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Results for “Biological Clocks”

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At least 235 records · Page 13Linked to original sources
PubMed1980

Biological clocks and puberty onset.

The ovulatory cycle in adult females appears to be a clock-driven process in which the regular gonadotrophin surges that induce ovulation are timed by the cycle of light and dark. In the absence of photoperiodic cues, the ovulatory surge appears to be driven by an endogenous rhythm, perhaps generated by the suprachiasmatic nuclei of the hypothalamus. There is now evidence that puberty onset is associated with the development of the capacity to reset biological rhythms in response to photoperiodic cues and to link neuroendocrine secretion to clock rhythms. This capacity emerges around 26-28 days of age in immature animals. A hypothesis is presented in which the initiation of responsiveness to photoperiodic signals is related to the onset of regular waves of follicular development in the ovaries that culminate in first ovulation 10 to 14 days later.

Animals↗
PubMed1982

Phase locking of biological clocks.

Radial isochron clocks (RICs) and their response to external signals and coupling with other RICs are studied. RICs are derived as phase approximations to self-sustained oscillators. Their response to single impulses (phase resetting) and to repetitive impulses is determined. This response may be harmonic or chaotic. Finally, the effect of coupling between clocks is studied. Simple coupling is shown to exhibit rhythm splitting like that observed in fish and small mammals. New phase locking results for general weakly coupled RIC systems are also derived.

Animals↗
PubMed1999

Circadian biology: clocks for the real world.

The circadian system of Neurospora crassa includes a molecular feedback loop that is entrainable by light. A recent study has shown that a second, elusive oscillator interacts with the feedback loop to drive output rhythms.

Animals↗
PubMed1977

[The biological clock: circadian organization of the cell (author's transl)].

Unicellular systems, and particularly individual cells, are advantageous in investigating the molecular mechanism of the circadian rhythm. The greeen alga Acetabularia has proven to be a uniquely suitable organism. Experiments are described which indicate that the circadian rhythm is due to a sequence of events which are casually interrelated. A model is presented which is based on a two-step mechanism, the first step of which is the synthesis of "essential polypeptides" on 80 S ribosomes. In a second step these polypeptides are consumed. The function of the "essential polypeptides" is to change a membrane component of the circadian clock. The membrane participates in a feed-back mechanism which produces a periodic alteration of the rate of synthesis of "essential polypeptides" and of photosynthesis. It is possible to draw parallels between this circadian rhythm model and other repetitive and non-repetitive types of temporal organization within the cell, e.g., the cell cycle; however, it must be stressed that in the case of the circadian rhythm, much stronger and more efficient mechanisms exist to keep the period constant. This is true particularly for temperature compensation. Such a conservatism is understandable if one recalls that during evolution, the circadian rhythm represents an adaptive advantage which permits the organism to adjust it activities in advance to periodic processes resulting from the earth's rotation.

Acetabularia↗
PubMed1995

Cellular analysis of a molluscan retinal biological clock.

The eye of the opisthobranch mollusc Bulla gouldiana expresses a circadian rhythm in optic nerve impulse frequency. The circadian rhythm is generated among approximately 100 neurons at the base of the retina referred to as basal retinal neurons. These cells are electrically coupled to one another and fire spontaneous action potentials in synchrony. Basal retinal neurons recorded intracellularly exhibit a circadian rhythm in membrane potential that appears to be driven by a circadian modulation of membrane conductance. Membrane conductance is relatively high during the subjective night and decreases after subjective dawn. Recent experiments in our laboratory indicate that individual basal retinal neurons in culture can express circadian rhythms in membrane conductance. When completely isolated, these cells continue to show circadian conductance changes. These studies provide the first direct demonstration that individual neurons can act as circadian pacemakers. Although the precise details of the mechanism generating the circadian periodicity remain obscure, our research indicates that several transmembrane ionic fluxes are not involved in rhythm generation, but that a transmembrane Ca2+ flux is critical for entrainment. Both transcription and translation appear to play critical roles in generating the circadian cycle.

Animals↗
PubMed1988

Light transducer for the biological clock: a function for rapid eye movements.

To test the hypothesis that the rapid eye movements (REMs) of paradoxical sleep (PS) transmit information about environmental light: dark cycles, spontaneous locomotor patterns were studied in rats following surgical disconnection of the extraocular muscles and thus deprived only of the eye movement component of REM sleep. Entrainment of the circadian spontaneous activity cycle to the light: dark cycle was abolished after sectioning the extra-ocular muscles of rats, but not in sham-operated controls. Endogenous (free running) circadian patterns of spontaneous activity were preserved in the operated animals. The results are consistent with our electrophysiologic studies showing that REMs induce light modulated electrical potentials in brain regions that influence circadian rhythms of activity.

Animals↗
PubMed2005

Circadian biology: clocks within clocks.

A small cluster of approximately 20,000 neurons in the ventral hypothalamus provide the body with key time-keeping signals and drive circadian rhythms. This circadian clock exhibits surprisingly complex substructures, with inputs from the retina, and outputs to other brain structures. Rather little is known of the neurotransmitters involved, or their regulation.

Animals↗
PubMed2005

Time microscopy for all kinds of data including circadian clock biology.

Data showing a rhythm to the naked eye prominently, barely or not at all were described as kinds A, B and C, respectively. Here, we document good agreement between estimates of maxima and minima with eyeballing and with the addition of point and interval estimates of parameters in kind A data. We also construct a chart that provides estimates of uncertainties that can be obtained objectively while they are more difficult to quantify subjectively; again there is agreement. Interval as point estimates of rhythm characteristics and parameter comparisons are useful in charting all kinds of data and become indispensable as we proceed from kind A to kind C data. Illustrations included herein from molecular biology apply equally to all aspects of transdisciplinary science.

ARNTL Transcription Factors↗