Class 2-3: Phenomenology of Circadian Rhythms

Flashcards covering key terms and concepts for Class 2-3 on the phenomenology of circadian rhythms, including rhythm parameters, actograms, entrainment, masking, and experimental protocols.

What are the three fundamental parameters used to describe a circadian rhythm?

Period, amplitude, and phase.

Period

The duration of one complete cycle of a circadian rhythm, typically about 24 hours.

Amplitude

The maximum extent of a circadian rhythm's fluctuation, representing the difference between the peak and trough of the rhythm.

Phase

The specific point in time within the circadian cycle, often defined relative to external cues such as light and dark cycles.

What are the three core components of circadian systems?

Inputs, clocks, and outputs.

Inputs

The signals received by circadian systems that influence their timing and synchronization, such as light, temperature, and social interactions.

Clocks

The internal mechanisms that generate rhythmic patterns of biological activity, allowing organisms to anticipate and adapt to changes in their environment.

Outputs

The physiological and behavioral responses coordinated by circadian systems, such as sleep-wake cycles, hormone release, and feeding patterns.

What is an actogram and what is it used for in circadian studies?

A graphical display used to visualize circadian activity data over successive days, showing how activity is distributed and allowing the determination of period length.

What do the experimental lighting condition abbreviations LD, DD, and LL stand for?

LD represents light:dark cycles, DD represents constant darkness, and LL represents constant light.

What is a zeitgeber?

An external environmental time cue (such as a light:dark cycle) that entrains or synchronizes an organism's endogenous circadian clock.

How are endogenous (free-running) rhythms uncovered experimentally?

By removing environmental cues, typically by placing the organism in constant conditions such as constant darkness (DD) or constant light (LL).

How do free-running rhythms differ from entrained rhythms?

Free-running rhythms reflect the clock's innate period running in the absence of environmental cues, whereas entrained rhythms match the period and phase of an external zeitgeber (e.g., a 24-hour light:dark cycle).

How did differences in the free-running periods of various organisms prove that circadian rhythms are endogenously driven?

Organisms display unique free-running periods that deviate from exactly 24 hours under constant conditions, showing that rhythms do not depend on external 24-hour geophysical cues.

How can researchers determine if an organism is entraining to light:dark cues?

By observing whether the rhythm adopts a period identical to the zeitgeber cycle and maintains a stable phase relationship relative to the light:dark transitions.

What is masking in the study of circadian rhythms?

The direct modification of an overt physiological or behavioral rhythm by external or internal factors, independent of the circadian clock mechanism.

What are the four properties of circadian rhythms?

The four properties are: persistence, entrainability, temperature compensation, and phase shifting.

How do homeostatic control and circadian control interact in regulating parameters such as body temperature and feeding?

Homeostatic control regulates levels based on immediate internal deficits or demands, while circadian control imposes a predictable, self-sustained rhythm over a 24-hour cycle.

Masking

is the influence of external or internal stimuli on physiological or behavioral rhythms. It occurs when these stimuli alter the expression of rhythms without affecting the underlying circadian clock.

What are the two primary experimental protocols used in humans to separate circadian control from masking factors?

Constant routine protocols and forced desynchrony protocols.

Constant routine protocols

are experimental designs that maintain a constant sleep-wake cycle to minimize the effects of external cues, isolating circadian rhythms from other influencing factors.

Forced desynchrony protocols

involve manipulating the relationship between the circadian clock and the sleep-wake cycle, typically by scheduling sleep at irregular times across the circadian cycle to observe the intrinsic circadian responses.

What would a measure look like in these protocols (forced desynchrony and constant routine) if it was arrhythmic? What would it look like if it were controlled by the circadian system? What would it look like if it was entirely due to masking factors? What would it look like if it were under both circadian and behavioral/homeostatic control?

An arrhythmic measure would show no consistent pattern in sleep or biological markers, while a circadian-controlled measure would display a periodic rhythm in sync with the clock. Measures influenced only by masking would lack rhythmicity related to the circadian system, and those under both circadian and behavioral control would show altered rhythms responding to external cues while still being periodic.

How can circadian control of a physiological or behavioral parameter be confirmed in animal models?

By demonstrating that the rhythm continues to oscillate with a circadian period when all external time cues are removed (under constant conditions).

What are the two major functional categories of advantages conferred by circadian rhythms?

Extrinsic advantages (adapting to and anticipating predictable environmental changes) and intrinsic advantages (coordinating internal biological and physiological processes temporally).

What is evidence of the benefit of circadian rhythms?

Evidence includes improved health outcomes, enhanced cognitive performance, and increased efficiency in daily activities, all seen when circadian rhythms are synchronized with environmental cues.