Negative feed-back

Of course the thyroid does not simply produce T-4 continuously. This is due to the "checks and balances" nature included called "feedback-mechanisms". In the care of thyroid function the feedback-mechanism or loop involves the hypothalamus (secretes TRH), pituitary gland (secretes TSH), thyroid gland (secretes T-4), and the liver (converts T-4 into T-3). A feedback-mechanism or loop can trigger the release of another hormone (positive feed-back), or inhibit its release (negative feed-back) thus maintaining that balance. This means high levels of T-4 or T-3 initiate a negative feed-back loop that tells the hypothalamus to produce less TRH, and low levels of T-4 or T-3 initiate a positive feed-back loop that tells the hypothalamus to produce more TRH. 

A tree has only limited ways of response. Any damage of the root system will have an effect to the leaf mass and any reducticm of photosynthesis by direct impact of air pollutants to leaves will reversely reduce the number of feeder roots and start a negative feed-back (Bossel et al., 1984). 

Figure 50-9 The regulatory feedback loop of the hypothalamic-pituitary-gonadal axis. Neural and sensory input from the brain elicits the release of Gn-RH. Gn-RH in turn stimulates the synthesis and release of the gonadotropins FSH and LH, which act on the gonads (ovary and testes) to elicit the ripening and ovulation of the ovary and steroidogenesis (estradiol and progesterone) in the female and spermatogenesis and testosterone production in the male. Inhibin formed by the ovaries and testes along with estradiol and testosterone negatively feeds back to the hypothalamic-pituitary axis to modulate Gn-RH, FSH, and LH release.

If AS < 0 a similar line of arguments will lead to the conclusion that DH will immediately assume a new higher value and thereafter decrease. We have thus demonstrated that there is negative feed-back in the system with respect to DH. 

The demonstration of negative feed-back for the two dependent variables, DH and S, is proof of the intrinsic stability of the steady state. 

By employing HCG intermittently athletes have maintained some degree of testicular function. Stimulated too much for too long of a period would result in another negative feed-back loop to the demise of the lads in the sack. 

In the case of emergency overheating and simultaneous postulated failure of emergency protection systems (EPS) down to the level which does not cause the core damage the reactor power decrease is ensured by reactivity negative feed backs. 

However, the reaction between zirconium and water which gives zirconium dioxide and hydrogen as products, generates heat (i.e. is exothermic) and so, if initiated, is likely to proceed rapidly. The rate of reaction will increase as the temperature of the components increases. An exothermic reaction, once started, will tend to raise the system temperature and increase the rate of the reaction. Such reactions can lead to instabilities of a positive feedback nature. Endothermic reactions, however, which require energy to be supplied—the reverse of exothermic—are therefore of a negative feed-back and inherently stable type. 

As early as 1966, Siperstein et al. showed negative feed-back control of cholesterol biosynthesis to be defective in transplantable hepatomas. More recently Bricker Levey (1972) demonstrated unsupressible cholesterol and fatty acid synthesis by cyclic nucleotides in hepatomas and indicated a deletion of this regulatory mechanism of lipogenesis. 

This, however, is only a matter of principle. In practice,the situation may be much more favorable. The reason of this lies in the fact that the SCF procedure is equivalent to a negative-feed-back scheme. The first iteration will generate, say, charges represented by the sequence... 

In proliferating cells purine nucleotides - necessary for DNA- and RNA-synthesis - can be formed by incorporation of small precursors (de novo pathway) and by utilisation of free purin bases (salvage pathway). The most important reactions of both metabolic pathways are demonstrated in figure 1. The first reaction of the de-novo pathway - the formation of phosphoribosylamine from glutamine and phosphoribosyIpyrophosphate (PRPP) by the enzyme PRPPamidotransferase - is the target of a negative feed back control mechanism by the endproducts IMP/GMP and AMP (1). 

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