EXAM 2--Biological rhythms

Biological rhythms

regular fluctuations in any living process

Ultradian rhythms

repeat more than once a day, such as bouts of activity, feeding, and hormone release

infradian rhythms

repeat less than once a day, such as reproductive cycles

circadian rhythms 

24 hour periods, generated by an endogenous (internal) clock 

light plays a large role in many of these rhythms

true

jean-jacques deMairan’s Experiment (1729)

opening and closing of a heliotrope plant in the absence of sunlight and found that the plant still raised leaves during the day and dropped its leaves during night

how do we study circadian rhythms?

wheel experiments with rodents 

evidence for the clock in these experiments incldues:

a free-running animal is maintaining its own cycle without external cues (cues like light) and the time between successive cycles may not exactly be 24 hours, but very close

phase shift

shift in activity in response to a synchronizing stimulus such as light

zeitgeber 

light, sound, temperature are all cues that an animal uses to synchronise with the environment its in 

Biological clock is located in the

suprachiasmatic nucleus (SCN) in the hypothalamus

damage to the SCN

eliminate circadian rhythms of drinking, locomotion, and hormone secretion

isolated (in vitro) SCN cells 

continue to show a circadian rhythm for days and weeks despite

hamsters with SCN lesions in dim light

showed no circadian rhythm

pineal gland in amphibians and birds

is sensitive to light and have photoreceptors on the pineal gland

the skull surface above the pineal gland is thinner in amphibians and birds to 

allow for better penetration of light 

melatonin is secreted from

the pineal gland to inform the brain about light

melatonin is released in

the absence of light

Retinohypothalamic pathway

contains retinal ganglion cells that project to the SCN

melanopsin

special light-sensitive photopigment

SCN cells in mammals make two proteins 

Clock and Cycle 

clock and cycle bind together to form a

dimer

clock/cycle dimer binds to

DNA and promotes transcription of two genes

clock/cycle binding to DNA produces 

period (per) and cryptochrome (cry) 

when per and cry bind together

they activate the neuron and produce downstream effects associated with wakefulness and alertness (melatonin is suppressed)

Per/Cry complex is important to

determine the time of day and inhibits the activity of the clock/cycle dimer, which slows the transcription of per and cry genes

Per and Cry eventually break down, releasing 

the Clock/Cycle from inhibition, allowing the cycle to start over again 

role of Glutamate in the Per and Cry system

axons of the retinohypothalamic tract release glutamate onto the neurons of the SCN, glutamate binding on the receptor leads to increased transcription of the per gene

peripheral circadian clock

can adopt to its own internal/external stimuli but is “conducted” by the SCN 

cardiovascular clock

blood pressure rhythm

kidney clock

controls filtration and excretion rates

loss of synchronization between the internal rhythms and external stimuli can 

induce diseases such as cardiovascular organ damage 

blue light before bedtime bad for sleep

light suppresses melatonin; light in the early evening causes a circadian delay; in the early morning causes a circadian advancement