Lecture 18: Biological Clocks Flashcards
circadian clocks
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
circadian clocks principles
- endogenous/persistent
- entrainment
- temperature independence
zeitgeber
a rhythmically occurring natural phenomenon which act as a cue in the regulation of the body’s circadian rhythm
circadian time: body temperature
fluctuates with higher temperatures during the day and lower ones at night
circadian rhythm: plasma growth hormone
spikes occur predominantly around midnight and early morning, aligning with the body’s natural growth
circadian rhythm: plasma cortisol
levels are highest in the early morning (helping with wakefulness and energy) and gradually decline throughout the day
circadian rhythm: urinary potassium
exhibit higher levels during the daytime and lower levels at night, possibly tied to activity patterns and metabolic processes
endogenous rhythm
gene expression follows a 24 hour pattern even without external cues, such as light
entrainable rhythm
gene expression adapts when exposed to regular light-dark cycles, illustrating how external factors synchronize internal rhythms
temperature compensation
gene expression remains consistent despite temperature changes, showcasing the robustness of these biological mechanism
entrainment example: effects of light-dark cycles on physiological rhythms in birds
- 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
endogenous example: constant light conditions
- 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
how light cycles impact the circadian rhythm of squirrels: 12 hours of darkness per day and constant darkness
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
how light cycles impact the circadian rhythm of bats: light/dark and constant darkness
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
drift
the internal circadian clock operates independently of the external synchronization
mechanism of entrainment
- 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
impact of optic nerve sectioning (ONX) on temperature regulation in quails
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
parietal eye
- photoreceptive organ associated with the pineal gland
- light sensitive organ found in some reptiles, amphibians, and fish
pineal gland
receives information about the state of the light-dark cycle from the environment and conveys this information via the hormone melatonin
how light and darkness signals impact melatonin production in the brain, connecting retina, SCN, paraventricular nucleus, pineal gland, and superior cervical ganglion
- 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
experiment testing the role of the pineal gland and its connection to melatonin in regulating biological rhythms
- 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
suprachiasmatic nucleus
- central pacemaker of the circadian timing system and regulates most circadian rhythms in the body
- considered the master clock
effect of destroying the suprachiasmatic nucleus in circadian rhythms
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
central and peripheral clocks
some organs can maintain their own internal clocks for a period of time
peripheral clocks
- 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
oxygen consumption x ambient temperature: snail, frog, fish, snake
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
temperature compensation
refers to the biological phenomenon where the period of an organism’s circadian rhythm remains relatively stable across a range of temperatures
temperature compensation and peripheral expression in flies
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
arctic ground squirrels
- train to intensity or spectral composition of light across the day
- can acclimate to new light/dark cycles much faster than other rodents
circatidal rhythms
biological synchronized with the tidal cycles of the ocean
moon, stars, fish image
- 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
circalunar rhythms
biological rhythms synchronized with lunar phases
circalunar clock of worms graphs
- 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
circannual rhythms
biological processes align with the annual photoperiod cycle (hours of daylight)
circannual rhythm: photoperiods and testicular width in African stonechats over time
- 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
climate change impacts the synchronization between birds nesting periods and food availability
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
comparative study of different bird species
- arrival and departure dates change over time
- some birds seem to be moving away from timeline whereas others are not
decreased fitness when birds do not match their resources
- climate change induced shifts in food availability
- birds that cannot adapt their timing to these changes risk decreased fitness, potentially leading to population declines
