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Lecture 10

Lecture 10 - Circadian Rhythms

Human Biology
Course Code
John Yeomans

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HMB200H1S L10; Feb. 08, 2012
Circadian Rhythms
Biology (how animals reproduce & survive)
Avoid predators, reproduce, when to find food
Ecological niche
Use day & night, light & dark
Circadian organization
Bhvr organization (all animals)
How hypothal coordinates & regulates bhvr
Very small structure in brain yet large effect
Critical genes & their evolution
Small # of critical genes
Shared among mice, flies, humans
Expression in hypothal & body of dif animals
Correlations in plants, flies, humans – shows
very important since conserved
Evolution of retina & vision
Strongly controlled by direct projection from
Specialized prot (not rhodopsin)
Changed understanding of how retina uses
opsins, how opsins evolved over time, how
evolved into full visual system
Suprachiasmatic nucleus
Tiny, in hypothal, receive retinal input
Critical nucleus for all circadian effects in
mammals, in particular circadian activity
Not in birds, flies
Pineal gland (birds)
Back of thal (epithalamus)
Release of melatonin
Bhvr’al genetics, higher f’ns in evolution
Dif circadian biology via dif genes
Measuring Rhythms in Hamsters
Circadian rhythms = most precise in hamsters
Love to run – long distances to find food, mates
Wake up in 5-10min of same time every day even
when in constant light or dark
Begin day w very intense activity intense signal of
when circadian day begins
Usually attach running wheel to continuous activity
record (or computer)
On running wheel – bar sticking out, activating
switch; 1 count every time wheel turns 360degrees
Switch activated pen to make black line across
piece of paper
Continuously moving paper; certain segment
length for one day
Some variability (more than hamster), but mostly
consistent daily
Cut up papers – see what time of day most active
Circa = about
Circadian = about 24hrs
Endogenous rhythm when in light or dark but not
exactly 24hrs
Constant light waking up later every day
Endogenous rhythm changing, moving to right;
so about 24.5hrs every day
Implies internal clock is changing, free-running
Also true of flies, almost all animals
When constant conditions exist
Very stable in most animals
24s hrs when light to dark every 24hrs
Activity Rhythms
Endogenous clock: Measured in constant conditions,
still 23-25 hr. “free running”
Rhythm is lost when SCN lesioned in mammals, or
pineal gland in birds.
Rhythm is restored by transplanting new SCN.
Period of donor SCN.
Tau mutant hamster has 20 hr rhythm.
Therefore, SCN is endogenous clock for activity.
Free running 24.1hr – only moves slighty to right
over 10 days
Normal in intact animal
SCN removal – tiny lesion <1mm diameter kills
SCN on both sides completely loses rhythm
less total activity, no pattern
Not 24hr cycle at all
Proves SCN maintains the rhythm, not that it’s
the clock
Take new SCN from other animal – mutant w
endogenous 20hr rhythm
Tau (time) mutant
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