Biological rhythms hormone secretion

For linear systems, the principle of superposition applies, and different oscillatory modes can evolve independently of one another. However, biological systems in general are not linear, and separation of different regulatory mechanisms may not be justified, even when they involve different time scales. One type of phenomenon that can arise from the interaction between two oscillatory modes is modulation of the amplitude and frequency of the faster mode in dependence of the phase of the slower mode. This type of phenomenon was demonstrated in Fig. 12.2c where the frequency of the myogenic mode fjast changes in step with the amplitude of the TGF-mediated mode. Similar modulation phenomena can be expected to occur in many other biological systems such as, for instance, the interaction between the circadian and the ultradian rhythms of hormone secretion [25]. 

The possible occurrence in the Dictyostelium system of complex oscillatory phenomena such as birhythmicity, bursting and chaos is discussed in chapter 6. The appearance of periodic behaviour in the course of development provides a model for the ontogenesis of biological rhythms. This aspect is treated in chapter 7. Finally, an additional interest of the intercellular communication system of Dictyostelium amoebae is that it allows us to address the question of the physiological function of the periodic phenomenon. This question is dealt with in chapter 8, where the discussion is extended to the role of pulsatile hormone secretion in higher organisms. 

Hormones transmit information from one body site to another. Transported in the bloodstream, the hormone secreted by an endocrine gland will activate biochemical processes in various "target" cells in the body that have specific reception for that hormone. Practically all hormonal secretions exhibit circadian rhythms [8], Consequently, the plasma concentration of a hormone has a peak at a characteristic time of day and a trough at another. The circadian timing system uses circadian rhythms in plasma hormone levels to transmit temporal information and ensure the internal synchronization of the multitude of rhythmic functions in various body tissues. Several researchers have shown that aberrations in circadian rhythm timing of hormones can be viewed as a biological dy sfunction[9,10,11 ]. 

Melatonin, a hormone secreted in the light-sensitive pineal gland in the vertebrate brain, is thought to function in the formation of biological rhythms (Campbell et al., 1999). Melatonin is... 

T.SH is the feedback of thyroid hormone activity on the pituitary, and to a lesser extent on the hypothalamus. Other factors also play a role. There is a diurnal rhythm with the serum TSH peaking around midday when the concentration is approximately 30% higher than at midnight, when it isaliLsnadir. Insy.stemic illness the normal regulation of TSH.Tj and T, secretion, and the subsequent metabolism of the thyroid hormones, is disturbed. Increased amounts of T are converted to the biologically inactive reverse Tj, rather than to T,. The resultant reduction in thyroid hormone activity does not result in an increased serum TSH concentration. TSH secretion is also suppressed, and the secretion of T and T, by the thyroid gland is therefore decreased. 

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