Circadian rhythms and sleep; chapter
28
Circadian rhythms; many functions and processes in living systems display a 24-hour rhythm. This circadian
rhythm is obviously not the only biological rhythm, as man other exists (tidal, ultradian, infradian, lunar
annual etc.)
In many cases the source of the rhythm appears to be obvious: the movement of the sun, the rotation of the
earth and the movement of the moon around the earth cause, seasonal, day/night and tidal rhythm.
Experiment e.g. voluntary activity in a running wheel; active during night phase. Rodents and rats are
nocturnal animals. It is obvious that this rat is active during the dark phase (it is nocturnal). This can change
due to environmental pressure (e.g. haunting). The rat does not sleep the whole day. There are brief periods
of activity during the light period.
How is it possible to determine what the source of the rhythm is? Take the lights off to see if this is
still the case.
Isolation experiment; one of the most spectacular observations during time-isolation experiments in caves,
laboratories and other settings is the way that subjects sleep-wake cycles shift relative to actual alternation
of day and nigh in the outside world.
Presence of external clues, your sleep rhythm is synchronized with external and internal cues.
Absence of any external circadian cues, the endogenous rhythm is longer driven exclusively by
internal cues.
With cues again, the prolonged endogenous rhythm returns to the normal 24-hour rhythm again as
soon as an external rhythmic circadian cue is provided.
Conclusions we have an internal clock. External sources such as light help to synchronize this internal clock
with the real world.
Circadian rhythm development -> human babies are not born with 24-h clock/entrainment mechanism that
is fully developed. No fully developed internal system that can synchronize with external cues. In the
beginning it is hunger that drives activity in the babies. The brain is more like a sponge when we are born, a
lot of neurons mature after being born. So internal stimulus sleep-wake cycle in babies before it
synchronizes with the external world is hunger.
The internal clock is found in many animals. Under normal day/night conditions, the internal clock is
entrained by an external stimulus that serves as a zeitgeber (time giver). Other zeitgeber can be sound.
Light is the zeitgeber, so idea is optical system helps sensing it. But the normal optical system is not
necessary. The optic tract is not important for the entrainment of the internal clock to the environment.
Where in mammals is the information from the retina about ambient light level processed? Suprachiasmatic
nucleus. Circadian rhythms were normal and synchronized to the light-dark period before an SCN lesion was
made, after lesions, the normal rhythm was disrupted. The SCN = the clock.
, Retinal ganglion cells (intrinsically photosensitive retinal
ganglion cells) primary role is to signal light for unconscious
visual reflexes, such as pupillary constriction. These special
cells also regulate a number of daily behaviour and
psychological rhythms.
Vision is separate function of the detection for circadian synchronization.
Neurons from the SCN contact the pineal gland in the brain. The pineal gland produces the hormone
melatonin during darkness. Melatonin promotes the calm down before the sleep.
The neurons of the SCN itself contain melatonin receptors which inhibit SCN activity as negative feedback
signal during the darkness.
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