13916184 aqa alevel psychology pya4 endogenous pacemakers and exogenous zeitgebers

8/6/2019 13916184 AQA ALevel Psychology PYA4 Endogenous Pacemakers and Exogenous Zeitgebers

1/16

The Role ofEndogenous

Pacemakers and Exogenous

Zeitgebers

http://xamiya.blogspot.com/


2/16

What are they?

Most organisms have an internal biological clockwhich controls the circadian rhythm (24hr cycle)

You biological clock is endogenous (part of theinternal organism)

Biological clock helps us to realise an event (i.e. thecoming of night)

Because your clock effectively instructs neuralstructures (paces them) over a 24hr cycle this isknown as an Endogenous Pacemaker

Seasons = clock to be re-set. To achieve this yourclock is re-set each day using cues in theenvironment (cues are outside the body referred toas exogenous.)

These exogenous cues (light, etc.) are known aszeitgebers.



3/16

Endogenous Pacemakers

Biological Clock

Ticking of clock created by reactions between

proteins. Darlington et al. (1998) describes

1. Proteins CLOCK & CYCLE bind together in the

morning

2. This causes the increase in the production of

proteins PER & TIM, during the day

3. As these increase, production of CLOCK-CYCLE

decrease

4. As they decrease production of PER-TIM fall and

CLOCK-CYCLE increase ready to begin cycle

again (new day)



4/16


Suprachiasmatic Nucleus (SCN)

Main endogenous pacemaker found in thehypothalamus. SCN is a pair of tiny clusters ofnerve cells. One half in left hemisphere of the brainand one on the right. Just above where the opticnerve from each eye cross over (optic chiasm).

SCN obtains info. on light from the optic nerve evenwhen shut. Special photoreceptors in the eyes pickup light signals and carry them to SCN.

If ourendogenous clock is running slow (i.e. sun risesearlier than day before), the morning lightautomatically shifts the clock ahead. So rhythm isin step with the world outside



5/16


Pineal Gland & Melatonin

SCN not only endogenous pacemaker. Pineal gland

contain light-sensitive cells.

Light = production of melatonin in the gland is

stopped. So, when levels of light fall, melatonin isproduced = inducing sleep

Therefore Light, the pineal gland and melatonin

regulate the sleep-wake cycle.

Pineal gland is really important as a pacemaker inbirds and reptiles.



6/16

AO2

SCN

IfSCN is damaged/removed from a organism, sleep-wake patterns are affected (i.e. DeCoursey et al.2000) suggesting the importance of the SCN incontrolling biological rhythms.

Morgan, 1995- if mutant hamsters are bred so theyhave a circadian rhythm of 20 instead of 24hrs andtransplanted into normal hamsters, the normal oneswill display the 20hr rhythm. Showing the role ofSCN as a pacemaker.

SCN seems to be the main clock, but body temp.rhythm continue to exist when the SCNs areremoved, suggesting another clock.

Folkard, 1996- Kate Aldcroft spent weeks in cave,after 25 days, temp. adopted a 24hr rhythm but hersleep rhythm adopted a 30hr cycle.



7/16

AO2

Pineal Gland & Melatonin

Moyer et al. 1997 - light causes pineal gland toproduce melatonin in lizards. If the gland isremoved, it continues to produce melatoninrhythmically in response to light for up to 10 days.

Potocki et al. 2000- levels of melatonin are invertedfor those who suffer from Smith-Magenis syndrome(difficult falling asleep). Taking supplements ofmelatonin at night can help with symptoms =

supporting role of melatonin in sleep-wake cycle.

Sackett & Korner, 1993 - melatonin circulates in theblood, so would expect Siamese twins to sharecircadian rhythms, but they do not; questioning therole of melatonin.



8/16

Question Time

1. What is an endogenous pacemaker?

2. How does Darlington et al. (1998) describe thereaction between proteins? (process)

3. What is the SCN? Where is it? What does it do?

4. Other than the SCN, what else is an endogenouspacemaker?

5. What did Morgan, 1995 find? (hamsters)

6. Who looked into Siamese twins?

7. Potocki et al. 2000 found?http://xamiya.blogspot.com/


9/16

Exogenous Zeitgebers

Social Cues

Biologists thought that social cues were the mainzeitgebers (external cues) for human circadianrhythm.

We eat meals at socially-determined meal times(12.00 - Lunch), we go to bed and wake up at timesdesignated appropriate for our age, etc..

Our daily rhythms appeared to be reset (entrained) by

social convention, not internal biology (SCN, pinealgland/melatonin)

However its been recognised that

Process of

resetting the

clock withexogenous

zeitgebers is

known as

entrainment



10/16


Light

Light is the dominant zeitgeber in humans. Afterdiscovery that exposure to bright light suppressesmelatonin production (Wever et al. 1983)

Hall, 2000 - certain proteins (cryptochromes) in thebody detect light explaining that shining a light

on the back of ppt. knees changed their circadianrhythms (Campbell & Murphy, 1998)

Importance of light as a time-giver can be difficult inblind people. Lack of info. on light messes up their

sleep patterns.One man (blind) had a circadian rhythm of 24.9hrs.

Exposed to various exogenous zeitgebers (socialcues, etc.) yet found great difficulty reducing hisinternal pace, showing light is the most importantzeitgeber. (Miles et al. 1977)



11/16


Temperature

Biological rhythms can be also entrained by

temperature. E.g. leaves on a deciduous trees

change colour and fall because of changes in temp.

Temperature is also a factor of hibernation.

However

No evidence to show that temperature affects human

biological rhythms



12/16

AO2

Social Cues

Social cues provide a means of controlling socialbehaviours (example in book, pg. 65)

LightLight cues sometimes are insufficient to override the

free-running clock (free of exogenous cues).

Kelly et al. 1999 US nuclear submariners live on an18hr day. All cues available (light, social, etc.)didnt shift the rhythm of melatonin production onto

their 18hr day, the rhythm remained tat 24hrs.On a biochemical level, its been found that light

destroys the TIM portion of the PER-TIM protein,which re-sets the biological clock



13/16

AO2

Why is Light important?

Animals want to stay active when its light, safe frompredators (vice versa). Animals have an internalclock that tells them when to wake up but theircircadian rhythm doesnt match day lengthbecause daylight changes over the course of theyear.

What about blind people?

Czeisler et al. 1995 found some blind people do

respond to bright light exposure with reducedmelatonin production.

Suggesting there are 2 pathways to the brain fromthe eyes.

1. For conscious vision

2. For light travelling to the SCNhttp://xamiya.blogspot.com/


14/16

Question Time

1. What is meant by exogenous zeitgebers?

2. What 3 zeitgebers help reset your clock?

3. Other than the SCN, what in the body alsodetects changes in light? (Hall, 2000)

4. What did Miles et al. 1977 find?

5. Who live on an 18hr day and what did Kelly et al.1999 find?

6. Who discovered that some blind people respondto bright light exposure?



15/16

AO2

Advantages of endogenous pacemakers &exogenous zeitgebers

We need pacemakers, otherwise there would beproblems. If an animals rhythm relied onenvironmental cues: we might sleep most of theday in winter.

DeCoursey et al. 2000- destroyed SCN in chipmunksand found they became more active ay night thannormal chipmunks and more likely to be eaten by

predators

Exogenous zeitgebers allow animals to respond tochanges in the environment, without them ananimal might be awake at unsuitable times.



16/16

AO2, finally

Disadvantage of Biological rhythms

Biological rhythms will not change when you want

them to, for example when travelling across time

zones.

It is possible that your mind can have some

influence

Born et al. 1999 found people who were told to wake

up earlier than usual had higher levels of the stress

hormone adrenocortotrophic (contributes to thewaking-up process) than normal at the designated

time and woke up earlier


13916184 aqa alevel psychology pya4 endogenous pacemakers and exogenous zeitgebers

Documents