Endogenous pacemakers: Flashcards
The suprachiasmatic nucleus:
In mammals, the main endogenous pacemaker is a tiny cluster of nerve cells called the suprachiasmatic nucleus (SCN), which lies in the hypothalamus.
The SCN plays an important role in generating the body’s circadian rhythm. It acts as the master clock, with links to other brain regions that control sleep and arousal, and has control over other biological clocks throughout the body.
Neurons within the SCN spontaneously synchronise with each other, so that their target neurons in sites elsewhere in the body receive correctly time-coordinated signals.
These peripheral clocks can maintain a circadian rhythm, but not for very long, which is why they must be controlled by the SCN.
This is possible because of the SCN’s built-in circadian rhythm, which only needs resetting when external light levels change.
The SCN receives information about light levels via the optic nerve,
This happens even when our eyes are shut, because light penetrates the eyelids.
If our biological clock is running slow (e.g. the sun rises earlier than on the previous day).
then morning light automatically adjusts the clock, putting its rhythm in step with the world outside.
The SCN also regulates the manufacture and secretion of melatonin in the pineal gland via an interconnecting neural pathway.
Endogenous pacemakers
Mechanisms within the body that govern the internal, biological bodily rhythms.
The term ‘endogenous’ refers to anything whose origins are within the organism.
These pacemakers are most probably the products of inherited genetic mechanisms and allow us to keep pace with changing cycles in the environment. The most important pacemaker in human beings is the suprachiasmatic nucleus.
The pineal gland:
The SCN sends signals to the pineal gland, directing it to increase production and secretion of the hormone melatonin at night and to decrease it as light levels increase in the morning.
Melatonin induces sleep by inhibiting the brain mechanisms that promote wakefulness.
The pineal and the SCN function jointly as endogenous pacemakers in the brain.
The sensitivity of the pineal gland and the SCN to light, and the role of melatonin in controlling sleep and activity, mean that, despite the endogenous nature of these clocks, their activity must be synchronised with the light-dark rhythm of the world outside.
EVALUATION OF ENDOGENOUS PACEMAKERS
The role of the SCN
Dangers of disrupted rhythms:
EVALUATION OF ENDOGENOUS PACEMAKERS:
The role of the SCN:
The importance of the SCN as an endogenous pacemaker was demonstrated in Morgan’s study of hamsters.
Morgan (1995) bred a strain of hamsters so that they had abnormal circadian rhythms of 20 hours rather than 24 hours.
SCN neurons from these abnormal hamsters were then transplanted into the brains of normal hamsters.
These normal hamsters then displayed the same abnormal circadian rhythm of 20 hours, showing that the transplanted SCN had imposed its pattern onto the recipients’ brains.
Morgan then transplanted SCN neurons from normal hamsters into the brains of the abnormal hamsters.
Rather than maintaining their abnormal circadian rhythm, the recipient hamsters then changed to a circadian pattern of 24 hours.
This confirms the importance of the SCN in setting circadian rhythms.
Dangers of disrupted rhythms:
Touitou et al. (2017) argue that exposure to artificial light at night results in a disruption of the circadian system, which has adverse effects on health.
For example, Touitou’s research has shown that teenagers spend increasing amounts of time on electronic media at night.
The LED bulbs of these devices are enriched with a blue light component very active on the circadian clock, which leads to suppression of melatonin secretion and circadian disruption
As a result, adolescent sleep becomes irregular, shortened and delayed.
In the long run, this combination of sleep deprivation and circadian disruption is detrimental to health, as shown by the many studies that have found increased rates of cardiovascular disorders and mood disorders such as depression.
