Regulation feedforward

Fig. 5.3. Process regulation (a) feedback control, (b) feedforward control.

The third and final control step is mediated by PK. This enzyme, like HK, exists as a number of isoenzymes in different tissues and, like the PFK reaction, is controlled by both the concentration of metabolites and covalent effects. Furthermore, PK also illustrates two other means of metabolic control, namely enzyme induction and feedforward, regulation. 

Figure 6.19 Regulation of the synthesis of glycogen from glucose in liver and muscle. Insulin is the major factor stimulating glycogen synthesis in muscle it increases glucose transport into the muscle and the activity of glycogen synthase, activity which is also activated by glucose 6-phosphate but inhibited by glycogen. The latter represents a feedback mechanism and the former a feedforward. The mechanism by which glycogen inhibits the activity is not known. The mechanism for the insulin effect is discussed in Chapter 12.

The control via activation or inhibition of the rate(s) of an enzyme-catalyzed reaction(s). This control includes the increase or decrease in the stability or half-life of the enzyme(s). There are many different means by which control can be achieved. These include 1. Substrate availability and reaction conditions (e.g., pH, temperature, ionic strength, lipid interface activation) 2. Magnitude of Vraax sud valucs) 3. Activation (particularly, feedforward activation) 4. Isozyme formation 5. Com-partmentalization and channeling 6. Oligomerization/ polymerization 7. Feedback inhibition and cooperativity (particularly, allosterism and/or hysteresis) 8. Covalent modification and 9. Gene regulation (induction repression)... 

PID controller the classical PID regulator (5.1) with a feedforward compensation of the desired reactor temperature (i.e., ypdes is added to the control input). 

Flocculant addition rate can be regulated in proportion to the thickener volumetric feed rate or solids mass flow in a feed-forward mode, or in a feed-back mode on either rake torque, underflow density, settling solids (sludge) bed level, or solids settling rate. Of these, feedforward on mass flow or feed-back on bed level are probably the most common. In some feed-forward schemes, the ratio multiplier is trimmed by one of the other parameters. 

In positive feedforward, earlier reactants in a metabolic sequence feed-forward positively on later steps. If A is accumulating, it makes sense to speed up downstream reactions to use it up, e.g., ft-uctose-l,6-bisphosphate activates pyruvate kinase in glycolysis. A combination of feedback and feedforward is used to regulate enzyme activity (Fig. 6.10). 

Intrinsic or Local Connections There are extensive translaminar cormections from superficial to deeper layers and vice versa. Layer II pyramidal cell axon collaterals synapse with deeper layer III pyramidal cells and with local inhibitory intemeurons in layers I and II. Local collaterals from deeper pyramidal cells synapse with local intemeurons or with more superficial pyramidal cells. Local projections from pyramidal cells to GABAergic intemeurons play an important role in regulating PC neuronal excitability, including the expression of LTP and seizures, via feedback and feedforward inhibition (Ekstrand et al., 2001a). 

The last enzyme in glycolysis, pyruvate kinase, is also subject to allosteric regulation. In this case, fructose-l,6-bisphosphate is the allosteric activator. It is interesting that fructose-l,6-bisphosphate is the product of the reaction catalyzed by phosphofructokinase. Thus, activation of phosphofructokinase results in the activation of pyruvate kinase. This is an example of feedforward activation because the product of an earlier reaction causes activation of an enzyme later in the pathway... 

Draw the feedforward and feedback control systems that regulate the flow through a pipe. Do you expect one of them to be significantly better than the other in maintaining the desired flow ... 

Pyruvate kinase is inhibited by ATP, activated by fructose-1,6-bisphosphate (feedforward activation), and inhibited by acetyl-CoA. It is also responsive to hormonally-regulated phosphorylation in the liver -the phosphorylated form of the enzyme is less active. Alanine is also an inhibitor of the enzyme. 

Feedback control - In this mechanism, the product of a series of reactions (like in an assembly line) inhibits the action of an earlier step in the process (usually the first step). Feedforward regulation occurs when a molecule in an assembly line reaction activates an enzyme ahead of it in the pathway. 

Figure 8.16 Decision boundaries of a feedforward neural network trained by a Bayesian regulation (a) and a conjugate gradient backpropagation algorithm (b).

Like any closed-loop system, the behavior of the respiratory control system is defined by the continual interaction of the controller and the peripheral processes being controlled. The latter include the respiratory mechanical system and the pulmonary gas exchange process. These peripheral processes have been extensively studied, and their quantitative relationships have been described in detail in previous reviews. Less well understood is the behavior of the respiratory controller and the way in which it processes afferent inputs. A confounding factor is that the controller may manifest itself in many different ways, depending on the modeling and experimental approaches being taken. Traditionally, the respiratory control system has been modeled as a closed-loop feedback/feedforward regulator whereby homeostasis of arterial blood gas and pH is maintained. Alternatively, the respiratory controller may be viewed as a... 

Microprocessor-based control (Figure 57.11b) in which the outlet-air tanperature is controlled by regulating the feed rate, with feedback from the moisture content measurement and feedforward from the atmospheric humidity, feed specific gravity, and inlet air-tanperature measurements... 

Sloviter, RS (1991a) Feedforward and feedback inhibition of hippocampal principal cell activity evoked by perforant path stimulation GABA-mediated mechanisms that regulate excitability in vivo. Hippocampus, 1 31-40. 

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