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Chapter 19: Circadian Rhythms (Sleep control (Circadian (Circadian…
Chapter 19: Circadian Rhythms
Sleep control
Homeostatic
sleepiness occurs after sleep debt
Allostatic
mechanisms to force wakefulness or sleep
a stressful situation that requires staying awake
during illness, immune signals promote daytime sleepiness
override normal homeostatic controls
Circadian
24-hour biological cycle
not noticeable because our environment also changes
travel across time zones: asynchrony
Circadian disorders
certain brain tumors: loss of separate sleep and awake phase
Elderly: advanced sleep phase syndrome (sleep earlier)
Adolescence: delayed sleep phase syndrome (sleep later)
Sometimes with blindness: free-running rhythm, 25 hours
Jet lag and shift work
sleep problems
gastrointensinal and metabolic problems
risk of diabetes and cardiovascular disease
Circadian basics
Biological rhythms
Lengths of biorhythms
Ultradian: less than a day
breathing, EEG waves
Infradian: many days
For humans, reproductive cycles
Circannual: once a year
For some animals, mating behavior
Circadian rhythm
24 hour period
self sustaining
entrained (cause synchrony with it) by Zeitgeber (environmental time cues) (usually light)
free running without zeitgebers
if time cues were removed, subject is free to set own schedule
sleep-wake cycles remained stable but lengthened to 25 hours
low point of body temperature shifts from end of sleep period to beginning
a true biological rhythm is self sustaining, endogenous (internal) to the organism
Circadian rhythms of physiological functions
growth hormone and cortisol levels are HIGHEST during sleep
cortisol is rising RIGHT BEFORE you wake up
growth hormone is highest at START of sleep
excretion of potassium by kidneys are highest during DAY
temperature DROPS during sleep
Early studies
melatonin is secreted in 24-hour pattern, with light as regulator
discovery of retinohypothalamic tract
target= suprachiasmatic nucleus (SCN)
Biological clock = SCN
termination of retinohypothalamic tract
lesions abolish rhythm
SCN lesions --> high-frequency rhythms of activity and temperature
Isolated SCN still has rhythm (even when input pathway is removed, clock continues its basic rhythm)
transplantation of SCN restores rhythm in an animal with an SCN lesion
light sensory
clock
output to brain (behavior)
special ganglion cells express melanopsin (rods and cones not needed)
axons to SCN
Peptides in SCN
VIP (vasointensinal peptide)
VP (vasopressin)
SCN neurons
neurons themselves are oscillators
rhythmic activity still occurs when SCN neurons are removed and each cell is grown in culture dish
In fetus, circadian pattern emerges BEFORE synapses
time keeping occurs with TTX (blocks action potentials of SCN)
SCN clock keeps running WITHOUT action potentials
Light is dominant stimulus
pacemaker (SCN) is slightly inaccurate
Luminosity: total amount of light (not color, shape, movement)
melanopsin releases the glutamate-neurotransmitter of retinal ganglion cell
SCN output (excites orexin neurons, inhibits VLPOA)
multi synaptic connections affect
motor performance
memory
hormone secretion
temperature regulation
reproduction
arousal/sleep
The Drosophilia clock
clock genes
Tim (timeless): a second gene that controls rhythmicity
PER and TIM dimerize
Per (period): the first clock gene to be cloned
activation of clock genes must be bound by protein transcription factors
CLK (clock)
CYC (cycle)
PER and TIM are clock genes
PER/TIM binds to CLK/CYK; shutting off transcription
negative feedback loop: products of PER and TIM inhibit their own synthesis
as level falls, inhibition is lifted
PER/TIM activity begins again
circadian oscillation of mRNA and protein levels
mutation can cause shorter OR longer rhythm
Biochemical basis of rhythm
PER/TIM is transcribed to produce mRNA, that's translated into proteins
After delay, new proteins send feedback, interact with transcription mechanism
cause decrease in gene expression
less protein is produced
gene expression increases
cycle starts again