3. Biological Rhythms

Question 1

How does the short term stopwatch in the brain work?

Answer 1

1. 1000s of neurons converge on the striatal spiny neurons in the brain and fire in an uncoordinated manner (Wright, 2014).
2. When something catches the attention of the cortex, the converging neurons fire ATST and continue simultaneously (Wright, 2014).
3. When this ends, dopamine is released and if this happens repeatedly, a time stamp of neuronal activity is created for the event (Wright, 2014).
4. After learning, the trigger event leads to dopamine release from the substantia nigra, and spiny neurons track impulses (Wright, 2014).
5. The end of the time stamp is recognised, and an electrical signal is sent from teh striatum to the thalamus to the cortex, so we can measure time (Wright, 2014).
6. 5-60% accuracy, and is affected by drugs eg. cocaine, which make it seem like time is expanding (Wright, 2014).

Question 2

How do animals cope with cyclical environmental changes?

Answer 2

1. Homeostatic mechanisms respind to environment
2. These allow tweaking to local conditions (Alcock, 2009).
3. Biological timekeepers are predicitive of likely environmental change eg. metabolic increase before waking
4. These allow cycle to continue without frequent assessment of environment
5. Our varying behaviour and physiology across teh day and year is due to an active biological clock

Question 3

Give some examples of the process circadian rhythms control.

Answer 3

1. Core temperature
2. Plasma melatonin
3. Locomotor activity
4. Cognitive performance
5. Alterness
6. Sleep onset
7. Hormone release

Question 4

What have we learnt about circadian rhythms from studies that involve rodent wheel running?

Answer 4

1. When a nocturnal rodent is in 12hrs light, 12hrs dark, it synchronises to the pattern, showing entrainment
2. Onset of activity closely linked to lights
3. If the 12 hour darkness phase is moved to the middle of the period, rodents show phase-shift
4. When allowed a free run, tneir behaviour remains organised in approx. 12 hour sessions
5. This moves step-wise, as theyir cycle is slightly longer than 24 hours

Question 5

How ahve animal studies proven that some environmental cues can be anticipated by the animal?

Answer 5

1. Squirrel monkey core body temperature monitored over 2 24hour cycles with one light and one dark phase each.
2. Temp rise begins before lights come on and decrease before lights go off
3. Continues if placed in new environment with no variation

Question 6

What is the basic layout of the circadian system?

Answer 6

Question 7

Where is the pacemaker located and how do we know this?

Answer 7

1. The suprachaismatic nuclei (SCN)
2. This was discovered by lesioning different parts of animal brains and seeing what happened
3. Damage to SCN makes animals arythmic
4. Located in hypothalamus at bottom of brain
5. Below paraventricular nuclei (PVN) and close to optic chiasms (OX), where optic nerves cross
6. Below 3rd ventricle
7. Bilateral, each is 600-700microns tall and 500microns wide
8. Masterclock may be in optic lobes, as in crickets, cutting connection to one lobe leads to free run, and cutting both prevents cycle (Alcock, 2009)
9. The clock communicates neuronally with body and receieved amd integrates hormone signals (Alock, 2009)

Question 8

What evidence is there to suggest that rhythm control occurs in the SCN only?

Answer 8

1. Lesioned rodents continue to run, but no cycle in constant dark
2. Grafting in foetal SCN tissue resumes cycling
3. Transplantaiton from mutant foetus makes the rhythm too short (Alcock, 2009)
4. In humans, tumours of the pituitary gland can press against SCN and cause damage
5. These patients lack an activity schedule, so are given an artificial one with two rest periods

Question 9

How do we know the SCN is inhibitory?

Answer 9

1. Oxyglucose was injected into rat brains and 2D images taken
2. This allows us to see areas of metabolic activity
3. During the day, the SCN was very metabollically activre, but not at night
4. As rats are nocturnal, the SCN must be inhibitory

Question 10

How do the cells in the SCN function?

Answer 10

1. Each lobe contains 8000-10 000 neurons
2. Cultured SCN neurons oscillate in isolation on a daily rhythm
3. There is a molecular based clock in each of the neurons that is largle conserved across species

Question 11

Why is it thought that the molecular clock is likely to be genetically controlled?

Answer 11

1. Period genes/proteins are important and mutations change period of the clock
2. per protein caused production of tau enzyme (Alcock, 2009)
3. tau enzyme degrades PER protein (Alcock, 2009)
4. A lack of PER eg. in young bees causes arythmic behaviour (Aclock, 2009)
5. PER and TIM proteins also have a feedback loop (Young, 2000)
6. When protein concs increase and proteins start binding, complexes enter nucelus and shut off genes (Young, 2000)
7. After a few hours, enzymes degrade complexes and cycle restarts (Young, 2000).
8. GE mice (bioluminescence) can be programmed to glow when period 2 protein is produced
9. This allows visualisation of individual cells cycling in the dish

Question 12

How does light govern the activities of the SCN?

Answer 12

1. SCN receives light information via the retinohypothalamic tract (RHT)
2. Input from eye infroms SCN of light levels
3. The cells that do this are less sensitivr than rods and cones (Wright, 2014)
4. Not a concious experience - this also occurs in blind people
5. This regulates release of melatonin from pineal gland by allowing it to occur when it is dark - stops inhinition
6. Naken mole rats live underground and lack a circadian rhythm (Alcock, 2009)

Question 13

What NETs are involved in biological time-keeping?

Answer 13

1. The key NET is thought to be melatonin
2. PKZ may be NET that allows clock to communicate with the rest of the body (Alcock, 2009)
3. It is produced in a circadian pattern and there are receptors in the right brain locations (Alcock, 2009)

Question 14

How can a non-functional circadian clock affect chipmunks in the wild?

Answer 14

1. Captured and lesioned SCN from area alsmost at carrying capacity, therefore many predators (De Coursey et al., 2000)
2. Increased risk of predation immediately on release due to lack of den and high competition (De Coursey et al., 2000)
3. Chipmunks with a damaged SCN showed high night time restlesness without leaving the burrow, so restlessness may have attracted predatory weasels (De Coursey at al., 2000)
4. Control suffered at first also - disturbance of experiment may have attracted predators (De Coursey et al., 2000)

Question 15

How can we test circadian rhythms in flies and what have we found out by doing this?

Answer 15

1. When the fly is active, it casts a shadow when passing through the IR light, which is recorded by the photostransistor (Young, 2000)
2. Drosophila are most active in the day and have a morning and evening activity peak
3. These are anticipated - there is a gradual increase in activity
4. When there is a PER gene mutation, there is no anticipation, just a direct 2-peak response
5. In constant dark, the fly shows no rhythm at all

Question 16

What species do we know of that have more than one clock, and how are these set up?

Answer 16

1. Drosophila head has several clocks, including in the oral region, antennae and brain
2. They have no SCN, just a network of about 150 neurones per hemisphere
3. 3 groups of dorsal neurones and 4 groups of ventral neurons
4. Mutations can be made to swtich off particular neurons
5. Mice appear to have a clock in the olfactory bulb - they smell things better at night (Alcock, 2009)

Question 17

What insects beside Drosophila have known circadian rhythms and what is known about them?

Answer 17

1. In constant light conditions, male crickets’ mating calls run on approx. 26 hour cycles
2. In light/dark, they are entrained to chirp in the dark and show anticipation

Question 18

Give some examples that show the complex relationship between internal and external factors in biological time-keeping.

Answer 18

1. The banner-tailed kangaroo rat follows the lunar cycle, and if they are well-fed they only forage in the dark (Alcock, 2009)
2. White crowned sparrows have growing gonads in mating season (all from Alcock, 2009)
3. Photosensitivity resets in the morning
4. Becomes more sensitive after 12hours
5. Only sensitive when photopreiod is 12+hours
6. Gonads then grow in lab or nature
7. Stonechats have physiologial reproductive changes to coincode with food abundance in rainy season, even in controlled conditions (Alcock, 2009)
8. Golden-mantled ground squirrels hibernate in controlled conditions

Question 19

Give some exmaples of non-circadian rhythms.

Answer 19

1. Ultraradian rhythms eg. 90 min human sleep cycles or pulse of growth hormone every 3 hours
2. Inraradian rhythms eg. bird migration, human menstural cycle and hibernation
3. There are elements of clock control in all three types

Question 20

Describe what is known and what is unknown about marine use of circadian rhythms.

Answer 20

1. Sea lice respond to circadian (24h) and circatidal (c.12h) rhythms
2. The circadian and circartidal clocks don’t havw the same mechanism - can destroy circadian clock and circatidal clock remains functional
3. Crabs have free-running circatidal rhythms (Wilcockson + Zhang, 2008)
4. Circalunidian clock hypothesis: tere is a lunar day clock that has 2 clocks running antiphase to produce two intervals (Wilcockson + Zhang, 2008)
5. Dedicated circatidal clock with circadian oscillator for day/night may be an alternative (Wilcockson + Zhang, 2008)
6. If this is the case, circadian clocks may have evolved from tidal osciallators.