Circadian rhythms

Circadian rhythms

-approximately 24-hr cycles of behavior and physiology that are demonstrated by virtually every animal

•The earth spins on its axis approximately once every 24hr, submitting plants and animals to highly predictable daily rhythms of sunlight and darkness

-Behavior and metabolism of most organisms follow a predictable schedule that is organized around a 24-hour day

IMPORTANTLY

these biological schedules are ADAPTABLE and able to gradually shift with the seasons as the days get longer/shorter

BEHAVIOR with a circadian rhythm

•going to sleep

-We tend to do it every 24hr at about the same time

-We also tend to wake up about every 24hr at about the same time

-> We continue to do this even if we are kept in total light or total darkness, with no outside cues about time

body functions are under circadian control

-Body temperature rises at the same time in early morning while we are still asleep (or same time if we're up), falls in the evening at the same time while we are still awake (or same time if we're sleeping)

-Levels of different hormones rise and fall, intestinal motility patterns increase and decrease, many other physiological variables follow circadian rhythms

"Free-running" rhythms

•predictable behavioral rhythms even when all external cues about time are removed

-Tend to drift a bit, but still have highly predictable circadian rhythms that occur even in the absence of zeitgebers

-Results are the same in all animals studied, including humans

- Primarily an experimental phenomenon

Entrained rhythms

circadian rhythms that are "set" by zeitgebers

Zeitgebers

Environmental cues, such as the light-dark cycle, that entrain circadian rhythms

- "time givers;" entrain the rhythms of plants and animals ("set" the biological clock)

- Usual situation; following scheduled time-keeper cues

jet lag

•: symptoms can include disruption of sleep, GI disturbances, decreased vigilance and attention span, general malaise

-Extent of symptoms depends on how many time zones crossed; individuals vary

Where is the biological clock in the brain?

•very specific region of the hypothalamus: Suprachiasmatic Nucleus (SCN)

Suprachiasmatic Nucleus (SCN)

-responsible for generating circadian 24-hr rhythms even without any sensory feedback about the environment, activity, etc.

•Our SCN is very accurate on its own

The SCN receives sensory information related to zeitgeber cues:

-Direct neural input from retinal neurons

-Direct neural input from thalamus

-Receives endocrine/hormonal input

Direct neural input from retinal neurons that respond to light

•for specific and very important light entrainment zeitgeber

Retinohypothalamic pathway: direct projection from retina to SCN

Evidence for SCN

•Normal free-running rhythms in hamsters can be short (23hr) or long (25hr)

•Hamsters with a damaged SCN will lose circadian rhythms

•Receive a transplant: will adopt a free-running rhythm the same as its host

What happens when our "biological clock" is damaged?

•No clock, no rhythm

-When the SCN is damaged, the zeitgebers have nothing to act on

•Plants and animals fail to function properly when deprived of zeitgebers

- •Major health consequences of shiftwork are disorders of digestion and of sleep

-Shift workers have 8x greater incidences of stomach ulcers

•Disruptions of mealtimes and rest periods are the most obvious causes of GI and sleep disorders

What is melatonin used for?

•can be used to provide an artificial zeitgeber to the SCN, to help entrain it to a new cycle

-Should take a pill at the time you would be going to sleep for a few days before making the shift

If Andrew has a damaged suprachiasmatic nucleus (SCN), he will still have normal sleep/wake patterns because he can see when it is light or dark outside

False

Electromyogram

brain wave activity

electrooculogram

eye movement

electroencephalogram

muscle activity

The EEG patterns taken from skull recordings of REM sleep are characterized by:

high-frequency waves, as if the person were awake

Narcolepsy

Slow-wave sleep disorder in which a person uncontrollably falls asleep at inappropriate times.

Insomnia

Slow-wave sleep disorder resulting in prolonged inability to sleep

Sleep apnea

Inability to breathe during sleep; must wake up to breathe