and hormones.
It begins on the first day of a woman’s period, when the womb lining is shed, to the day before her next period.
In the brain the Pituitary gland releases FSH which activates the release of oestrogen from the ovaries and causes an egg to mature. At the middle of the cycle the increase of oestrogen increases the levels of LH (released by the pituitary gland) which promote the release of an egg. Oestrogen develops the lining of the womb and progesterone helps it grow thicker, readying the womb for pregnancy. At the end of the cycle if pregnancy does not occur, the egg is absorbed into the body, the womb lining comes away and leaves the body and the cycle begins again.
Although the menstrual cycle is mainly an endogenous system, which is regulated by the pituitary gland and the hormones of oestrogen and progesterone. It can be affected by exogenous factors such as light, stress and pheromones which can lead to an inconsistent infradian rhythm.
Evaluations (3)
1) RTC Russell - synchronised menstrual cycle
2) Discussion: Low control over extraneous variables
3) Evolutionary past
AO3: Synchronised
Research to contradict that infradian rhythms such as the menstrual cycle are mainly governed by an endogenous system was conducted by Russell. They asked a sample of women to wear cotton pads under their arm. The pads were then rubbed on the upper lip of 5 sexually inactive women. This process was repeated daily for five months. They found that 4 out of the 5 women developed menstrual cycles that synchronised to within one day of the donor cycle. This shows that the infradian rhythm of the menstrual cycle can be affected by exogenous factors as well as controlled by endogenous factors which may influence the length of the infradian rhythm. Therefore, reduces the validity of role of endogenous pacemakers in regulating the infradian rhythm and emphasises the importance of an interaction with exogenous zeitgebers.
AO3: Discussion - extraneous variables
However, Russell’s research into the effect of exogenous zeitgebers on the menstrual cycle was a field experiment and so has low control over extraneous variables. The research took part in the participant’s natural environment, where other exogenous factors, such as light, changes in diet and stress, could also have affected the infradian rhythm. Therefore, it is difficult to establish cause and effect between the influence of pheromones on maintaining an infradian rhythm. This weakens the extent to which Russell’s research can contradict the importance of endogenous pacemakers on maintaining infradian rhythms.
AO3: Evolution
The role of exogenous zeitgebers in infradian rhythms, such as pheromones can be explained by evolutionary psychologists. It would have been advantageous in our evolutionary past for a social group to synchronise pregnancies so that many women would be breast feeding at the same time so one mother could take over caring for an orphaned child. This demonstrates that exogenous zeitgebers play an important role in maintaining the infradian rhythm, to enhance survival chances, as evolution would predict. Therefore, showing the importance of the interaction of endogenous and exogenous factors in infradian rhythms.
