Complete and elaborate summary of the fifth task of the elective course PSY3349 - Sleep and sleep disorders. Summary contains all resources on the reference list, including figures.
Humans are diurnal – active during the day. Most rodents, including hamsters, are nocturnal – active
during dark periods. In either case, almost all physiological measures (hormone levels, body
temperature, drug sensitivity) change in a regular repeating fashion over the course of the day.
A hamster placed in a dimly lit room continues to show a daily rhythm in wheel running despite the
absence of day versus night, suggesting that the animal has an internal clock. But even if the light is
always dim, the animal may detect other external cues (e.g. outside noises, temperature, barometric
pressure) that signal the time of day. Arguing for an internal clock, however, is the fact that in constant
light or dark, the circadian rhythm is not exactly 24 hours; activity starts a few minutes later each day.
The animal is said to be free running in the absence of external cues is not exactly 24 hours long.
The free-running period is the animal’s natural rhythm. A period is the time between two similar
points of successive cycles (sunset to sunset). Because the free-running period does not quite match
the period of earth’s rotation, the free-
running period cannot simply be reflecting
an external cue; the animal has some sort
of endogenous clock.
Normally, the internal clock is set by light.
The shift of activity produced by a
synchronizing stimulus is referred to as
phase shift. Any cue that an animal uses
to synchronize its activity with the
environment is called a zeitgeber; light
acts as a powerful zeitgeber.
The hypothalamus houses a circadian
clock
The suprachiasmatic nucleus (SCN)
serves as the biological clock. Lesions
confined to the SCN portion of the
hypothalamus interfere with circadian
rhythms of drinking, locomotor behavior
and hormone secretion (figure).
Certain retinal ganglion cells send their axons along the retinohypothalamic pathway. This tiny
pathway carries information about light to the hypothalamus to entrain behavior. The retinal ganglion
cells themselves contain a special photopigment called melanopsin, which makes them sensitive to
light.
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, PSY3349 Sleep and Sleep Disorders
These ganglion cells send their axons to the SCN,
but other melanopsin-containing retinal ganglion
cells project to the brainstem. Melanopsin is most
sensitive to light frequencies in the blue range,
which explains why blue light has the largest effect
on human circadian systems.
Several genes provide a molecular clock, regulating each other’s expression such that their protein
products wax and wane in a cycle that takes about 24 hours. Cells in the SCN make the two proteins
Clock and Bmal1 (also called Cycle). These two proteins bind together to form a dimer (pair of
molecules joined together). This dimer binds to the cell’s DNA to promote the transcription of two
other genes (per and cryptochrome; cry). The resulting Per and Cry proteins then dimerize, and now
they inhibit expression of the Clock/Cycle genes that began the whole process (figure below).
Because the Per/Cry
proteins either degrade or
become chemically
modified with time,
eventually the inhibition I
lifted, starting the whole
cycle over again. The
entire cycle takes about
24 hours to complete, and
it is this cycle that drives
the 24-hour activity of
the SCN cells. Each SCN
neuron uses this
mechanism to keep time
approximately, and then
all the neurons
communicate with each
other through electrical
synapses.
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