Negative feedback system

Figure 1.2. A block diagram representation of a single-input single-output negative feedback system. Labels within the boxes are general. Labels outside the boxes apply to the simplified pH control discussion.

Figure 5.2. Information around the summing point in a negative feedback system.

Say we want to control the liquid level in a tank by manipulating the inlet flow rate (Fig. 5.8). If the liquid level drops below the set point, the controller will increase its output signal to open up the inlet valve and increase liquid flow. The changes in controlled variable and controller output are in opposite directions. This is a consequence of how the error is defined in a negative feedback system. 

Glass, L. and Malta, C., Chaos in multi-looped negative feedback systems, Journal of Theoretical Biology, Vol. 145, No. 2, 1990, pp. 217-223. 

There is evidence that, in humans, a luminal, protease-mediated, negative feedback system may be operative under certain circumstances (Slaff et al., 1984), but it is controversial whether (and rather unlikely that) this mechanism contributes to the pathogenesis of pain in patients with chronic pancreatitis. Several controlled therapeutic trials in patients with chronic pancreatitis have yielded conflicting results. Moreover, experimental data suggest that hormonally-induced inhibition of pancreatic secretion alone is ineffective in painful pancreatitis. It is more likely that amelioration of pain following enzyme administration originates from correction of disturbed motor function, such as ileal brake... 

Q10 Many endocrine secretions are controlled by negative feedback systems. When the thyroid is stimulated and thyroid hormone concentration increases, it inhibits production of TSH to reduce further stimulation of the gland. As thyroid hormone secretion then diminishes, the negative feedback on the anterior pituitary is reduced and TSH secretion increases again. Basically, in... 

When self-regulating physiological systems (generally controlled by negative feedback systems, e.g. endocrine, cardiovascular) are subject to interference, their control mechanisms respond to minimise the effects of the interference and to restore the previous steady state or rhythm this is homeostasis. The previous state may be a normal function, e.g. ovulation (a rare example of a positive feedback mechanism), or an abnormal function, e.g. 

FIGURE 74-3. Negative feedback system involved in the regulation of cortisol secretion under normal conditions. CRH, corticotropin-releasing hormone ACTH, adrenocorticotropic hormone. 

The summing block that follows has one negative and one positive input, since it represents a negative feedback system. This implies that another 180° shift will occur after the signal leaves the H block. So we conclude that in general, a system would be unstable if a disturbance goes around the loop and comes back to the same point, with the same magnitude and the same phase. 

The arterial blood pressure within the mammalian organism, whether normal or abnormal, is ultimately determined by cardiac output and peripheral resistance to flow. Of the several physical and chemical variables of the cardiovascular system, blood pressure is ordinarily the most constant. This is especially remarkable if one considers the large variations encountered in cardiac output and in heart rate. This feat is accomplished by a negative feedback system. The chief mechanism involves the baroreceptor reflex. 

However, if the initial event is a decrease in pressure, the opposite effects occur. These then are the essential aspects of a negative feedback system. When pathological factors affect the system, hypertension may result. 

Regulation of Secretion Plasma Ca +is the major regulator of PTH secretion hypocalcemia stimulates and hypercalcemia inhibits PTH secretion. Sustained hypocalcemia also induces parathyroid hypertrophy and hyperplasia. Changes in Ca modulate PTH secretion by parathyroid cells via the calcium-sensing receptor (CaSR), a GPCR that couples with G -PLC and G. Occupancy of the CaSR by Ca inhibits PTH secretion thus, the extracellular concentration of Ca is controlled by an endocrine negative-feedback system, the afferent limb of which senses the ambient activity of Ca and the efferent hmb of which releases PTH that then acts to increase Ca. The active vitamin D metabolite calcitriol directly suppresses PTH gene expression. 

In each of the models studied here, the domains of chaos and birhythmicity are much more reduced than the domain of complex periodic oscillations, which is itself smaller than the domain where simple periodic oscillations occur. This observation, which is corroborated by results obtained on the occurrence of complex oscillations in multi-looped negative feedback systems (Glass Malta, 1990), accounts for... 

After the surgery and onset of SIRS, negative feedback systems tend to limit or neutralize the auto-destructive inflammatory process. While this is beneficial from a self-destructive perspective, it is often associated with major post-surgical infections [19],... 

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