Physiological clock

Bunning, E. 1973. The Physiological Clock. Circadian Rhythms and Biological Chronometry. Third edn. Springer, Berlin. 

Bunning,E. The physiological clock. The English Univ. Press London. Heidelberg, New... 

Behind the visible phenomenon of petal movement there must be a chronometric mechanism, which goes through similar oscillations, i.e. an oscillator. Once this physiological clock is set, it continues in this fixed rhythm. In the example the timing was determined by the light-dark alternation first presented to the plant. The timing factors are called the pace setters. 

Physiological clocks like this are widely distributed in living creatures of all kinds. They are coupled to the most varied rhythms. As far as 24 hour rhythms are concerned, one used to speak of endogenous diurnal rhythms. It was soon noticed, however, that these rhythms usually show periods of 18-21 hours, not exactly 24, under constant external conditions. For this reason the designation circadian rhythms (circa = about, dies = day) is preferred nowadays. 

Let us return to our special problem, photoperiodism in flower formation. How is time measured in this instance, by an hour-glass or by an oscillating clock, the physiological clock in its narrower sense ... 

We do not know the nature of physiological clocks. However, results in recent years do, at least, allow hypotheses to be put forward for animals as well as plants. One of these hypotheses, which is based on experiments... 

This hypothesis certainly rests on a weak foundation. But let us not lose sight of the crucially important point the fact that one can even attempt an explanation at the molecular biological level of so complex a phenomenon as the physiological clock is a better indication than many others of the revolution which has occurred in modern botany. 

BUnning, E. The Physiological Clock. Springer, New York 1967, 2. ed. Busch, H. Histones and other Nuclear Proteins. Academic Press, New York and London 1965. 

However, despite its enormous importance to human physiology, no pharmacological compounds targeting the components of the circadian clock system have been identified to date. There are, nevertheless, two therapeutic approaches that are currently used for treatment of circadian-related disorders - full-spectrum and bright light therapy and melatonin therapy. Melatonin is a hormone that is produced by the pineal gland in... 

Circadian rhythm 24-Hour cycles of behavior and physiology that are generated by endogenous biological clocks (pacemakers). 

Time is readily sensed physiologically, and the so-called flow of time is measured by the regularity of motions and changes occurring within and around us. Plato s interpretation of Heraclitus analogy of fife to a river is that all things are in flux. We are immersed in a universe of processes that act as clocks, and even in solitude, we can still sense heartbeat, pulse, heat flow and the motions of other internal processes. Consciousness itself appears... 

Circadian clock-controlled rhythms provide most organisms with an orchestrated temporal programme that allows for appropriate timing of physiology (i.e. blood pressure, hormonal levels) and behaviour (i.e. alertness, sleep-wake cycle). The mammalian central circadian pacemaker resides in the suprachiasmatic nucleus (SCN) of the brain (Weaver 1998). At the molecular level, the core oscillator driving the mammahan clock consists of interconnected autoregulatory... 

In mammalian tissue culture cells, robust circadian gene expression can be entrained by 12 h temperature cycles with an amplitude of 4 °C (e.g. 37 °C versus 33 °C) (Brown et al 2002). We thus wondered whether physiological temperature rhythms, themselves circadian and with an amplitude of —4 °C in most mammals, could also sustain cyclic clock-gene transcription. To this end, we engineered a... 

Circadian clocks are molecular time-keeping mechanisms found in a broad range of cell types from a variety of organisms. The primary roles of these clocks are to maintain their own 24 hour molecular rhythm and to drive the rhythmic expression of genes that control output processes in physiology, metabohsm and behaviour. Core features of the clock are its ability to synchronize to daily environmental speitgehers (e.g. hght—dark or temperature cycles), and then maintain rhythmic function when placed in constant conditions. 

Studies of mammals subjected to SCN destruction and transplantation have revealed that the hypothalamic SCN contains a master circadian oscillator which is involved in a number of behaviours and hormonal secretions. The circadian oscillatory activity of SCN neurons is directly demonstrated by the measurement of [ " C] glucose metabolic activity and field potentials assessed by electro-physiological devices. The clock oscillatory genes mPer1 and mPer2 are expressed rhythmically in most neurons in the SCN. Thus, thousands of clock cells in the SCN might generate the rhythm. 

Interestingly, clock regulation of these physiologies seems to have evolved to exert transcriptional control of key rate-limiting steps. In the SCN for example, transcriptional regulation of metaUothionein 1 activator, a component of aU... 

---