Feedback interactions

Sunda, W. G. and Huntsman, S. A. (1992). Feedback interactions between zinc and phytoplankton in seawater, Limnol. Oceanogr., 37, 25-40. 

As shown notably by the thermodynamic school of Brussels,9,10 systems maintained far from equilibrium and endowed with appropriate non-linearities and feedback interactions may display such nontrivial behaviors as sustained oscillations and multiple steady states (see papers by I. Prigogine and G. Nicolis in this volume). Even though the structures studied are much simpler than biological systems, this type of work provides a firm fundamental basis, not sufficient but absolutely necessary, for the future understanding of such processes as cell differentiation. Clearly, the sustained oscillations and multiple steady states displayed by simpler systems are related, respectively, with the two types of biological regulation described above. 

Luyben (1993a) provided valuable insights into the characteristics of recycle systems and their design, control, and economics, and illustrated the challenges caused by the feedback interactions in such systems, within a multi-loop linear control framework. Also, in the context of steady-state operation, it was shown (Luyben 1994) that the steady-state recycle flow rate is very sensitive to disturbances in feed flow rate and feed composition and that, when certain control configurations are used, the recycle flow rate increases considerably facing feed flow rate disturbances. This behavior was termed the snowball effect. ... 

The control challenges posed by the feedback interactions induced by the recycle were also illustrated in studies carried out on other problems, such as supercritical fluid extraction (Ramchandran et al. 1992) and recycle reactors (Kanadibhotla and Riggs 1995, Antoniades and Christofides 2001). 

Example of Feedback Interaction Between Zinc and Phytoplankton in Seawater... 

Excitatory Amino Acids (EAAs). Excitatory amino acids are claimed to be the principal excitatory transmitters in the reticular formation (34). EAAs including glutamate are involved in the positive feedback interaction between the cholinergic and the reticular neurons. 

Fig. 2 Feedback interactions in a hypothalamic-pituitary-endocrine gland system.

The Tribal type evolved through the evolution of Tribal cultures the human type and the cultural structure were, of course, co>evolving as a feedback-interacting system. The resultant human type is characterized by rejection of creativity, and by complete rejection of personal responsibility. The Tribal rituals determine all Right Living Ways he is not responsible for anything, so long as he follows the commands of the Tribal rituals. He accepts external command—and is rewarded with security. If he obeys the Tribal rituals, the tribe will defend and protect him. 

As chemistry penetrates biology, biotechnology, and genome smdies, the reaction systems being considered become more and more complex, with many more species, many more catalysts, more complex connectivities, and positive and negative feedforward and feedback interactions. New experiments and theories are necessary for investigating such systems, and this book is an introduction to the field. Much, bul certainly not all, of the contents of the book is the result of our research studies in the last fifteen years, and it is time to assemble this work into a coherent presentation. 

Figure 1 A schematic representation of a gene regulatory network involving modules of molecular classes (shown in boxes) the modules shown are the transcriptional units in the genome (G), primary transcripts (Ro), mature transcripts (R), primary proteins (Po), modified proteins (P), and metabolites (M). The labeled steps shown in black lines are transcription (x), RNA processing (p), translation (a), protein modification (p), metabolic pathways (ti), and genome replication (a). The feedback interactions shown in gray lines are discussed in the text. Filled circles represent either inhibition or activation.

In the hormone delivery oscillator considered below, neither imposed electric fields or currents, nor pH or salt gradients are used, since they are difficult to impose in a physiological environment Instead, feedback interactions between an enzyme-catalyzed chemical reaction and transport across the membrane are key. Previous efforts coupling enzymes and membrane transport in artificial membranes, leading to oscillations, will now be reviewed. 

That is, because the block diagonal rearrangement means it is possible to assume an association between state Xj and eigenvalue Xj, a sufficiently large Py indicates the dynamics of state Xj are present in the natural response of state Xi. This indicates an interaction from state Xj to state Xi. The structure of the non-zeroes in the UPSR matrix indicates a maximum possible level of feedback interaction within a process. However once the UPSR matrix is actually calculated many of the structural feedback loops prove to be very weak, and can be regarded as zero. This type of structure is very common in process systems. 

Figure 4.6 The feedback interaction between the source and the resonator is an important aspect of the waveguide technique...

To illustrate what information is missing let us look at the complexity and diversity of the feedback functionality. There are four possible types of feedback Interaction Feedback to inform users that the system has heard their request Progress Feedback for tasks that take some time to finish System Status Display to inform users about any change in the system status, and Warnings to inform users about irreversible actions. Additionally, each feedback type has its own peculiarities. For example, a developer needs many details to build a system that provides a satisfactory System Status Feedback what states to report, what information to display for each state how prominent the information should be in each case (for example, should the application keep control of the system while reporting, or should the system let the user work on other tasks while status reporting), etc. 

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