Lecture 18: Biological Clocks

Question 1

circadian clocks

Answer 1

also known as one’s internal alarm clock is a biochemical oscillator that cycles with a stable phase and is synchronized with solar time
- circa = about, dia = day

Question 2

circadian clocks principles

Answer 2

• endogenous/persistent
• entrainment
• temperature independence

Question 3

zeitgeber

Answer 3

a rhythmically occurring natural phenomenon which act as a cue in the regulation of the body’s circadian rhythm

Question 4

circadian time: body temperature

Answer 4

fluctuates with higher temperatures during the day and lower ones at night

Question 5

circadian rhythm: plasma growth hormone

Answer 5

spikes occur predominantly around midnight and early morning, aligning with the body’s natural growth

Question 6

circadian rhythm: plasma cortisol

Answer 6

levels are highest in the early morning (helping with wakefulness and energy) and gradually decline throughout the day

Question 7

circadian rhythm: urinary potassium

Answer 7

exhibit higher levels during the daytime and lower levels at night, possibly tied to activity patterns and metabolic processes

Question 8

endogenous rhythm

Answer 8

gene expression follows a 24 hour pattern even without external cues, such as light

Question 9

entrainable rhythm

Answer 9

gene expression adapts when exposed to regular light-dark cycles, illustrating how external factors synchronize internal rhythms

Question 10

temperature compensation

Answer 10

gene expression remains consistent despite temperature changes, showcasing the robustness of these biological mechanism

Question 11

entrainment example: effects of light-dark cycles on physiological rhythms in birds

Answer 11

• the bird is exposed to a natural cycle of light and darkness
• oxygen consumption: shows a rhythmic pattern with peaks during light periods and drop during dark periods
• activity: the birds movement follows a clear cycle, with increased activity during light periods and rest during rest during dark periods
• so the light-dark cycle entrains the physiological rhythms

Question 12

endogenous example: constant light conditions

Answer 12

• although the rhythms persist, they gradually drift out of alignment since the 24 hour external cycle is absent
• this drift underscores the influence of environmental cues (like light) on maintaining consistent biological rhythms

Question 13

how light cycles impact the circadian rhythm of squirrels: 12 hours of darkness per day and constant darkness

Answer 13

12 hours of darkness per day: the squirrel’s activity starts consistently at the onset of darkness, displaying a clear and regular pattern of behavior in sync with the environmental light-dark cycle
constant darkness: without light cues, the squirrel’s activity patterns gradually shift, beginning slightly later each day; this drift reveals that the squirrel’s internal clock operates independently but loses synchronization without external cues

Question 14

how light cycles impact the circadian rhythm of bats: light/dark and constant darkness

Answer 14

light/dark: demonstrates synchronized activity periods, indicating that bat’s behaviors are entrained to the external light-dark cycle
constant darkness: bat activity periods gradually shift later each day when there are no external light cues; illustrates drift

Question 15

drift

Answer 15

the internal circadian clock operates independently of the external synchronization

Question 16

mechanism of entrainment

Answer 16

• the human suprachiasmatic nucleus governs circadian rhythms by processing light signals
• the SCN, located in the hypothalamus, acts as the brain’s internal clock, regulating physiological and behavioral rhythms
• light exposure influences the SCN, which then coordinates biological rhythms
• light exposure influences the SCN, which then coordinated biological rhythms like sleep-wake cycles and hormone secretion

Question 17

impact of optic nerve sectioning (ONX) on temperature regulation in quails

Answer 17

in intact quails, temperature variations follow a consistent circadian rhythm aligned with light cues
- after ONX, the quails still exhibit temperature cycles but with altered patterns, suggesting the persistence of endogenous rhythms independent of external light signals; however, these rhythms may become desynchronized or less precise

in constant darkness, already experienced ONX
- despite the absence of light cues, the bird’s activity patterns persist, showcasing the resilience of the internal biological clock
- temperature fluctuations follow cyclic patterns, even in the absence of light cues, indicating that circadian temperature regulation is maintained but may not be perfectly aligned with a 24 hour cycle

Question 18

parietal eye

Answer 18

• photoreceptive organ associated with the pineal gland
• light sensitive organ found in some reptiles, amphibians, and fish

Question 19

pineal gland

Answer 19

receives information about the state of the light-dark cycle from the environment and conveys this information via the hormone melatonin

Question 20

how light and darkness signals impact melatonin production in the brain, connecting retina, SCN, paraventricular nucleus, pineal gland, and superior cervical ganglion

Answer 20

• pathway of light signals: light detected by the retina is transmitted to the SCN, which acts as the master clock controlling biological rhythms; signals are further relayed through the paraventricular nucleus and superior cervical ganglion before reaching the pineal gland; the pineal gland is responsible for producing melatonin, a hormone crucial for sleep regulation
• in continuous darkness, melatonin levels rise during the night and peak in the early hours, slowly declining as daytime approaches
• in the presence of light pulse, melatonin production is interrupted, leading to a sharp drop, which highlights the sensitive of melatonin levels to environmental light exposure

Question 21

experiment testing the role of the pineal gland and its connection to melatonin in regulating biological rhythms

Answer 21

• removing pineal gland from birds, which is essential for producing melatonin, regulating sleep-wake cycles, and other circadian rhythms
• melatonin injections were administered at specific times to mimic the gland’s role in rhythmic regulation
• mineral oil was used as a control and injected at similar times to observe its lack of effect
• melatonin is crucial in maintaining biological rhythms, even in the absence of the pineal gland
• mineral oil injections serve as a baseline, melatonin injections appear to restore rhythmic behaviors

Question 22

suprachiasmatic nucleus

Answer 22

• central pacemaker of the circadian timing system and regulates most circadian rhythms in the body
• considered the master clock

Question 23

effect of destroying the suprachiasmatic nucleus in circadian rhythms

Answer 23

before SCN destruction
- the graph displays consistent black activity bars, indicating well-organized, free-running rhythms; these patterns reflect a robust internal clock, even without environmental cues like light

after SCN destruction
- following SCN destruction, the activity patterns become irregular and fragmented; this disorganization highlights the SCN’s critical role in generating and maintaining circadian rhythms

Question 24

central and peripheral clocks

Answer 24

some organs can maintain their own internal clocks for a period of time

Question 25

peripheral clocks

Answer 25

- organs such as the thyroid gland, skeletal muscle myotubes, endocrine pancreas, and skin fibroblasts possess their own independent internal clocks - can function autonomously for extended periods but rely on synchronization from the SCN to align with environmental cycles

Question 26

oxygen consumption x ambient temperature: snail, frog, fish, snake

Answer 26

snails: display a nearly horizontal trend, indicating minimal metabolic change across temperatures; this suggests effective temperature compensation, likely due to physiological mechanisms that stabilize metabolic rate fish and frogs: show moderate increases in oxygen consumption as ambient temperature rises, reflecting typical ectothermic responses snakes: exhibit a steep increase in metabolic rate with temperature, demonstrating low temperature compensation and high sensitivity to environmental changes - temperature affects metabolic processes differently across species, emphasizing evolutionary adaptations in maintaining homeostasis

Question 27

temperature compensation

Answer 27

refers to the biological phenomenon where the period of an organism's circadian rhythm remains relatively stable across a range of temperatures

Question 28

temperature compensation and peripheral expression in flies

Answer 28

head (brain region): suprachiasmatic-like structures in Drosophila, which function as the master circadian clock controlling the overall rhythm thorax (middle body section): suggests circadian expression in flight muscles or metabolic tissue, possibly regulating energy use and activity patterns; temperature compensation abdomen (lower body): likely targeting the digestive or reproductive tissues, showing that circadian rhythm influence metabolism, digestion, or

Question 29

arctic ground squirrels

Answer 29

- train to intensity or spectral composition of light across the day - can acclimate to new light/dark cycles much faster than other rodents

Question 30

circatidal rhythms

Answer 30

biological synchronized with the tidal cycles of the ocean

Question 31

moon, stars, fish image

Answer 31

- the stars and moon emit light that penetrates the water's surface - a fish in the water is lit from above, creating a sharp shadow underneath it - the contrast between the fish and its shadow makes the fish's silhouette more visible to other organisms, possible affecting behaviors like hunting or avoiding predators

Question 32

circalunar rhythms

Answer 32

biological rhythms synchronized with lunar phases

Question 33

circalunar clock of worms graphs

Answer 33

- more present when dark because they aren't seen as well in the dark - less present when more light because their shadow makes them vulnerable - reproductive maturity in marine worms is precisely timed with specific lunar phases - synchronizing maturation with lunar phases likely aids in reproductive success, ensuring that both males and females are ready to reproduce simultaneously - even in free running periods without direct lunar cues, the graph suggests internal biological clock persists, maintaining rhythmic patterns

Question 34

circannual rhythms

Answer 34

biological processes align with the annual photoperiod cycle (hours of daylight)

Question 35

circannual rhythm: photoperiods and testicular width in African stonechats over time

Answer 35

- stonechats exhibit circannual rhythms, testicular growth aligns with the annual photoperiod cycle - ensures that reproduction occurs during optimal environmental conditions - testicular growth peaks align with longer daylight hours, which are likely associated with favorable conditions for mating and raising offspring 12-year graph: - annual changes in reproductive cycles and molting patterns - increases from January, peaking in June when environmental conditions are likely optimal for reproduction; after this peak, it decreases during the latter half of the year - flight feather molt is consistent in June and July, directly following the reproductive phase; timing ensures that the bird is ready for flight during migration or foraging - body feather molt happens occasionally in July, likely serving as a secondary maintenance phase - testicular growth follows endogenous circannual rhythm, meaning their reproductive cycle is driven by their internal biological clock; this rhythm persist even in environments without external changes in photoperiod, underscoring the bird's ability to self-regulate based on intrinsic genetic and physiological mechanisms - while they have this, it is entrained by changes in photoperiod; longer daylight hours act as a cue, signaling the birds to synchronize their internal clock with external seasonal variations, ensuring reproductive readiness when conditions are optimal - over 12 years, we see drift as testicular growth doesn't perfectly align with the calendar year

Question 36

climate change impacts the synchronization between birds nesting periods and food availability

Answer 36

match: the peaks of lines align perfectly so that the birds nest when food is most abundant meaning they can feed their young mismatch: noticeable gap and not lining up well; food availability and nesting gap, this misalignment can lead to food shortages for chicks, affecting their survival and growth

Question 37

comparative study of different bird species

Answer 37

- arrival and departure dates change over time - some birds seem to be moving away from timeline whereas others are not

Question 38

decreased fitness when birds do not match their resources

Answer 38

- climate change induced shifts in food availability - birds that cannot adapt their timing to these changes risk decreased fitness, potentially leading to population declines